scheme48/scheme/srfi/srfi-13.scm

;;; SRFI 13 string library reference implementation		-*- Scheme -*-
;;; Olin Shivers 7/2000
;;;
;;; Copyright (c) 1988-1994 Massachusetts Institute of Technology.
;;; Copyright (c) 1998, 1999, 2000 Olin Shivers. All rights reserved.
;;;   The details of the copyrights appear at the end of the file. Short
;;;   summary: BSD-style open source.

;;; Exports:
;;; string-map string-map!
;;; string-fold       string-unfold
;;; string-fold-right string-unfold-right 
;;; string-tabulate string-for-each string-for-each-index
;;; string-every string-any
;;; string-hash string-hash-ci
;;; string-compare string-compare-ci
;;; string=    string<    string>    string<=    string>=    string<>
;;; string-ci= string-ci< string-ci> string-ci<= string-ci>= string-ci<> 
;;; string-downcase  string-upcase  string-titlecase  
;;; string-downcase! string-upcase! string-titlecase! 
;;; string-take string-take-right
;;; string-drop string-drop-right
;;; string-pad string-pad-right
;;; string-trim string-trim-right string-trim-both
;;; string-filter string-delete
;;; string-index string-index-right 
;;; string-skip  string-skip-right
;;; string-count
;;; string-prefix-length string-prefix-length-ci
;;; string-suffix-length string-suffix-length-ci
;;; string-prefix? string-prefix-ci?
;;; string-suffix? string-suffix-ci?
;;; string-contains string-contains-ci
;;; string-copy! substring/shared
;;; string-reverse string-reverse! reverse-list->string
;;; string-concatenate string-concatenate/shared string-concatenate-reverse
;;; string-append/shared
;;; xsubstring string-xcopy!
;;; string-null?
;;; string-join
;;; string-tokenize
;;; string-replace
;;; 
;;; R5RS extended:
;;; string->list string-copy string-fill! 
;;;
;;; R5RS re-exports:
;;; string? make-string string-length string-ref string-set! 
;;;
;;; R5RS re-exports (also defined here but commented-out):
;;; string string-append list->string
;;;
;;; Low-level routines:
;;; make-kmp-restart-vector string-kmp-partial-search kmp-step
;;; string-parse-start+end
;;; string-parse-final-start+end
;;; let-string-start+end
;;; check-substring-spec
;;; substring-spec-ok?

;;; Imports
;;; This is a fairly large library. While it was written for portability, you
;;; must be aware of its dependencies in order to run it in a given scheme
;;; implementation. Here is a complete list of the dependencies it has and the
;;; assumptions it makes beyond stock R5RS Scheme:
;;;
;;; This code has the following non-R5RS dependencies:
;;; - (RECEIVE (var ...) mv-exp body ...) multiple-value binding macro;
;;;
;;; - Various imports from the char-set library for the routines that can
;;;   take char-set arguments;
;;;   
;;; - An ASSERTION-VIOLATION procedure;
;;;   
;;; - BITWISE-AND for the hash functions;
;;;   
;;; - A simple CHECK-ARG procedure for checking parameter values; it is 
;;;   (lambda (pred val proc) 
;;;     (if (pred val) val (assertion-violation 'check-arg "Bad arg" val pred proc)))
;;;   
;;; - :OPTIONAL and LET-OPTIONALS* macros for parsing, defaulting & 
;;;   type-checking optional parameters from a rest argument;
;;;   
;;; - CHAR-CASED? and CHAR-TITLECASE for the STRING-TITLECASE & 
;;;   STRING-TITLECASE! procedures. The former returns true iff a character is
;;;   one that has case distinctions; in ASCII it returns true on a-z and A-Z.
;;;   CHAR-TITLECASE is analagous to CHAR-UPCASE and CHAR-DOWNCASE. In ASCII &
;;;   Latin-1, it is the same as CHAR-UPCASE.
;;;
;;; The code depends upon a small set of core string primitives from R5RS:
;;;     MAKE-STRING STRING-REF STRING-SET! STRING? STRING-LENGTH SUBSTRING 
;;; (Actually, SUBSTRING is not a primitive, but we assume that an 
;;; implementation's native version is probably faster than one we could
;;; define, so we import it from R5RS.)
;;;
;;; The code depends upon a small set of R5RS character primitives:
;;;   char? char=? char-ci=? char<? char-ci<?
;;;   char-upcase char-downcase
;;;   char->integer (for the hash functions)
;;;   
;;; We assume the following:
;;; - CHAR-DOWNCASE o CHAR-UPCASE = CHAR-DOWNCASE
;;; - CHAR-CI=? is equivalent to
;;;     (lambda (c1 c2) (char=? (char-downcase (char-upcase c1))
;;;                             (char-downcase (char-upcase c2))))
;;; - CHAR-UPCASE, CHAR-DOWNCASE and CHAR-TITLECASE are locale-insensitive
;;;   and consistent with Unicode's 1-1 char-mapping spec.
;;; These things are typically true, but if not, you would need to modify
;;; the case-mapping and case-insensitive routines.

;;; Enough introductory blather. On to the source code. (But see the end of
;;; the file for further notes on porting & performance tuning.)

; Start S48 additions

(define (check-arg pred val caller)
  (if (not (pred val))
      (assertion-violation caller "invalid argument" val))
  val)

(define-syntax :optional
  (syntax-rules ()
    ((:optional rest default-exp)
     (let ((maybe-arg rest))
       (if (pair? maybe-arg)
	   (if (null? (cdr maybe-arg)) (car maybe-arg)
	       (apply assertion-violation ':optional
		      "too many optional arguments" maybe-arg))
	   default-exp)))

    ((:optional rest default-exp arg-test)
     (let ((maybe-arg rest))
       (if (pair? maybe-arg)
	   (if (null? (cdr maybe-arg))
	       (let ((val (car maybe-arg)))
		 (if (arg-test val)
		     val
		     (assertion-violation ':optional
					  "Optional argument failed test"
					  arg-test val)))
	       (apply assertion-violation ':optional
		      "too many optional arguments"
		      maybe-arg))
	   default-exp)))))

(define-syntax let-optionals*
  (syntax-rules ()
    ((let-optionals* arg (opt-clause ...) body ...)
     (let ((rest arg))
       (%let-optionals* rest (opt-clause ...) body ...)))))

(define-syntax %let-optionals*
  (syntax-rules ()
    ((%let-optionals* arg (((var ...) xparser) opt-clause ...) body ...)
     (call-with-values (lambda () (xparser arg))
       (lambda (rest var ...)
         (%let-optionals* rest (opt-clause ...) body ...))))
    
    ((%let-optionals* arg ((var default) opt-clause ...) body ...)
     (call-with-values (lambda () (if (null? arg) (values default '())
				      (values (car arg) (cdr arg))))
       (lambda (var rest)
	 (%let-optionals* rest (opt-clause ...) body ...))))

    ((%let-optionals* arg ((var default test) opt-clause ...) body ...)
     (call-with-values (lambda ()
			 (if (null? arg) (values default '())
			     (let ((var (car arg)))
			       (if test (values var (cdr arg))
				   (assertion-violation 'let-opt
							"arg failed LET-OPT test" var)))))
       (lambda (var rest)
	 (%let-optionals* rest (opt-clause ...) body ...))))

    ((%let-optionals* arg ((var default test supplied?) opt-clause ...) body ...)
     (call-with-values (lambda ()
			 (if (null? arg) (values default #f '())
			     (let ((var (car arg)))
			       (if test (values var #t (cdr arg))
				   (assertion-violation 'let-opt
							"arg failed LET-OPT test" var)))))
       (lambda (var supplied? rest)
	 (%let-optionals* rest (opt-clause ...) body ...))))

    ((%let-optionals* arg (rest) body ...)
     (let ((rest arg)) body ...))

    ((%let-optionals* arg () body ...)
     (if (null? arg) (begin body ...)
	 (assertion-violation 'let-opt "Too many arguments in let-opt" arg)))))

(define (char-cased? c)
  (or (char-lower-case? c)
      (char-upper-case? c)
      (char-title-case? c)))
	 
; End S48 additions

;;; Support for START/END substring specs
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; This macro parses optional start/end arguments from arg lists, defaulting
;;; them to 0/(string-length s), and checks them for correctness.

(define-syntax let-string-start+end
  (syntax-rules ()
    ((let-string-start+end (start end) proc s-exp args-exp body ...)
     (receive (start end) (string-parse-final-start+end 'proc s-exp args-exp)
       body ...))
    ((let-string-start+end (start end rest) proc s-exp args-exp body ...)
     (receive (rest start end) (string-parse-start+end 'proc s-exp args-exp)
       body ...))))

;;; This one parses out a *pair* of final start/end indices. 
;;; Not exported; for internal use.
(define-syntax let-string-start+end2
  (syntax-rules ()
    ((l-s-s+e2 (start1 end1 start2 end2) proc s1 s2 args body ...)
     (let ((procv proc)) ; Make sure PROC is only evaluated once.
       (let-string-start+end (start1 end1 rest) procv s1 args
         (let-string-start+end (start2 end2) procv s2 rest
           body ...))))))


;;; Returns three values: rest start end

(define (string-parse-start+end proc s args)
  (if (not (string? s)) (assertion-violation proc "Non-string value" s))
  (let ((slen (string-length s)))
    (if (pair? args)

	(let ((start (car args))
	      (args (cdr args)))
	  (if (and (integer? start) (exact? start) (>= start 0))
	      (receive (end args)
		  (if (pair? args)
		      (let ((end (car args))
			    (args (cdr args)))
			(if (and (integer? end) (exact? end) (<= end slen))
			    (values end args)
			    (assertion-violation proc "Illegal substring END spec"
						 end s)))
		      (values slen args))
		(if (<= start end) (values args start end)
		    (assertion-violation proc "Illegal substring START/END spec"
					 start end s)))
	      (assertion-violation proc "Illegal substring START spec" start s)))

	(values '() 0 slen))))

(define (string-parse-final-start+end proc s args)
  (receive (rest start end) (string-parse-start+end proc s args)
    (if (pair? rest)
	(assertion-violation proc "Extra arguments to procedure" rest)
	(values start end))))

(define (substring-spec-ok? s start end)
  (and (string? s)
       (integer? start)
       (exact? start)
       (integer? end)
       (exact? end)
       (<= 0 start)
       (<= start end)
       (<= end (string-length s))))

(define (check-substring-spec proc s start end)
  (if (not (substring-spec-ok? s start end))
      (assertion-violation proc "Illegal substring spec." s start end)))


;;; Defined by R5RS, so commented out here.
;(define (string . chars)
;  (let* ((len (length chars))
;         (ans (make-string len)))
;    (do ((i 0 (+ i 1))
;	 (chars chars (cdr chars)))
;	((>= i len))
;      (string-set! ans i (car chars)))
;    ans))
;
;(define (string . chars) (string-unfold null? car cdr chars))



;;; substring/shared S START [END] 
;;; string-copy      S [START END]
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;; All this goop is just arg parsing & checking surrounding a call to the
;;; actual primitive, %SUBSTRING/SHARED.

