guile/doc/ref/data-rep.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2010
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Data Representation
@section Data Representation

Scheme is a latently-typed language; this means that the system cannot,
in general, determine the type of a given expression at compile time.
Types only become apparent at run time.  Variables do not have fixed
types; a variable may hold a pair at one point, an integer at the next,
and a thousand-element vector later.  Instead, values, not variables,
have fixed types.

In order to implement standard Scheme functions like @code{pair?} and
@code{string?} and provide garbage collection, the representation of
every value must contain enough information to accurately determine its
type at run time.  Often, Scheme systems also use this information to
determine whether a program has attempted to apply an operation to an
inappropriately typed value (such as taking the @code{car} of a string).

Because variables, pairs, and vectors may hold values of any type,
Scheme implementations use a uniform representation for values --- a
single type large enough to hold either a complete value or a pointer
to a complete value, along with the necessary typing information.

The following sections will present a simple typing system, and then
make some refinements to correct its major weaknesses. We then conclude
with a discussion of specific choices that Guile has made regarding
garbage collection and data representation.

@menu
* A Simple Representation::     
* Faster Integers::             
* Cheaper Pairs::               
* Conservative GC::          
* The SCM Type in Guile::
@end menu

@node A Simple Representation
@subsection A Simple Representation

The simplest way to represent Scheme values in C would be to represent
each value as a pointer to a structure containing a type indicator,
followed by a union carrying the real value. Assuming that @code{SCM} is
the name of our universal type, we can write:

@example
enum type @{ integer, pair, string, vector, ... @};

typedef struct value *SCM;

struct value @{
  enum type type;
  union @{
    int integer;
    struct @{ SCM car, cdr; @} pair;
    struct @{ int length; char *elts; @} string;
    struct @{ int length; SCM  *elts; @} vector;
    ...
  @} value;
@};
@end example
with the ellipses replaced with code for the remaining Scheme types.

This representation is sufficient to implement all of Scheme's
semantics.  If @var{x} is an @code{SCM} value:
@itemize @bullet
@item
  To test if @var{x} is an integer, we can write @code{@var{x}->type == integer}.
@item
  To find its value, we can write @code{@var{x}->value.integer}.
@item
  To test if @var{x} is a vector, we can write @code{@var{x}->type == vector}.
@item
  If we know @var{x} is a vector, we can write
  @code{@var{x}->value.vector.elts[0]} to refer to its first element.
@item
  If we know @var{x} is a pair, we can write
  @code{@var{x}->value.pair.car} to extract its car.
@end itemize


@node Faster Integers
@subsection Faster Integers

Unfortunately, the above representation has a serious disadvantage.  In
order to return an integer, an expression must allocate a @code{struct
value}, initialize it to represent that integer, and return a pointer to
it.  Furthermore, fetching an integer's value requires a memory
reference, which is much slower than a register reference on most
processors.  Since integers are extremely common, this representation is
too costly, in both time and space.  Integers should be very cheap to
create and manipulate.

One possible solution comes from the observation that, on many
architectures, heap-allocated data (i.e., what you get when you call
@code{malloc}) must be aligned on an eight-byte boundary. (Whether or
not the machine actually requires it, we can write our own allocator for
@code{struct value} objects that assures this is true.) In this case,
the lower three bits of the structure's address are known to be zero.

This gives us the room we need to provide an improved representation
for integers.  We make the following rules:
@itemize @bullet
@item
If the lower three bits of an @code{SCM} value are zero, then the SCM
value is a pointer to a @code{struct value}, and everything proceeds as
before.
@item
Otherwise, the @code{SCM} value represents an integer, whose value
appears in its upper bits.
@end itemize

Here is C code implementing this convention:
@example
enum type @{ pair, string, vector, ... @};

typedef struct value *SCM;

struct value @{
  enum type type;
  union @{
    struct @{ SCM car, cdr; @} pair;
    struct @{ int length; char *elts; @} string;
    struct @{ int length; SCM  *elts; @} vector;
    ...
  @} value;
@};

#define POINTER_P(x) (((int) (x) & 7) == 0)
#define INTEGER_P(x) (! POINTER_P (x))

#define GET_INTEGER(x)  ((int) (x) >> 3)
#define MAKE_INTEGER(x) ((SCM) (((x) << 3) | 1))
@end example

Notice that @code{integer} no longer appears as an element of @code{enum
type}, and the union has lost its @code{integer} member.  Instead, we
use the @code{POINTER_P} and @code{INTEGER_P} macros to make a coarse
classification of values into integers and non-integers, and do further
type testing as before.

