Nim/tests/showoff/tdrdobbs_examples.nim

discard """
  output: '''108
11 -1 1936
0.4
true
truefalse'''
"""

proc `++`(x: var int; y: int = 1; z: int = 0) =
  x = x + y + z

var g = 70
++g
g ++ 7
g.`++`(10, 20)
echo g


#let lv = stdin.readline
#var vv = stdin.readline
#vv = "abc" # valid, reassignment allowed
#lv = "abc" # fails at compile time

#proc square(x: int): int = x*x

template square(x: int): int =
  # ensure 'x' is only evaluated once:
  let y = x
  y * y

proc mostSignificantBit(n: int): int =
  # naive algorithm:
  var n = n
  while n != 0:
    n = n shr 1
    result += 1
  result -= 1

const msb3999 = mostSignificantBit(3999)

echo msb3999, " ", mostSignificantBit(0), " ", square(44)

proc filter[T](a: openarray[T], predicate: proc (x: T): bool): seq[T] =
  result = @[] # @[] constructs the empty seq
  for x in a:
    if predicate(x): result.add(x)

proc map[T, S](a: openarray[T], fn: proc (x: T): S): seq[S] =
  newSeq(result, a.len)
  for i in 0 .. <a.len: result[i] = fn(a[i])


type
  FormulaKind = enum
    fkVar,        ## element is a variable like 'X'
    fkLit,        ## element is a literal like 0.1
    fkAdd,        ## element is an addition operation
    fkMul,        ## element is a multiplication operation
    fkExp         ## element is an exponentiation operation

type
  Formula = ref object
    case kind: FormulaKind
    of fkVar: name: string
    of fkLit: value: float
    of fkAdd, fkMul, fkExp: left, right: Formula

from math import pow

proc evaluate(n: Formula, varToVal: proc (name: string): float): float =
  case n.kind
  of fkVar: varToVal(n.name)
  of fkLit: n.value
  of fkAdd: evaluate(n.left, varToVal) + evaluate(n.right, varToVal)
  of fkMul: evaluate(n.left, varToVal) * evaluate(n.right, varToVal)
  of fkExp: pow(evaluate(n.left, varToVal), evaluate(n.right, varToVal))

echo evaluate(Formula(kind: fkLit, value: 0.4), nil)

proc isPolyTerm(n: Formula): bool =
  n.kind == fkMul and n.left.kind == fkLit and (let e = n.right;
    e.kind == fkExp and e.left.kind == fkVar and e.right.kind == fkLit)

proc isPolynomial(n: Formula): bool =
  isPolyTerm(n) or
    (n.kind == fkAdd and isPolynomial(n.left) and isPolynomial(n.right))

let myFormula = Formula(kind: fkMul,
                        left: Formula(kind: fkLit, value: 2.0),
                        right: Formula(kind: fkExp,
                          left: Formula(kind: fkVar, name: "x"),
                          right: Formula(kind: fkLit, value: 5.0)))

echo isPolyTerm(myFormula)

proc pat2kind(pattern: string): FormulaKind =
  case pattern
  of "^": fkExp
  of "*": fkMul
  of "+": fkAdd
  of "x": fkVar
  of "c": fkLit
  else:   fkVar # no error reporting for reasons of simplicity

import macros

proc matchAgainst(n, pattern: NimNode): NimNode {.compileTime.} =
  template `@`(current, field: untyped): untyped =
    newDotExpr(current, newIdentNode(astToStr(field)))

  template `==@`(n, pattern: untyped): untyped =
    newCall("==", n@kind, newIdentNode($pat2kind($pattern.ident)))

  case pattern.kind
  of CallNodes:
    result = newCall("and",
      n ==@ pattern[0],
      matchAgainst(n@left, pattern[1]))
    if pattern.len == 3:
      result = newCall("and", result.copy,
        matchAgainst(n@right, pattern[2]))
  of nnkIdent:
    result = n ==@ pattern
  of nnkPar:
    result = matchAgainst(n, pattern[0])
  else:
    error "invalid pattern"

macro `=~` (n: Formula, pattern: untyped): bool =
  result = matchAgainst(n, pattern)

proc isPolyTerm2(n: Formula): bool = n =~ c * x^c

echo isPolyTerm2(myFormula), isPolyTerm2(Formula(kind: fkLit, value: 0.7))