(define (substring/shared s start . maybe-end)
  (check-arg string? s 'substring/shared)
  (let ((slen (string-length s)))
    (check-arg (lambda (start) (and (integer? start) (exact? start) (<= 0 start)))
	       start 'substring/shared)
    (%substring/shared s start
		       (:optional maybe-end slen
				  (lambda (end) (and (integer? end)
						     (exact? end)
						     (<= start end)
						     (<= end slen)))))))

;;; Split out so that other routines in this library can avoid arg-parsing
;;; overhead for END parameter.
(define (%substring/shared s start end)
  (if (and (zero? start) (= end (string-length s))) s
      (substring s start end)))

(define (string-copy s . maybe-start+end)
  (let-string-start+end (start end) string-copy s maybe-start+end
    (substring s start end)))

;This library uses the R5RS SUBSTRING, but doesn't export it.
;Here is a definition, just for completeness.
;(define (substring s start end)
;  (check-substring-spec 'substring s start end)
;  (let* ((slen (- end start))
;         (ans (make-string slen)))
;    (do ((i 0 (+ i 1))
;         (j start (+ j 1)))
;        ((>= i slen) ans)
;      (string-set! ans i (string-ref s j)))))

;;; Basic iterators and other higher-order abstractions
;;; (string-map proc s [start end])
;;; (string-map! proc s [start end])
;;; (string-fold kons knil s [start end])
;;; (string-fold-right kons knil s [start end])
;;; (string-unfold       p f g seed [base make-final])
;;; (string-unfold-right p f g seed [base make-final])
;;; (string-for-each       proc s [start end])
;;; (string-for-each-index proc s [start end])
;;; (string-every char-set/char/pred s [start end])
;;; (string-any   char-set/char/pred s [start end])
;;; (string-tabulate proc len)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; You want compiler support for high-level transforms on fold and unfold ops.
;;; You'd at least like a lot of inlining for clients of these procedures.
;;; Don't hold your breath.

(define (string-map proc s . maybe-start+end)
  (check-arg procedure? proc 'string-map)
  (let-string-start+end (start end) string-map s maybe-start+end
    (%string-map proc s start end)))

(define (%string-map proc s start end)	; Internal utility
  (let* ((len (- end start))
	 (ans (make-string len)))
    (do ((i (- end 1) (- i 1))
	 (j (- len 1) (- j 1)))
	((< j 0))
      (string-set! ans j (proc (string-ref s i))))
    ans))

(define (string-map! proc s . maybe-start+end)
  (check-arg procedure? proc 'string-map!)
  (let-string-start+end (start end) string-map! s maybe-start+end
    (%string-map! proc s start end)))

(define (%string-map! proc s start end)
  (do ((i (- end 1) (- i 1)))
      ((< i start))
    (string-set! s i (proc (string-ref s i)))))

(define (string-fold kons knil s . maybe-start+end)
  (check-arg procedure? kons 'string-fold)
  (let-string-start+end (start end) string-fold s maybe-start+end
    (let lp ((v knil) (i start))
      (if (< i end) (lp (kons (string-ref s i) v) (+ i 1))
	  v))))

(define (string-fold-right kons knil s . maybe-start+end)
  (check-arg procedure? kons 'string-fold-right)
  (let-string-start+end (start end) string-fold-right s maybe-start+end
    (let lp ((v knil) (i (- end 1)))
      (if (>= i start) (lp (kons (string-ref s i) v) (- i 1))
	  v))))

;;; (string-unfold p f g seed [base make-final])
;;; This is the fundamental constructor for strings. 
;;; - G is used to generate a series of "seed" values from the initial seed:
;;;     SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...
;;; - P tells us when to stop -- when it returns true when applied to one 
;;;   of these seed values.
;;; - F maps each seed value to the corresponding character 
;;;   in the result string. These chars are assembled into the
;;;   string in a left-to-right order.
;;; - BASE is the optional initial/leftmost portion of the constructed string;
;;;   it defaults to the empty string "".
;;; - MAKE-FINAL is applied to the terminal seed value (on which P returns
;;;   true) to produce the final/rightmost portion of the constructed string.
;;;   It defaults to (LAMBDA (X) "").
;;;
;;; In other words, the following (simple, inefficient) definition holds:
;;; (define (string-unfold p f g seed base make-final)
;;;   (string-append base
;;;                  (let recur ((seed seed))
;;;                    (if (p seed) (make-final seed)
;;;                        (string-append (string (f seed))
;;;                                       (recur (g seed)))))))
;;; 
;;; STRING-UNFOLD is a fairly powerful constructor -- you can use it to
;;; reverse a string, copy a string, convert a list to a string, read
;;; a port into a string, and so forth. Examples:
;;; (port->string port) =
;;;   (string-unfold (compose eof-object? peek-char)
;;;                  read-char values port)
;;;
;;; (list->string lis) = (string-unfold null? car cdr lis)
;;; 
;;; (tabulate-string f size) = (string-unfold (lambda (i) (= i size)) f add1 0)

;;; A problem with the following simple formulation is that it pushes one
;;; stack frame for every char in the result string -- an issue if you are
;;; using it to read a 100kchar string. So we don't use it -- but I include
;;; it to give a clear, straightforward description of what the function
;;; does.

;(define (string-unfold p f g seed base make-final)
;  (let ((ans (let recur ((seed seed) (i (string-length base)))
;               (if (p seed)
;                   (let* ((final (make-final seed))
;                          (ans (make-string (+ i (string-length final)))))
;                     (string-copy! ans i final)
;                     ans)
;
;                   (let* ((c (f seed))
;                          (s (recur (g seed) (+ i 1))))
;                     (string-set! s i c)
;                     s)))))
;    (string-copy! ans 0 base)
;    ans))

;;; The strategy is to allocate a series of chunks into which we stash the
;;; chars as we generate them. Chunk size goes up in powers of two starting
;;; with 40 and levelling out at 4k, i.e.
;;;     40 40 80 160 320 640 1280 2560 4096 4096 4096 4096 4096...
;;; This should work pretty well for short strings, 1-line (80 char) strings,
;;; and longer ones. When done, we allocate an answer string and copy the
;;; chars over from the chunk buffers.

(define (string-unfold p f g seed . base+make-final)
  (check-arg procedure? p 'string-unfold)
  (check-arg procedure? f 'string-unfold)
  (check-arg procedure? g 'string-unfold)
  (let-optionals* base+make-final
                  ((base       ""              (string? base))
		   (make-final (lambda (x) "") (procedure? make-final)))
    (let lp ((chunks '())		; Previously filled chunks
	     (nchars 0)			; Number of chars in CHUNKS
	     (chunk (make-string 40))	; Current chunk into which we write
	     (chunk-len 40)
	     (i 0)			; Number of chars written into CHUNK
	     (seed seed))
      (let lp2 ((i i) (seed seed))
	(if (not (p seed))
	    (let ((c (f seed))
		  (seed (g seed)))
	      (if (< i chunk-len)
		  (begin (string-set! chunk i c)
			 (lp2 (+ i 1) seed))

		  (let* ((nchars2 (+ chunk-len nchars))
			 (chunk-len2 (min 4096 nchars2))
			 (new-chunk (make-string chunk-len2)))
		    (string-set! new-chunk 0 c)
		    (lp (cons chunk chunks) (+ nchars chunk-len)
			new-chunk chunk-len2 1 seed))))

	    ;; We're done. Make the answer string & install the bits.
	    (let* ((final (make-final seed))
		   (flen (string-length final))
		   (base-len (string-length base))
		   (j (+ base-len nchars i))
		   (ans (make-string (+ j flen))))
	      (%string-copy! ans j final 0 flen)	; Install FINAL.
	      (let ((j (- j i)))
		(%string-copy! ans j chunk 0 i)		; Install CHUNK[0,I).
		(let lp ((j j) (chunks chunks))		; Install CHUNKS.
		  (if (pair? chunks)
		      (let* ((chunk  (car chunks))
			     (chunks (cdr chunks))
			     (chunk-len (string-length chunk))
			     (j (- j chunk-len)))
			(%string-copy! ans j chunk 0 chunk-len)
			(lp j chunks)))))
	      (%string-copy! ans 0 base 0 base-len)	; Install BASE.
	      ans))))))

(define (string-unfold-right p f g seed . base+make-final)
  (let-optionals* base+make-final
                  ((base       ""              (string? base))
		   (make-final (lambda (x) "") (procedure? make-final)))
    (let lp ((chunks '())		; Previously filled chunks
	     (nchars 0)			; Number of chars in CHUNKS
	     (chunk (make-string 40))	; Current chunk into which we write
	     (chunk-len 40)
	     (i 40)			; Number of chars available in CHUNK
	     (seed seed))
      (let lp2 ((i i) (seed seed))	; Fill up CHUNK from right
	(if (not (p seed))		; to left.
	    (let ((c (f seed))
		  (seed (g seed)))
	      (if (> i 0)
		  (let ((i (- i 1)))
		    (string-set! chunk i c)
		    (lp2 i seed))

		  (let* ((nchars2 (+ chunk-len nchars))
			 (chunk-len2 (min 4096 nchars2))
			 (new-chunk (make-string chunk-len2))
			 (i (- chunk-len2 1)))
		    (string-set! new-chunk i c)
		    (lp (cons chunk chunks) (+ nchars chunk-len)
			new-chunk chunk-len2 i seed))))

	    ;; We're done. Make the answer string & install the bits.
	    (let* ((final (make-final seed))
		   (flen (string-length final))
		   (base-len (string-length base))
		   (chunk-used (- chunk-len i))
		   (j (+ base-len nchars chunk-used))
		   (ans (make-string (+ j flen))))
	      (%string-copy! ans 0 final 0 flen)	; Install FINAL.
	      (%string-copy! ans flen chunk i chunk-len); Install CHUNK[I,).
	      (let lp ((j (+ flen chunk-used))		; Install CHUNKS.
		       (chunks chunks))		
		  (if (pair? chunks)
		      (let* ((chunk  (car chunks))
			     (chunks (cdr chunks))
			     (chunk-len (string-length chunk)))
			(%string-copy! ans j chunk 0 chunk-len)
			(lp (+ j chunk-len) chunks))
		      (%string-copy! ans j base 0 base-len))); Install BASE.
	      ans))))))