Here's how we would answer the questions posed above (again, assume
@var{x} is an @code{SCM} value):
@itemize @bullet
@item
  To test if @var{x} is an integer, we can write @code{INTEGER_P (@var{x})}.
@item
  To find its value, we can write @code{GET_INTEGER (@var{x})}.
@item
  To test if @var{x} is a vector, we can write:
@example
  @code{POINTER_P (@var{x}) && @var{x}->type == vector}
@end example
  Given the new representation, we must make sure @var{x} is truly a
  pointer before we dereference it to determine its complete type.
@item
  If we know @var{x} is a vector, we can write
  @code{@var{x}->value.vector.elts[0]} to refer to its first element, as
  before.
@item
  If we know @var{x} is a pair, we can write
  @code{@var{x}->value.pair.car} to extract its car, just as before.
@end itemize

This representation allows us to operate more efficiently on integers
than the first.  For example, if @var{x} and @var{y} are known to be
integers, we can compute their sum as follows:
@example
MAKE_INTEGER (GET_INTEGER (@var{x}) + GET_INTEGER (@var{y}))
@end example
Now, integer math requires no allocation or memory references. Most real
Scheme systems actually implement addition and other operations using an
even more efficient algorithm, but this essay isn't about
bit-twiddling. (Hint: how do you decide when to overflow to a bignum?
How would you do it in assembly?)


@node Cheaper Pairs
@subsection Cheaper Pairs

However, there is yet another issue to confront. Most Scheme heaps
contain more pairs than any other type of object; Jonathan Rees said at
one point that pairs occupy 45% of the heap in his Scheme
implementation, Scheme 48. However, our representation above spends
three @code{SCM}-sized words per pair --- one for the type, and two for
the @sc{car} and @sc{cdr}. Is there any way to represent pairs using
only two words?

Let us refine the convention we established earlier.  Let us assert
that:
@itemize @bullet
@item
  If the bottom three bits of an @code{SCM} value are @code{#b000}, then
  it is a pointer, as before.
@item
  If the bottom three bits are @code{#b001}, then the upper bits are an
  integer.  This is a bit more restrictive than before.
@item
  If the bottom two bits are @code{#b010}, then the value, with the bottom
  three bits masked out, is the address of a pair.
@end itemize

Here is the new C code:
@example
enum type @{ string, vector, ... @};

typedef struct value *SCM;

struct value @{
  enum type type;
  union @{
    struct @{ int length; char *elts; @} string;
    struct @{ int length; SCM  *elts; @} vector;
    ...
  @} value;
@};

struct pair @{
  SCM car, cdr;
@};

#define POINTER_P(x) (((int) (x) & 7) == 0)

#define INTEGER_P(x)  (((int) (x) & 7) == 1)
#define GET_INTEGER(x)  ((int) (x) >> 3)
#define MAKE_INTEGER(x) ((SCM) (((x) << 3) | 1))

#define PAIR_P(x) (((int) (x) & 7) == 2)
#define GET_PAIR(x) ((struct pair *) ((int) (x) & ~7))
@end example

Notice that @code{enum type} and @code{struct value} now only contain
provisions for vectors and strings; both integers and pairs have become
special cases.  The code above also assumes that an @code{int} is large
enough to hold a pointer, which isn't generally true.


Our list of examples is now as follows:
@itemize @bullet
@item
  To test if @var{x} is an integer, we can write @code{INTEGER_P
  (@var{x})}; this is as before.
@item
  To find its value, we can write @code{GET_INTEGER (@var{x})}, as
  before.
@item
  To test if @var{x} is a vector, we can write:
@example
  @code{POINTER_P (@var{x}) && @var{x}->type == vector}
@end example
  We must still make sure that @var{x} is a pointer to a @code{struct
  value} before dereferencing it to find its type.
@item
  If we know @var{x} is a vector, we can write
  @code{@var{x}->value.vector.elts[0]} to refer to its first element, as
  before.
@item
  We can write @code{PAIR_P (@var{x})} to determine if @var{x} is a
  pair, and then write @code{GET_PAIR (@var{x})->car} to refer to its
  car.
@end itemize

This change in representation reduces our heap size by 15%.  It also
makes it cheaper to decide if a value is a pair, because no memory
references are necessary; it suffices to check the bottom two bits of
the @code{SCM} value.  This may be significant when traversing lists, a
common activity in a Scheme system.