(define (string-for-each proc s . maybe-start+end)
  (check-arg procedure? proc 'string-for-each)
  (let-string-start+end (start end) string-for-each s maybe-start+end
    (let lp ((i start))
      (if (< i end)
	  (begin (proc (string-ref s i)) 
		 (lp (+ i 1)))))))

(define (string-for-each-index proc s . maybe-start+end)
  (check-arg procedure? proc 'string-for-each-index)
  (let-string-start+end (start end) string-for-each-index s maybe-start+end
    (let lp ((i start))
      (if (< i end) (begin (proc i) (lp (+ i 1)))))))

(define (string-every criterion s . maybe-start+end)
  (let-string-start+end (start end) string-every s maybe-start+end
    (cond ((char? criterion)
	   (let lp ((i start))
	     (or (>= i end)
		 (and (char=? criterion (string-ref s i))
		      (lp (+ i 1))))))

	  ((char-set? criterion)
	   (let lp ((i start))
	     (or (>= i end)
		 (and (char-set-contains? criterion (string-ref s i))
		      (lp (+ i 1))))))

	  ((procedure? criterion)		; Slightly funky loop so that
	   (or (= start end)			; final (PRED S[END-1]) call
	       (let lp ((i start))		; is a tail call.
		 (let ((c (string-ref s i))
		       (i1 (+ i 1)))
		   (if (= i1 end) (criterion c)	; Tail call.
		       (and (criterion c) (lp i1)))))))

	  (else (assertion-violation 'string-every
				     "Second param is neither char-set, char, or predicate procedure."
				     criterion)))))


(define (string-any criterion s . maybe-start+end)
  (let-string-start+end (start end) string-any s maybe-start+end
    (cond ((char? criterion)
	   (let lp ((i start))
	     (and (< i end)
		  (or (char=? criterion (string-ref s i))
		      (lp (+ i 1))))))

	  ((char-set? criterion)
	   (let lp ((i start))
	     (and (< i end)
		  (or (char-set-contains? criterion (string-ref s i))
		      (lp (+ i 1))))))

	  ((procedure? criterion)		; Slightly funky loop so that
	   (and (< start end)			; final (PRED S[END-1]) call
		(let lp ((i start))		; is a tail call.
		  (let ((c (string-ref s i))
			(i1 (+ i 1)))
		    (if (= i1 end) (criterion c)	; Tail call
			(or (criterion c) (lp i1)))))))

	  (else (assertion-violation 'string-any
				     "Second param is neither char-set, char, or predicate procedure."
				     criterion)))))


(define (string-tabulate proc len)
  (check-arg procedure? proc 'string-tabulate)
  (check-arg (lambda (val) (and (integer? val) (exact? val) (<= 0 val)))
	     len 'string-tabulate)
  (let ((s (make-string len)))
    (do ((i (- len 1) (- i 1)))
	((< i 0))
      (string-set! s i (proc i)))
    s))



;;; string-prefix-length[-ci] s1 s2 [start1 end1 start2 end2]
;;; string-suffix-length[-ci] s1 s2 [start1 end1 start2 end2]
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Find the length of the common prefix/suffix.
;;; It is not required that the two substrings passed be of equal length.
;;; This was microcode in MIT Scheme -- a very tightly bummed primitive.
;;; %STRING-PREFIX-LENGTH is the core routine of all string-comparisons,
;;; so should be as tense as possible.

(define (%string-prefix-length s1 start1 end1 s2 start2 end2)
  (let* ((delta (min (- end1 start1) (- end2 start2)))
	 (end1 (+ start1 delta)))

    (if (and (eq? s1 s2) (= start1 start2))	; EQ fast path
	delta

	(let lp ((i start1) (j start2))		; Regular path
	  (if (or (>= i end1)
		  (not (char=? (string-ref s1 i)
			       (string-ref s2 j))))
	      (- i start1)
	      (lp (+ i 1) (+ j 1)))))))

(define (%string-suffix-length s1 start1 end1 s2 start2 end2)
  (let* ((delta (min (- end1 start1) (- end2 start2)))
	 (start1 (- end1 delta)))

    (if (and (eq? s1 s2) (= end1 end2))		; EQ fast path
	delta

	(let lp ((i (- end1 1)) (j (- end2 1)))	; Regular path
	  (if (or (< i start1)
		  (not (char=? (string-ref s1 i)
			       (string-ref s2 j))))
	      (- (- end1 i) 1)
	      (lp (- i 1) (- j 1)))))))

(define (%string-prefix-length-ci s1 start1 end1 s2 start2 end2)
  (let* ((delta (min (- end1 start1) (- end2 start2)))
	 (end1 (+ start1 delta)))

    (if (and (eq? s1 s2) (= start1 start2))	; EQ fast path
	delta

	(let lp ((i start1) (j start2))		; Regular path
	  (if (or (>= i end1)
		  (not (char-ci=? (string-ref s1 i)
				  (string-ref s2 j))))
	      (- i start1)
	      (lp (+ i 1) (+ j 1)))))))

(define (%string-suffix-length-ci s1 start1 end1 s2 start2 end2)
  (let* ((delta (min (- end1 start1) (- end2 start2)))
	 (start1 (- end1 delta)))

    (if (and (eq? s1 s2) (= end1 end2))		; EQ fast path
	delta

	(let lp ((i (- end1 1)) (j (- end2 1)))	; Regular path
	  (if (or (< i start1)
		  (not (char-ci=? (string-ref s1 i)
				  (string-ref s2 j))))
	      (- (- end1 i) 1)
	      (lp (- i 1) (- j 1)))))))


(define (string-prefix-length s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-prefix-length s1 s2 maybe-starts+ends
    (%string-prefix-length s1 start1 end1 s2 start2 end2)))

(define (string-suffix-length s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-suffix-length s1 s2 maybe-starts+ends
    (%string-suffix-length s1 start1 end1 s2 start2 end2)))

(define (string-prefix-length-ci s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-prefix-length-ci s1 s2 maybe-starts+ends
    (%string-prefix-length-ci s1 start1 end1 s2 start2 end2)))

(define (string-suffix-length-ci s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-suffix-length-ci s1 s2 maybe-starts+ends
    (%string-suffix-length-ci s1 start1 end1 s2 start2 end2)))


;;; string-prefix?    s1 s2 [start1 end1 start2 end2]
;;; string-suffix?    s1 s2 [start1 end1 start2 end2]
;;; string-prefix-ci? s1 s2 [start1 end1 start2 end2]
;;; string-suffix-ci? s1 s2 [start1 end1 start2 end2]
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; These are all simple derivatives of the previous counting funs.

(define (string-prefix? s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-prefix? s1 s2 maybe-starts+ends
    (%string-prefix? s1 start1 end1 s2 start2 end2)))

(define (string-suffix? s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-suffix? s1 s2 maybe-starts+ends
    (%string-suffix? s1 start1 end1 s2 start2 end2)))

(define (string-prefix-ci? s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-prefix-ci? s1 s2 maybe-starts+ends
    (%string-prefix-ci? s1 start1 end1 s2 start2 end2)))

(define (string-suffix-ci? s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-suffix-ci? s1 s2 maybe-starts+ends
    (%string-suffix-ci? s1 start1 end1 s2 start2 end2)))


;;; Here are the internal routines that do the real work.

(define (%string-prefix? s1 start1 end1 s2 start2 end2)
  (let ((len1 (- end1 start1)))
    (and (<= len1 (- end2 start2))	; Quick check
	 (= (%string-prefix-length s1 start1 end1
				   s2 start2 end2)
	    len1))))

(define (%string-suffix? s1 start1 end1 s2 start2 end2)
  (let ((len1 (- end1 start1)))
    (and (<= len1 (- end2 start2))	; Quick check
	 (= len1 (%string-suffix-length s1 start1 end1
					s2 start2 end2)))))

(define (%string-prefix-ci? s1 start1 end1 s2 start2 end2)
  (let ((len1 (- end1 start1)))
    (and (<= len1 (- end2 start2))	; Quick check
	 (= len1 (%string-prefix-length-ci s1 start1 end1
					   s2 start2 end2)))))

(define (%string-suffix-ci? s1 start1 end1 s2 start2 end2)
  (let ((len1 (- end1 start1)))
    (and (<= len1 (- end2 start2))	; Quick check
	 (= len1 (%string-suffix-length-ci s1 start1 end1
					   s2 start2 end2)))))


;;; string-compare    s1 s2 proc< proc= proc> [start1 end1 start2 end2]
;;; string-compare-ci s1 s2 proc< proc= proc> [start1 end1 start2 end2]
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Primitive string-comparison functions.
;;; Continuation order is different from MIT Scheme.
;;; Continuations are applied to s1's mismatch index;
;;; in the case of equality, this is END1.

(define (%string-compare s1 start1 end1 s2 start2 end2
			   proc< proc= proc>)
  (let ((size1 (- end1 start1))
	(size2 (- end2 start2)))
    (let ((match (%string-prefix-length s1 start1 end1 s2 start2 end2)))
      (if (= match size1)
	  ((if (= match size2) proc= proc<) end1)
	  ((if (= match size2)
	       proc>
	       (if (char<? (string-ref s1 (+ start1 match))
			   (string-ref s2 (+ start2 match)))
		   proc< proc>))
	   (+ match start1))))))

(define (%string-compare-ci s1 start1 end1 s2 start2 end2
			      proc< proc= proc>)
  (let ((size1 (- end1 start1))
	(size2 (- end2 start2)))
    (let ((match (%string-prefix-length-ci s1 start1 end1 s2 start2 end2)))
      (if (= match size1)
	  ((if (= match size2) proc= proc<) end1)
	  ((if (= match size2) proc>
	       (if (char-ci<? (string-ref s1 (+ start1 match))
			      (string-ref s2 (+ start2 match)))
		   proc< proc>))
	   (+ start1 match))))))

(define (string-compare s1 s2 proc< proc= proc> . maybe-starts+ends)
  (check-arg procedure? proc< 'string-compare)
  (check-arg procedure? proc= 'string-compare)
  (check-arg procedure? proc> 'string-compare)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-compare s1 s2 maybe-starts+ends
    (%string-compare s1 start1 end1 s2 start2 end2 proc< proc= proc>)))

(define (string-compare-ci s1 s2 proc< proc= proc> . maybe-starts+ends)
  (check-arg procedure? proc< 'string-compare-ci)
  (check-arg procedure? proc= 'string-compare-ci)
  (check-arg procedure? proc> 'string-compare-ci)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-compare-ci s1 s2 maybe-starts+ends
    (%string-compare-ci s1 start1 end1 s2 start2 end2 proc< proc= proc>)))



;;; string=          string<>		string-ci=          string-ci<>
;;; string<          string>		string-ci<          string-ci>
;;; string<=         string>=		string-ci<=         string-ci>=
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Simple definitions in terms of the previous comparison funs.
;;; I sure hope the %STRING-COMPARE calls get integrated.

(define (string= s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string= s1 s2 maybe-starts+ends
    (and (= (- end1 start1) (- end2 start2))			; Quick filter
	 (or (and (eq? s1 s2) (= start1 start2))		; Fast path
	     (%string-compare s1 start1 end1 s2 start2 end2	; Real test
			      (lambda (i) #f)
			      (lambda (i) #t)
			      (lambda (i) #f))))))

(define (string<> s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string<> s1 s2 maybe-starts+ends
    (or (not (= (- end1 start1) (- end2 start2)))		; Fast path
	(and (not (and (eq? s1 s2) (= start1 start2)))		; Quick filter
	     (%string-compare s1 start1 end1 s2 start2 end2	; Real test
			      (lambda (i) #t)
			      (lambda (i) #f)
			      (lambda (i) #t))))))

(define (string< s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string< s1 s2 maybe-starts+ends
    (if (and (eq? s1 s2) (= start1 start2))			; Fast path
	(< end1 end2)

	(%string-compare s1 start1 end1 s2 start2 end2 		; Real test
			 (lambda (i) #t)
			 (lambda (i) #f)
			 (lambda (i) #f)))))

(define (string> s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string> s1 s2 maybe-starts+ends
    (if (and (eq? s1 s2) (= start1 start2))			; Fast path
	(> end1 end2)

	(%string-compare s1 start1 end1 s2 start2 end2 		; Real test
			 (lambda (i) #f)
			 (lambda (i) #f)
			 (lambda (i) #t)))))

(define (string<= s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string<= s1 s2 maybe-starts+ends
    (if (and (eq? s1 s2) (= start1 start2))			; Fast path
	(<= end1 end2)

	(%string-compare s1 start1 end1 s2 start2 end2 		; Real test
			 (lambda (i) #t)
			 (lambda (i) #t)
			 (lambda (i) #f)))))

(define (string>= s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string>= s1 s2 maybe-starts+ends
    (if (and (eq? s1 s2) (= start1 start2))			; Fast path
	(>= end1 end2)

	(%string-compare s1 start1 end1 s2 start2 end2 		; Real test
			 (lambda (i) #f)
			 (lambda (i) #t)
			 (lambda (i) #t)))))

(define (string-ci= s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-ci= s1 s2 maybe-starts+ends
    (and (= (- end1 start1) (- end2 start2))			; Quick filter
	 (or (and (eq? s1 s2) (= start1 start2))		; Fast path
	     (%string-compare-ci s1 start1 end1 s2 start2 end2	; Real test
				 (lambda (i) #f)
				 (lambda (i) #t)
				 (lambda (i) #f))))))

(define (string-ci<> s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-ci<> s1 s2 maybe-starts+ends
    (or (not (= (- end1 start1) (- end2 start2)))		; Fast path
	(and (not (and (eq? s1 s2) (= start1 start2)))		; Quick filter
	     (%string-compare-ci s1 start1 end1 s2 start2 end2	; Real test
				 (lambda (i) #t)
				 (lambda (i) #f)
				 (lambda (i) #t))))))

(define (string-ci< s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-ci< s1 s2 maybe-starts+ends
    (if (and (eq? s1 s2) (= start1 start2))			; Fast path
	(< end1 end2)

	(%string-compare-ci s1 start1 end1 s2 start2 end2	; Real test
			    (lambda (i) #t)
			    (lambda (i) #f)
			    (lambda (i) #f)))))

(define (string-ci> s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-ci> s1 s2 maybe-starts+ends
    (if (and (eq? s1 s2) (= start1 start2))			; Fast path
	(> end1 end2)

	(%string-compare-ci s1 start1 end1 s2 start2 end2	; Real test
			    (lambda (i) #f)
			    (lambda (i) #f)
			    (lambda (i) #t)))))

(define (string-ci<= s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-ci<= s1 s2 maybe-starts+ends
    (if (and (eq? s1 s2) (= start1 start2))			; Fast path
	(<= end1 end2)

	(%string-compare-ci s1 start1 end1 s2 start2 end2	; Real test
			    (lambda (i) #t)
			    (lambda (i) #t)
			    (lambda (i) #f)))))

(define (string-ci>= s1 s2 . maybe-starts+ends)
  (let-string-start+end2 (start1 end1 start2 end2) 
			 string-ci>= s1 s2 maybe-starts+ends
    (if (and (eq? s1 s2) (= start1 start2))			; Fast path
	(>= end1 end2)

	(%string-compare-ci s1 start1 end1 s2 start2 end2	; Real test
			    (lambda (i) #f)
			    (lambda (i) #t)
			    (lambda (i) #t)))))


;;; Hash
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Compute (c + 37 c + 37^2 c + ...) modulo BOUND, with sleaze thrown in
;;; to keep the intermediate values small. (We do the calculation with just
;;; enough bits to represent BOUND, masking off high bits at each step in
;;; calculation. If this screws up any important properties of the hash
;;; function I'd like to hear about it. -Olin)
;;;
;;; If you keep BOUND small enough, the intermediate calculations will 
;;; always be fixnums. How small is dependent on the underlying Scheme system; 
;;; we use a default BOUND of 2^22 = 4194304, which should hack it in
;;; Schemes that give you at least 29 signed bits for fixnums. The core 
;;; calculation that you don't want to overflow is, worst case,
;;;     (+ 65535 (* 37 (- bound 1)))
;;; where 65535 is the max character code. Choose the default BOUND to be the
;;; biggest power of two that won't cause this expression to fixnum overflow, 
;;; and everything will be copacetic.

(define (%string-hash s char->int bound start end)
  (let ((iref (lambda (s i) (char->int (string-ref s i))))
	;; Compute a 111...1 mask that will cover BOUND-1:
	(mask (let lp ((i #x10000)) ; Let's skip first 16 iterations, eh?
		(if (>= i bound) (- i 1) (lp (+ i i))))))
    (let lp ((i start) (ans 0))
      (if (>= i end) (modulo ans bound)
	  (lp (+ i 1) (bitwise-and mask (+ (* 37 ans) (iref s i))))))))

(define (string-hash s . maybe-bound+start+end)
  (let-optionals* maybe-bound+start+end ((bound 4194304 (and (integer? bound)
							     (exact? bound)
							     (<= 0 bound)))
					 rest)
    (let ((bound (if (zero? bound) 4194304 bound)))	; 0 means default.
      (let-string-start+end (start end) string-hash s rest
        (%string-hash s char->integer bound start end)))))

(define (string-hash-ci s . maybe-bound+start+end)
  (let-optionals* maybe-bound+start+end ((bound 4194304 (and (integer? bound)
							     (exact? bound)
							     (<= 0 bound)))
					 rest)
    (let ((bound (if (zero? bound) 4194304 bound)))	; 0 means default.
      (let-string-start+end (start end) string-hash-ci s rest
        (%string-hash s (lambda (c) (char->integer (char-downcase c)))
		      bound start end)))))

;;; Case hacking
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; string-upcase  s [start end]
;;; string-upcase! s [start end]
;;; string-downcase  s [start end]
;;; string-downcase! s [start end]
;;;
;;; string-titlecase  s [start end]
;;; string-titlecase! s [start end]
;;;   Capitalize every contiguous alpha sequence: capitalise
;;;   first char, lowercase rest.

(define (string-upcase  s . maybe-start+end)
  (let-string-start+end (start end) string-upcase s maybe-start+end
    (%string-map char-upcase s start end)))

(define (string-upcase! s . maybe-start+end)
  (let-string-start+end (start end) string-upcase! s maybe-start+end
    (%string-map! char-upcase s start end)))

(define (string-downcase  s . maybe-start+end)
  (let-string-start+end (start end) string-downcase s maybe-start+end
    (%string-map char-downcase s start end)))

(define (string-downcase! s . maybe-start+end)
  (let-string-start+end (start end) string-downcase! s maybe-start+end
    (%string-map! char-downcase s start end)))

(define (%string-titlecase! s start end)
  (let lp ((i start))
    (cond ((string-index s char-cased? i end) =>
           (lambda (i)
	     (string-set! s i (char-titlecase (string-ref s i)))
	     (let ((i1 (+ i 1)))
	       (cond ((string-skip s char-cased? i1 end) =>
		      (lambda (j)
			(string-downcase! s i1 j)
			(lp (+ j 1))))
		     (else (string-downcase! s i1 end)))))))))

(define (string-titlecase! s . maybe-start+end)
  (let-string-start+end (start end) string-titlecase! s maybe-start+end
    (%string-titlecase! s start end)))

(define (string-titlecase s . maybe-start+end)
  (let-string-start+end (start end) string-titlecase! s maybe-start+end
    (let ((ans (substring s start end)))
      (%string-titlecase! ans 0 (- end start))
      ans)))


;;; Cutting & pasting strings
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; string-take string nchars
;;; string-drop string nchars
;;;
;;; string-take-right string nchars
;;; string-drop-right string nchars
;;;
;;; string-pad string k [char start end] 
;;; string-pad-right string k [char start end] 
;;; 
;;; string-trim       string [char/char-set/pred start end] 
;;; string-trim-right string [char/char-set/pred start end] 
;;; string-trim-both  string [char/char-set/pred start end] 
;;;
;;; These trimmers invert the char-set meaning from MIT Scheme -- you
;;; say what you want to trim.

(define (string-take s n)
  (check-arg string? s 'string-take)
  (check-arg (lambda (val) (and (integer? n) (exact? n)
				(<= 0 n (string-length s))))
	     n 'string-take)
  (%substring/shared s 0 n))

(define (string-take-right s n)
  (check-arg string? s 'string-take-right)
  (let ((len (string-length s)))
    (check-arg (lambda (val) (and (integer? n) (exact? n) (<= 0 n len)))
	       n 'string-take-right)
    (%substring/shared s (- len n) len)))

(define (string-drop s n)
  (check-arg string? s 'string-drop)
  (let ((len (string-length s)))
    (check-arg (lambda (val) (and (integer? n) (exact? n) (<= 0 n len)))
	       n 'string-drop)
  (%substring/shared s n len)))

(define (string-drop-right s n)
  (check-arg string? s 'string-drop-right)
  (let ((len (string-length s)))
    (check-arg (lambda (val) (and (integer? n) (exact? n) (<= 0 n len)))
	       n 'string-drop-right)
    (%substring/shared s 0 (- len n))))


(define (string-trim s . criterion+start+end)
  (let-optionals* criterion+start+end ((criterion char-set:whitespace) rest)
    (let-string-start+end (start end) string-trim s rest
      (cond ((string-skip s criterion start end) =>
	     (lambda (i) (%substring/shared s i end)))
	    (else "")))))

(define (string-trim-right s . criterion+start+end)
  (let-optionals* criterion+start+end ((criterion char-set:whitespace) rest)
    (let-string-start+end (start end) string-trim-right s rest
      (cond ((string-skip-right s criterion start end) =>
	     (lambda (i) (%substring/shared s start (+ 1 i))))
	    (else "")))))

(define (string-trim-both s . criterion+start+end)
  (let-optionals* criterion+start+end ((criterion char-set:whitespace) rest)
    (let-string-start+end (start end) string-trim-both s rest
      (cond ((string-skip s criterion start end) =>
	     (lambda (i)
	       (%substring/shared s i (+ 1 (string-skip-right s criterion i end)))))
	    (else "")))))


(define (string-pad-right s n . char+start+end)
  (let-optionals* char+start+end ((char #\space (char? char)) rest)
    (let-string-start+end (start end) string-pad-right s rest
      (check-arg (lambda (n) (and (integer? n) (exact? n) (<= 0 n)))
		 n 'string-pad-right)
      (let ((len (- end start)))
	(if (<= n len)
	    (%substring/shared s start (+ start n))
	    (let ((ans (make-string n char)))
	      (%string-copy! ans 0 s start end)
	      ans))))))

(define (string-pad s n . char+start+end)
  (let-optionals* char+start+end ((char #\space (char? char)) rest)
    (let-string-start+end (start end) string-pad s rest
      (check-arg (lambda (n) (and (integer? n) (exact? n) (<= 0 n)))
		 n 'string-pad)
      (let ((len (- end start)))
	(if (<= n len)
	    (%substring/shared s (- end n) end)
	    (let ((ans (make-string n char)))
	      (%string-copy! ans (- n len) s start end)
	      ans))))))



;;; Filtering strings
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; string-delete char/char-set/pred string [start end]
;;; string-filter char/char-set/pred string [start end]
;;;
;;; If the criterion is a char or char-set, we scan the string twice with
;;;   string-fold -- once to determine the length of the result string, 
;;;   and once to do the filtered copy.
;;; If the criterion is a predicate, we don't do this double-scan strategy, 
;;;   because the predicate might have side-effects or be very expensive to
;;;   compute. So we preallocate a temp buffer pessimistically, and only do
;;;   one scan over S. This is likely to be faster and more space-efficient
;;;   than consing a list.

(define (string-delete criterion s . maybe-start+end)
  (let-string-start+end (start end) string-delete s maybe-start+end
    (if (procedure? criterion)
	(let* ((slen (- end start))
	       (temp (make-string slen))
	       (ans-len (string-fold (lambda (c i)
				       (if (criterion c) i
					   (begin (string-set! temp i c)
						  (+ i 1))))
				     0 s start end)))
	  (if (= ans-len slen) temp (substring temp 0 ans-len)))

	(let* ((cset (cond ((char-set? criterion) criterion)
			   ((char? criterion) (char-set criterion))
			   (else
			    (assertion-violation 'string-delete "string-delete criterion not predicate, char or char-set" criterion))))
	       (len (string-fold (lambda (c i) (if (char-set-contains? cset c)
						   i
						   (+ i 1)))
				 0 s start end))
	       (ans (make-string len)))
	  (string-fold (lambda (c i) (if (char-set-contains? cset c)
					 i
					 (begin (string-set! ans i c)
						(+ i 1))))
		       0 s start end)
	  ans))))

(define (string-filter criterion s . maybe-start+end)
  (let-string-start+end (start end) string-filter s maybe-start+end
    (if (procedure? criterion)
	(let* ((slen (- end start))
	       (temp (make-string slen))
	       (ans-len (string-fold (lambda (c i)
				       (if (criterion c)
					   (begin (string-set! temp i c)
						  (+ i 1))
					   i))
				     0 s start end)))
	  (if (= ans-len slen) temp (substring temp 0 ans-len)))

	(let* ((cset (cond ((char-set? criterion) criterion)
			   ((char? criterion) (char-set criterion))
			   (else (assertion-violation 'string-filter "string-filter criterion not predicate, char or char-set" criterion))))

	       (len (string-fold (lambda (c i) (if (char-set-contains? cset c)
						   (+ i 1)
						   i))
				 0 s start end))
	       (ans (make-string len)))
	  (string-fold (lambda (c i) (if (char-set-contains? cset c)
					 (begin (string-set! ans i c)
						(+ i 1))
					 i))
		       0 s start end)
	  ans))))


;;; String search
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; string-index       string char/char-set/pred [start end]
;;; string-index-right string char/char-set/pred [start end]
;;; string-skip        string char/char-set/pred [start end]
;;; string-skip-right  string char/char-set/pred [start end]
;;; string-count       string char/char-set/pred [start end]
;;;     There's a lot of replicated code here for efficiency.
;;;     For example, the char/char-set/pred discrimination has
;;;     been lifted above the inner loop of each proc.

(define (string-index str criterion . maybe-start+end)
  (let-string-start+end (start end) string-index str maybe-start+end
    (cond ((char? criterion)
	   (let lp ((i start))
	     (and (< i end)
		  (if (char=? criterion (string-ref str i)) i
		      (lp (+ i 1))))))
	  ((char-set? criterion)
	   (let lp ((i start))
	     (and (< i end)
		  (if (char-set-contains? criterion (string-ref str i)) i
		      (lp (+ i 1))))))
	  ((procedure? criterion)
	   (let lp ((i start))
	     (and (< i end)
		  (if (criterion (string-ref str i)) i
		      (lp (+ i 1))))))
	  (else (assertion-violation 'string-index
				     "Second param is neither char-set, char, or predicate procedure."
				     criterion)))))

(define (string-index-right str criterion . maybe-start+end)
  (let-string-start+end (start end) string-index-right str maybe-start+end
    (cond ((char? criterion)
	   (let lp ((i (- end 1)))
	     (and (>= i start)
		  (if (char=? criterion (string-ref str i)) i
		      (lp (- i 1))))))
	  ((char-set? criterion)
	   (let lp ((i (- end 1)))
	     (and (>= i start)
		  (if (char-set-contains? criterion (string-ref str i)) i
		      (lp (- i 1))))))
	  ((procedure? criterion)
	   (let lp ((i (- end 1)))
	     (and (>= i start)
		  (if (criterion (string-ref str i)) i
		      (lp (- i 1))))))
	  (else (assertion-violation
		 'string-index-right
		 "Second param is neither char-set, char, or predicate procedure."
		 criterion)))))

(define (string-skip str criterion . maybe-start+end)
  (let-string-start+end (start end) string-skip str maybe-start+end
    (cond ((char? criterion)
	   (let lp ((i start))
	     (and (< i end)
		  (if (char=? criterion (string-ref str i))
		      (lp (+ i 1))
		      i))))
	  ((char-set? criterion)
	   (let lp ((i start))
	     (and (< i end)
		  (if (char-set-contains? criterion (string-ref str i))
		      (lp (+ i 1))
		      i))))
	  ((procedure? criterion)
	   (let lp ((i start))
	     (and (< i end)
		  (if (criterion (string-ref str i)) (lp (+ i 1))
		      i))))
	  (else (assertion-violation
		 'string-skip
		 "Second param is neither char-set, char, or predicate procedure."
		 criterion)))))

(define (string-skip-right str criterion . maybe-start+end)
  (let-string-start+end (start end) string-skip-right str maybe-start+end
    (cond ((char? criterion)
	   (let lp ((i (- end 1)))
	     (and (>= i start)
		  (if (char=? criterion (string-ref str i))
		      (lp (- i 1))
		      i))))
	  ((char-set? criterion)
	   (let lp ((i (- end 1)))
	     (and (>= i start)
		  (if (char-set-contains? criterion (string-ref str i))
		      (lp (- i 1))
		      i))))
	  ((procedure? criterion)
	   (let lp ((i (- end 1)))
	     (and (>= i start)
		  (if (criterion (string-ref str i)) (lp (- i 1))
		      i))))
	  (else (assertion-violation 'string-skip-right
				     "CRITERION param is neither char-set or char."
				     criterion)))))


(define (string-count s criterion . maybe-start+end)
  (let-string-start+end (start end) string-count s maybe-start+end
    (cond ((char? criterion)
	   (do ((i start (+ i 1))
		(count 0 (if (char=? criterion (string-ref s i))
			     (+ count 1)
			     count)))
	       ((>= i end) count)))

	  ((char-set? criterion)
	   (do ((i start (+ i 1))
		(count 0 (if (char-set-contains? criterion (string-ref s i))
			     (+ count 1)
			     count)))
	       ((>= i end) count)))

	  ((procedure? criterion)
	   (do ((i start (+ i 1))
		(count 0 (if (criterion (string-ref s i)) (+ count 1) count)))
	       ((>= i end) count)))

	  (else (assertion-violation
		 'string-count
		 "CRITERION param is neither char-set or char."
		 criterion)))))



;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; string-fill! string char [start end]
;;; 
;;; string-copy! to tstart from [fstart fend]
;;; 	Guaranteed to work, even if s1 eq s2.

(define (string-fill! s char . maybe-start+end)
  (check-arg char? char 'string-fill!)
  (let-string-start+end (start end) string-fill! s maybe-start+end
    (do ((i (- end 1) (- i 1)))
	((< i start))
      (string-set! s i char))))

(define (string-copy! to tstart from . maybe-fstart+fend)
  (let-string-start+end (fstart fend) string-copy! from maybe-fstart+fend
    (check-arg integer? tstart 'string-copy!)
    (check-substring-spec 'string-copy! to tstart (+ tstart (- fend fstart)))
    (%string-copy! to tstart from fstart fend)))

;;; Library-internal routine
(define (%string-copy! to tstart from fstart fend)
  (if (> fstart tstart)
      (do ((i fstart (+ i 1))
	   (j tstart (+ j 1)))
	  ((>= i fend))
	(string-set! to j (string-ref from i)))

      (do ((i (- fend 1)                    (- i 1))
	   (j (+ -1 tstart (- fend fstart)) (- j 1)))
	  ((< i fstart))
	(string-set! to j (string-ref from i)))))



;;; Returns starting-position in STRING or #f if not true.
;;; This implementation is slow & simple. It is useful as a "spec" or for
;;; comparison testing with fancier implementations.
;;; See below for fast KMP version.

;(define (string-contains string substring . maybe-starts+ends)
;  (let-string-start+end2 (start1 end1 start2 end2) 
;                         string-contains string substring maybe-starts+ends
;    (let* ((len (- end2 start2))
;	   (i-bound (- end1 len)))
;      (let lp ((i start1))
;	(and (< i i-bound)
;	     (if (string= string substring i (+ i len) start2 end2)
;		 i
;		 (lp (+ i 1))))))))


;;; Searching for an occurrence of a substring
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (string-contains text pattern . maybe-starts+ends)
  (let-string-start+end2 (t-start t-end p-start p-end)
                         string-contains text pattern maybe-starts+ends
    (%kmp-search pattern text char=? p-start p-end t-start t-end)))

(define (string-contains-ci text pattern . maybe-starts+ends)
  (let-string-start+end2 (t-start t-end p-start p-end)
                         string-contains-ci text pattern maybe-starts+ends
    (%kmp-search pattern text char-ci=? p-start p-end t-start t-end)))


;;; Knuth-Morris-Pratt string searching
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; See
;;;     "Fast pattern matching in strings"
;;;     SIAM J. Computing 6(2):323-350 1977
;;;     D. E. Knuth, J. H. Morris and V. R. Pratt
;;; also described in
;;;     "Pattern matching in strings"
;;;     Alfred V. Aho
;;;     Formal Language Theory - Perspectives and Open Problems
;;;     Ronald V. Brook (editor)
;;; This algorithm is O(m + n) where m and n are the 
;;; lengths of the pattern and string respectively

;;; KMP search source[start,end) for PATTERN. Return starting index of
;;; leftmost match or #f.

(define (%kmp-search pattern text c= p-start p-end t-start t-end)
  (let ((plen (- p-end p-start))
	(rv (make-kmp-restart-vector pattern c= p-start p-end)))

    ;; The search loop. TJ & PJ are redundant state.
    (let lp ((ti t-start) (pi 0)
	     (tj (- t-end t-start)) ; (- tlen ti) -- how many chars left.
	     (pj plen))		 ; (- plen pi) -- how many chars left.

      (if (= pi plen)
	  (- ti plen)			; Win.
	  (and (<= pj tj)		; Lose.
	       (if (c= (string-ref text ti) ; Search.
		       (string-ref pattern (+ p-start pi)))
		   (lp (+ 1 ti) (+ 1 pi) (- tj 1) (- pj 1)) ; Advance.
		   
		   (let ((pi (vector-ref rv pi))) ; Retreat.
		     (if (= pi -1)
			 (lp (+ ti 1) 0  (- tj 1) plen) ; Punt.
			 (lp ti       pi tj       (- plen pi))))))))))

;;; (make-kmp-restart-vector pattern [c= start end]) -> integer-vector
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Compute the KMP restart vector RV for string PATTERN.  If
;;; we have matched chars 0..i-1 of PATTERN against a search string S, and
;;; PATTERN[i] doesn't match S[k], then reset i := RV[i], and try again to
;;; match S[k].  If RV[i] = -1, then punt S[k] completely, and move on to
;;; S[k+1] and PATTERN[0] -- no possible match of PAT[0..i] contains S[k].
;;;
;;; In other words, if you have matched the first i chars of PATTERN, but
;;; the i+1'th char doesn't match, RV[i] tells you what the next-longest
;;; prefix of PATTERN is that you have matched.
;;;
;;; - C= (default CHAR=?) is used to compare characters for equality.
;;;   Pass in CHAR-CI=? for case-folded string search.
;;;
;;; - START & END restrict the pattern to the indicated substring; the
;;;   returned vector will be of length END - START. The numbers stored
;;;   in the vector will be values in the range [0,END-START) -- that is,
;;;   they are valid indices into the restart vector; you have to add START
;;;   to them to use them as indices into PATTERN.
;;;
;;; I've split this out as a separate function in case other constant-string
;;; searchers might want to use it.
;;;
;;; E.g.:
;;;    a b d  a b x
;;; #(-1 0 0 -1 1 2)

(define (make-kmp-restart-vector pattern . maybe-c=+start+end)
  (let-optionals* maybe-c=+start+end
                  ((c= char=? (procedure? c=))
		   ((start end) (lambda (args)
				  (string-parse-start+end 'make-kmp-restart-vector
							  pattern args))))
    (let* ((rvlen (- end start))
	   (rv (make-vector rvlen -1)))
      (if (> rvlen 0)
	  (let ((rvlen-1 (- rvlen 1))
		(c0 (string-ref pattern start)))

	    ;; Here's the main loop. We have set rv[0] ... rv[i].
	    ;; K = I + START -- it is the corresponding index into PATTERN.
	    (let lp1 ((i 0) (j -1) (k start))	
	      (if (< i rvlen-1)
		  ;; lp2 invariant:
		  ;;   pat[(k-j) .. k-1] matches pat[start .. start+j-1]
		  ;;   or j = -1.
		  (let lp2 ((j j))
		    (cond ((= j -1)
			   (let ((i1 (+ 1 i)))
			     (if (not (c= (string-ref pattern (+ k 1)) c0))
				 (vector-set! rv i1 0))
			     (lp1 i1 0 (+ k 1))))
			  ;; pat[(k-j) .. k] matches pat[start..start+j].
			  ((c= (string-ref pattern k) (string-ref pattern (+ j start)))
			   (let* ((i1 (+ 1 i))
				  (j1 (+ 1 j)))
			     (vector-set! rv i1 j1)
			     (lp1 i1 j1 (+ k 1))))

			  (else (lp2 (vector-ref rv j)))))))))
      rv)))


;;; We've matched I chars from PAT. C is the next char from the search string.
;;; Return the new I after handling C. 
;;;
;;; The pattern is (VECTOR-LENGTH RV) chars long, beginning at index PAT-START
;;; in PAT (PAT-START is usually 0). The I chars of the pattern we've matched
;;; are 
;;;     PAT[PAT-START .. PAT-START + I].
;;;
;;; It's *not* an oversight that there is no friendly error checking or
;;; defaulting of arguments. This is a low-level, inner-loop procedure
;;; that we want integrated/inlined into the point of call.

(define (kmp-step pat rv c i c= p-start)
  (let lp ((i i))
    (if (c= c (string-ref pat (+ i p-start)))	; Match =>
	(+ i 1)					;   Done.
	(let ((i (vector-ref rv i)))		; Back up in PAT.
	  (if (= i -1) 0			; Can't back up further.
	      (lp i))))))			; Keep trying for match.

;;; Zip through S[start,end), looking for a match of PAT. Assume we've
;;; already matched the first I chars of PAT when we commence at S[start].
;;; - <0:  If we find a match *ending* at index J, return -J.
;;; - >=0: If we get to the end of the S[start,end) span without finding
;;;   a complete match, return the number of chars from PAT we'd matched
;;;   when we ran off the end.
;;;
;;; This is useful for searching *across* buffers -- that is, when your
;;; input comes in chunks of text. We hand-integrate the KMP-STEP loop
;;; for speed.

(define (string-kmp-partial-search pat rv s i . c=+p-start+s-start+s-end)
  (check-arg vector? rv 'string-kmp-partial-search)
  (let-optionals* c=+p-start+s-start+s-end
		  ((c=      char=? (procedure? c=))
		   (p-start 0 (and (integer? p-start) (exact? p-start) (<= 0 p-start)))
		   ((s-start s-end) (lambda (args)
				      (string-parse-start+end 'string-kmp-partial-search
							      s args))))
    (let ((patlen (vector-length rv)))
      (check-arg (lambda (i) (and (integer? i) (exact? i) (<= 0 i) (< i patlen)))
		 i 'string-kmp-partial-search)

      ;; Enough prelude. Here's the actual code.
      (let lp ((si s-start)		; An index into S.
	       (vi i))			; An index into RV.
	(cond ((= vi patlen) (- si))	; Win.
	      ((= si s-end) vi)		; Ran off the end.
	      (else			; Match s[si] & loop.
	       (let ((c (string-ref s si)))
		 (lp (+ si 1)	
		     (let lp2 ((vi vi))	; This is just KMP-STEP.
		       (if (c= c (string-ref pat (+ vi p-start)))
			   (+ vi 1)
			   (let ((vi (vector-ref rv vi)))
			     (if (= vi -1) 0
				 (lp2 vi)))))))))))))


;;; Misc
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; (string-null? s)
;;; (string-reverse  s [start end])
;;; (string-reverse! s [start end])
;;; (reverse-list->string clist)
;;; (string->list s [start end])

(define (string-null? s) (zero? (string-length s)))

(define (string-reverse s . maybe-start+end)
  (let-string-start+end (start end) string-reverse s maybe-start+end
    (let* ((len (- end start))
	   (ans (make-string len)))
      (do ((i start (+ i 1))
	   (j (- len 1) (- j 1)))
	  ((< j 0))
	(string-set! ans j (string-ref s i)))
      ans)))

(define (string-reverse! s . maybe-start+end)
  (let-string-start+end (start end) string-reverse! s maybe-start+end
    (do ((i (- end 1) (- i 1))
	 (j start (+ j 1)))
	((<= i j))
      (let ((ci (string-ref s i)))
	(string-set! s i (string-ref s j))
	(string-set! s j ci)))))


(define (reverse-list->string clist)
  (let* ((len (length clist))
	 (s (make-string len)))
    (do ((i (- len 1) (- i 1))   (clist clist (cdr clist)))
	((not (pair? clist)))
      (string-set! s i (car clist)))
    s))


;(define (string->list s . maybe-start+end)
;  (apply string-fold-right cons '() s maybe-start+end))

(define (string->list s . maybe-start+end)
  (let-string-start+end (start end) string->list s maybe-start+end
    (do ((i (- end 1) (- i 1))
	 (ans '() (cons (string-ref s i) ans)))
	((< i start) ans))))

;;; Defined by R5RS, so commented out here.
;(define (list->string lis) (string-unfold null? car cdr lis))


;;; string-concatenate        string-list -> string
;;; string-concatenate/shared string-list -> string
;;; string-append/shared s ... -> string
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; STRING-APPEND/SHARED has license to return a string that shares storage
;;; with any of its arguments. In particular, if there is only one non-empty
;;; string amongst its parameters, it is permitted to return that string as
;;; its result. STRING-APPEND, by contrast, always allocates new storage.
;;;
;;; STRING-CONCATENATE & STRING-CONCATENATE/SHARED are passed a list of
;;; strings, which they concatenate into a result string. STRING-CONCATENATE
;;; always allocates a fresh string; STRING-CONCATENATE/SHARED may (or may
;;; not) return a result that shares storage with any of its arguments. In
;;; particular, if it is applied to a singleton list, it is permitted to
;;; return the car of that list as its value.

(define (string-append/shared . strings) (string-concatenate/shared strings))

(define (string-concatenate/shared strings)
  (let lp ((strings strings) (nchars 0) (first #f))
    (cond ((pair? strings)			; Scan the args, add up total
	   (let* ((string  (car strings))	; length, remember 1st 
		  (tail (cdr strings))		; non-empty string.
		  (slen (string-length string)))
	     (if (zero? slen)
		 (lp tail nchars first)
		 (lp tail (+ nchars slen) (or first strings)))))

	  ((zero? nchars) "")

	  ;; Just one non-empty string! Return it.
	  ((= nchars (string-length (car first))) (car first))

	  (else (let ((ans (make-string nchars)))
		  (let lp ((strings first) (i 0))
		    (if (pair? strings)
			(let* ((s (car strings))
			       (slen (string-length s)))
			  (%string-copy! ans i s 0 slen)
			  (lp (cdr strings) (+ i slen)))))
		  ans)))))
			

; Alas, Scheme 48's APPLY blows up if you have many, many arguments.
;(define (string-concatenate strings) (apply string-append strings))

;;; Here it is written out. I avoid using REDUCE to add up string lengths
;;; to avoid non-R5RS dependencies.
(define (string-concatenate strings)
  (let* ((total (do ((strings strings (cdr strings))
		     (i 0 (+ i (string-length (car strings)))))
		    ((not (pair? strings)) i)))
	 (ans (make-string total)))
    (let lp ((i 0) (strings strings))
      (if (pair? strings)
	  (let* ((s (car strings))
		 (slen (string-length s)))
	    (%string-copy! ans i s 0 slen)
	    (lp (+ i slen) (cdr strings)))))
    ans))
	  

;;; Defined by R5RS, so commented out here.
;(define (string-append . strings) (string-concatenate strings))

;;; string-concatenate-reverse        string-list [final-string end] -> string
;;; string-concatenate-reverse/shared string-list [final-string end] -> string
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Return
;;;   (string-concatenate 
;;;     (reverse
;;;       (cons (substring final-string 0 end) string-list)))

(define (string-concatenate-reverse string-list . maybe-final+end)
  (let-optionals* maybe-final+end ((final "" (string? final))
				   (end (string-length final)
					(and (integer? end)
					     (exact? end)
					     (<= 0 end (string-length final)))))
    (let ((len (let lp ((sum 0) (lis string-list))
		 (if (pair? lis)
		     (lp (+ sum (string-length (car lis))) (cdr lis))
		     sum))))

      (%finish-string-concatenate-reverse len string-list final end))))

(define (string-concatenate-reverse/shared string-list . maybe-final+end)
  (let-optionals* maybe-final+end ((final "" (string? final))
				   (end (string-length final)
					(and (integer? end)
					     (exact? end)
					     (<= 0 end (string-length final)))))
    ;; Add up the lengths of all the strings in STRING-LIST; also get a
    ;; pointer NZLIST into STRING-LIST showing where the first non-zero-length
    ;; string starts.
    (let lp ((len 0) (nzlist #f) (lis string-list))
      (if (pair? lis)
	  (let ((slen (string-length (car lis))))
	    (lp (+ len slen)
		(if (or nzlist (zero? slen)) nzlist lis)
		(cdr lis)))

	  (cond ((zero? len) (substring/shared final 0 end))

		;; LEN > 0, so NZLIST is non-empty.

		((and (zero? end) (= len (string-length (car nzlist))))
		 (car nzlist))

		(else (%finish-string-concatenate-reverse len nzlist final end)))))))

(define (%finish-string-concatenate-reverse len string-list final end)
  (let ((ans (make-string (+ end len))))
    (%string-copy! ans len final 0 end)
    (let lp ((i len) (lis string-list))
      (if (pair? lis)
	  (let* ((s   (car lis))
		 (lis (cdr lis))
		 (slen (string-length s))
		 (i (- i slen)))
	    (%string-copy! ans i s 0 slen)
	    (lp i lis))))
    ans))




;;; string-replace s1 s2 start1 end1 [start2 end2] -> string
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Replace S1[START1,END1) with S2[START2,END2).

(define (string-replace s1 s2 start1 end1 . maybe-start+end)
  (check-substring-spec 'string-replace s1 start1 end1)
  (let-string-start+end (start2 end2) string-replace s2 maybe-start+end
    (let* ((slen1 (string-length s1))
	   (sublen2 (- end2 start2))
	   (alen (+ (- slen1 (- end1 start1)) sublen2))
	   (ans (make-string alen)))
      (%string-copy! ans 0 s1 0 start1)
      (%string-copy! ans start1 s2 start2 end2)
      (%string-copy! ans (+ start1 sublen2) s1 end1 slen1)
      ans)))


;;; string-tokenize s [token-set start end] -> list
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Break S up into a list of token strings, where a token is a maximal
;;; non-empty contiguous sequence of chars belonging to TOKEN-SET.
;;; (string-tokenize "hello, world") => ("hello," "world")

(define (string-tokenize s . token-chars+start+end)
  (let-optionals* token-chars+start+end
                  ((token-chars char-set:graphic (char-set? token-chars)) rest)
    (let-string-start+end (start end) string-tokenize s rest
      (let lp ((i end) (ans '()))
	(cond ((and (< start i) (string-index-right s token-chars start i)) =>
	       (lambda (tend-1)
		 (let ((tend (+ 1 tend-1)))
		   (cond ((string-skip-right s token-chars start tend-1) =>
			  (lambda (tstart-1)
			    (lp tstart-1
				(cons (substring s (+ 1 tstart-1) tend)
				      ans))))
			 (else (cons (substring s start tend) ans))))))
	      (else ans))))))


;;; xsubstring s from [to start end] -> string
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; S is a string; START and END are optional arguments that demarcate
;;; a substring of S, defaulting to 0 and the length of S (e.g., the whole
;;; string). Replicate this substring up and down index space, in both the
;;  positive and negative directions. For example, if S = "abcdefg", START=3, 
;;; and END=6, then we have the conceptual bidirectionally-infinite string
;;;     ...  d  e  f  d  e  f  d  e  f  d  e  f  d  e  f  d  e  f  d  e  f ...
;;;     ... -9 -8 -7 -6 -5 -4 -3 -2 -1  0  1  2  3  4  5  6  7  8  9 ...
;;; XSUBSTRING returns the substring of this string beginning at index FROM,
;;; and ending at TO (which defaults to FROM+(END-START)).
;;; 
;;; You can use XSUBSTRING in many ways:
;;; - To rotate a string left:  (xsubstring "abcdef" 2)  => "cdefab"
;;; - To rotate a string right: (xsubstring "abcdef" -2) => "efabcd"
;;; - To replicate a string:    (xsubstring "abc" 0 7) => "abcabca"
;;;
;;; Note that 
;;;   - The FROM/TO indices give a half-open range -- the characters from
;;;     index FROM up to, but not including index TO.
;;;   - The FROM/TO indices are not in terms of the index space for string S.
;;;     They are in terms of the replicated index space of the substring
;;;     defined by S, START, and END.
;;;
;;; It is an error if START=END -- although this is allowed by special
;;; dispensation when FROM=TO.

(define (xsubstring s from . maybe-to+start+end)
  (check-arg (lambda (val) (and (integer? val) (exact? val)))
	     from 'xsubstring)
  (receive (to start end)
           (if (pair? maybe-to+start+end)
	       (let-string-start+end (start end) xsubstring s (cdr maybe-to+start+end)
		 (let ((to (car maybe-to+start+end)))
		   (check-arg (lambda (val) (and (integer? val)
						 (exact? val)
						 (<= from val)))
			      to 'xsubstring)
		   (values to start end)))
	       (let ((slen (string-length (check-arg string? s 'xsubstring))))
		 (values (+ from slen) 0 slen)))
    (let ((slen   (- end start))
	  (anslen (- to  from)))
      (cond ((zero? anslen) "")
	    ((zero? slen) (assertion-violation 'xsubstring
					       "Cannot replicate empty (sub)string"
					       s from to start end))

	    ((= 1 slen)		; Fast path for 1-char replication.
	     (make-string anslen (string-ref s start)))

	    ;; Selected text falls entirely within one span.
	    ((= (floor (/ from slen)) (floor (/ to slen)))
	     (substring s (+ start (modulo from slen))
			  (+ start (modulo to   slen))))

	    ;; Selected text requires multiple spans.
	    (else (let ((ans (make-string anslen)))
		    (%multispan-repcopy! ans 0 s from to start end)
		    ans))))))


;;; string-xcopy! target tstart s sfrom [sto start end] -> unspecific
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Exactly the same as xsubstring, but the extracted text is written
;;; into the string TARGET starting at index TSTART.
;;; This operation is not defined if (EQ? TARGET S) -- you cannot copy
;;; a string on top of itself.

(define (string-xcopy! target tstart s sfrom . maybe-sto+start+end)
  (check-arg (lambda (val) (and (integer? val) (exact? val)))
	     sfrom 'string-xcopy!)
  (receive (sto start end)
           (if (pair? maybe-sto+start+end)
	       (let-string-start+end (start end) string-xcopy! s (cdr maybe-sto+start+end)
		 (let ((sto (car maybe-sto+start+end)))
		   (check-arg (lambda (val) (and (integer? val) (exact? val)))
			      sto 'string-xcopy!)
		   (values sto start end)))
	       (let ((slen (string-length s)))
		 (values (+ sfrom slen) 0 slen)))

    (let* ((tocopy (- sto sfrom))
	   (tend (+ tstart tocopy))
	   (slen (- end start)))
      (check-substring-spec 'string-xcopy! target tstart tend)
      (cond ((zero? tocopy))
	    ((zero? slen)
	     (assertion-violation 'string-xcopy!
				  "Cannot replicate empty (sub)string"
				  target tstart s sfrom sto start end))

	    ((= 1 slen)			; Fast path for 1-char replication.
	     (string-fill! target (string-ref s start) tstart tend))

	    ;; Selected text falls entirely within one span.
	    ((= (floor (/ sfrom slen)) (floor (/ sto slen)))
	     (%string-copy! target tstart s 
			    (+ start (modulo sfrom slen))
			    (+ start (modulo sto   slen))))

	    ;; Multi-span copy.
	    (else (%multispan-repcopy! target tstart s sfrom sto start end))))))

;;; This is the core copying loop for XSUBSTRING and STRING-XCOPY!
;;; Internal -- not exported, no careful arg checking.
(define (%multispan-repcopy! target tstart s sfrom sto start end)
  (let* ((slen (- end start))
	 (i0 (+ start (modulo sfrom slen)))
	 (total-chars (- sto sfrom)))

    ;; Copy the partial span @ the beginning
    (%string-copy! target tstart s i0 end)
		    
    (let* ((ncopied (- end i0))			; We've copied this many.
	   (nleft (- total-chars ncopied))	; # chars left to copy.
	   (nspans (quotient nleft slen)))	; # whole spans to copy
			   
      ;; Copy the whole spans in the middle.
      (do ((i (+ tstart ncopied) (+ i slen))	; Current target index.
	   (nspans nspans (- nspans 1)))	; # spans to copy
	  ((zero? nspans)
	   ;; Copy the partial-span @ the end & we're done.
	   (%string-copy! target i s start (+ start (- total-chars (- i tstart)))))

	(%string-copy! target i s start end))))); Copy a whole span.



;;; (string-join string-list [delimiter grammar]) => string
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Paste strings together using the delimiter string.
;;;
;;; (join-strings '("foo" "bar" "baz") ":") => "foo:bar:baz"
;;;
;;; DELIMITER defaults to a single space " "
;;; GRAMMAR is one of the symbols {prefix, infix, strict-infix, suffix} 
;;; and defaults to 'infix.
;;;
;;; I could rewrite this more efficiently -- precompute the length of the
;;; answer string, then allocate & fill it in iteratively. Using 
;;; STRING-CONCATENATE is less efficient.

(define (string-join strings . delim+grammar)
  (let-optionals* delim+grammar ((delim " " (string? delim))
				 (grammar 'infix))
    (let ((buildit (lambda (lis final)
		     (let recur ((lis lis))
		       (if (pair? lis)
			   (cons delim (cons (car lis) (recur (cdr lis))))
			   final)))))

      (cond ((pair? strings)
	     (string-concatenate
	      (case grammar

		((infix strict-infix)
		 (cons (car strings) (buildit (cdr strings) '())))

		((prefix) (buildit strings '()))

		((suffix)
		 (cons (car strings) (buildit (cdr strings) (list delim))))

		(else (assertion-violation 'string-join
					   "Illegal join grammar"
					   grammar)))))

	     ((not (null? strings))
	      (assertion-violation 'string-join
				   "STRINGS parameter not list." strings))

	     ;; STRINGS is ()

	     ((eq? grammar 'strict-infix)
	      (assertion-violation 'string-join
				   "Empty list cannot be joined with STRICT-INFIX grammar."))

	     (else "")))))		; Special-cased for infix grammar.


;;; Porting & performance-tuning notes
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; See the section at the beginning of this file on external dependencies.
;;;
;;; The biggest issue with respect to porting is the LET-OPTIONALS* macro.
;;; There are many, many optional arguments in this library; the complexity
;;; of parsing, defaulting & type-testing these parameters is handled with the
;;; aid of this macro. There are about 15 uses of LET-OPTIONALS*. You can
;;; rewrite the uses, port the hairy macro definition (which is implemented
;;; using a Clinger-Rees low-level explicit-renaming macro system), or port
;;; the simple, high-level definition, which is less efficient.
;;;
;;; There is a fair amount of argument checking. This is, strictly speaking,
;;; unnecessary -- the actual body of the procedures will blow up if, say, a
;;; START/END index is improper. However, the error message will not be as
;;; good as if the error were caught at the "higher level." Also, a very, very
;;; smart Scheme compiler may be able to exploit having the type checks done
;;; early, so that the actual body of the procedures can assume proper values.
;;; This isn't likely; this kind of compiler technology isn't common any 
;;; longer.
;;; 
;;; The overhead of optional-argument parsing is irritating. The optional
;;; arguments must be consed into a rest list on entry, and then parsed out.
;;; Function call should be a matter of a few register moves and a jump; it
;;; should not involve heap allocation! Your Scheme system may have a superior
;;; non-R5RS optional-argument system that can eliminate this overhead. If so,
;;; then this is a prime candidate for optimising these procedures,
;;; *especially* the many optional START/END index parameters.
;;;
;;; Note that optional arguments are also a barrier to procedure integration.
;;; If your Scheme system permits you to specify alternate entry points
;;; for a call when the number of optional arguments is known in a manner
;;; that enables inlining/integration, this can provide performance 
;;; improvements.
;;;
;;; There is enough *explicit* error checking that *all* string-index
;;; operations should *never* produce a bounds error. Period. Feel like
;;; living dangerously? *Big* performance win to be had by replacing
;;; STRING-REF's and STRING-SET!'s with unsafe equivalents in the loops. 
;;; Similarly, fixnum-specific operators can speed up the arithmetic done on 
;;; the index values in the inner loops. The only arguments that are not
;;; completely error checked are
;;;   - string lists (complete checking requires time proportional to the
;;;     length of the list)
;;;   - procedure arguments, such as char->char maps & predicates.
;;;     There is no way to check the range & domain of procedures in Scheme.
;;; Procedures that take these parameters cannot fully check their
;;; arguments. But all other types to all other procedures are fully
;;; checked.
;;;
;;; This does open up the alternate possibility of simply *removing* these 
;;; checks, and letting the safe primitives raise the errors. On a dumb
;;; Scheme system, this would provide speed (by eliminating the redundant
;;; error checks) at the cost of error-message clarity.
;;;
;;; See the comments preceding the hash function code for notes on tuning
;;; the default bound so that the code never overflows your implementation's
;;; fixnum size into bignum calculation.
;;;
;;; In an interpreted Scheme, some of these procedures, or the internal
;;; routines with % prefixes, are excellent candidates for being rewritten
;;; in C. Consider STRING-HASH, %STRING-COMPARE, the 
;;; %STRING-{SUF,PRE}FIX-LENGTH routines, STRING-COPY!, STRING-INDEX &
;;; STRING-SKIP (char-set & char cases), SUBSTRING and SUBSTRING/SHARED,
;;; %KMP-SEARCH, and %MULTISPAN-REPCOPY!.
;;;
;;; It would also be nice to have the ability to mark some of these
;;; routines as candidates for inlining/integration.
;;; 
;;; All the %-prefixed routines in this source code are written
;;; to be called internally to this library. They do *not* perform
;;; friendly error checks on the inputs; they assume everything is
;;; proper. They also do not take optional arguments. These two properties
;;; save calling overhead and enable procedure integration -- but they
;;; are not appropriate for exported routines.


;;; Copyright details
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; The prefix/suffix and comparison routines in this code had (extremely
;;; distant) origins in MIT Scheme's string lib, and was substantially
;;; reworked by Olin Shivers (shivers@ai.mit.edu) 9/98. As such, it is
;;; covered by MIT Scheme's open source copyright. See below for details.
;;; 
;;; The KMP string-search code was influenced by implementations written
;;; by Stephen Bevan, Brian Dehneyer and Will Fitzgerald. However, this
;;; version was written from scratch by myself.
;;;
;;; The remainder of this code was written from scratch by myself for scsh.
;;; The scsh copyright is a BSD-style open source copyright. See below for
;;; details.
;;;     -Olin Shivers

;;; MIT Scheme copyright terms
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; This material was developed by the Scheme project at the Massachusetts
;;; Institute of Technology, Department of Electrical Engineering and
;;; Computer Science.  Permission to copy and modify this software, to
;;; redistribute either the original software or a modified version, and
;;; to use this software for any purpose is granted, subject to the
;;; following restrictions and understandings.
;;; 
;;; 1. Any copy made of this software must include this copyright notice
;;; in full.
;;; 
;;; 2. Users of this software agree to make their best efforts (a) to
;;; return to the MIT Scheme project any improvements or extensions that
;;; they make, so that these may be included in future releases; and (b)
;;; to inform MIT of noteworthy uses of this software.
;;; 
;;; 3. All materials developed as a consequence of the use of this
;;; software shall duly acknowledge such use, in accordance with the usual
;;; standards of acknowledging credit in academic research.
;;; 
;;; 4. MIT has made no warrantee or representation that the operation of
;;; this software will be error-free, and MIT is under no obligation to
;;; provide any services, by way of maintenance, update, or otherwise.
;;; 
;;; 5. In conjunction with products arising from the use of this material,
;;; there shall be no use of the name of the Massachusetts Institute of
;;; Technology nor of any adaptation thereof in any advertising,
;;; promotional, or sales literature without prior written consent from
;;; MIT in each case.

;;; Scsh copyright terms
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; All rights reserved.
;;; 
;;; Redistribution and use in source and binary forms, with or without
;;; modification, are permitted provided that the following conditions
;;; are met:
;;; 1. Redistributions of source code must retain the above copyright
;;;    notice, this list of conditions and the following disclaimer.
;;; 2. Redistributions in binary form must reproduce the above copyright
;;;    notice, this list of conditions and the following disclaimer in the
;;;    documentation and/or other materials provided with the distribution.
;;; 3. The name of the authors may not be used to endorse or promote products
;;;    derived from this software without specific prior written permission.
;;; 
;;; THIS SOFTWARE IS PROVIDED BY THE AUTHORS ``AS IS'' AND ANY EXPRESS OR
;;; IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
;;; OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
;;; IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY DIRECT, INDIRECT,
;;; INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
;;; NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
;;; DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
;;; THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
;;; (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
;;; THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.