Again, most real Scheme systems use a slightly different implementation;
for example, if GET_PAIR subtracts off the low bits of @code{x}, instead
of masking them off, the optimizer will often be able to combine that
subtraction with the addition of the offset of the structure member we
are referencing, making a modified pointer as fast to use as an
unmodified pointer.


@node Conservative GC
@subsection Conservative Garbage Collection

Aside from the latent typing, the major source of constraints on a
Scheme implementation's data representation is the garbage collector.
The collector must be able to traverse every live object in the heap, to
determine which objects are not live, and thus collectable.

There are many ways to implement this. Guile's garbage collection is
built on a library, the Boehm-Demers-Weiser conservative garbage
collector (BDW-GC). The BDW-GC ``just works'', for the most part. But
since it is interesting to know how these things work, we include here a
high-level description of what the BDW-GC does.

Garbage collection has two logical phases: a @dfn{mark} phase, in which
the set of live objects is enumerated, and a @dfn{sweep} phase, in which
objects not traversed in the mark phase are collected. Correct
functioning of the collector depends on being able to traverse the
entire set of live objects.

In the mark phase, the collector scans the system's global variables and
the local variables on the stack to determine which objects are
immediately accessible by the C code. It then scans those objects to
find the objects they point to, and so on. The collector logically sets
a @dfn{mark bit} on each object it finds, so each object is traversed
only once.

When the collector can find no unmarked objects pointed to by marked
objects, it assumes that any objects that are still unmarked will never
be used by the program (since there is no path of dereferences from any
global or local variable that reaches them) and deallocates them.

In the above paragraphs, we did not specify how the garbage collector
finds the global and local variables; as usual, there are many different
approaches.  Frequently, the programmer must maintain a list of pointers
to all global variables that refer to the heap, and another list
(adjusted upon entry to and exit from each function) of local variables,
for the collector's benefit.

The list of global variables is usually not too difficult to maintain,
since global variables are relatively rare. However, an explicitly
maintained list of local variables (in the author's personal experience)
is a nightmare to maintain. Thus, the BDW-GC uses a technique called
@dfn{conservative garbage collection}, to make the local variable list
unnecessary.

The trick to conservative collection is to treat the stack as an
ordinary range of memory, and assume that @emph{every} word on the stack
is a pointer into the heap.  Thus, the collector marks all objects whose
addresses appear anywhere in the stack, without knowing for sure how
that word is meant to be interpreted.

In addition to the stack, the BDW-GC will also scan static data
sections. This means that global variables are also scanned when looking
for live Scheme objects.

Obviously, such a system will occasionally retain objects that are
actually garbage, and should be freed. In practice, this is not a
problem. The alternative, an explicitly maintained list of local
variable addresses, is effectively much less reliable, due to programmer
error. Interested readers should see the BDW-GC web page at
@uref{http://www.hpl.hp.com/personal/Hans_Boehm/gc}, for more
information.


@node The SCM Type in Guile
@subsection The SCM Type in Guile

Guile classifies Scheme objects into two kinds: those that fit entirely
within an @code{SCM}, and those that require heap storage.

The former class are called @dfn{immediates}.  The class of immediates
includes small integers, characters, boolean values, the empty list, the
mysterious end-of-file object, and some others.

The remaining types are called, not surprisingly, @dfn{non-immediates}.
They include pairs, procedures, strings, vectors, and all other data
types in Guile. For non-immediates, the @code{SCM} word contains a
pointer to data on the heap, with further information about the object
in question is stored in that data.

This section describes how the @code{SCM} type is actually represented
and used at the C level. Interested readers should see
@code{libguile/tags.h} for an exposition of how Guile stores type
information.

In fact, there are two basic C data types to represent objects in
Guile: @code{SCM} and @code{scm_t_bits}.

@menu
* Relationship between SCM and scm_t_bits::
* Immediate objects::
* Non-immediate objects::
* Allocating Cells::
* Heap Cell Type Information::
* Accessing Cell Entries::
@end menu


@node Relationship between SCM and scm_t_bits
@subsubsection Relationship between @code{SCM} and @code{scm_t_bits}

A variable of type @code{SCM} is guaranteed to hold a valid Scheme
object.  A variable of type @code{scm_t_bits}, on the other hand, may
hold a representation of a @code{SCM} value as a C integral type, but
may also hold any C value, even if it does not correspond to a valid
Scheme object.

For a variable @var{x} of type @code{SCM}, the Scheme object's type
information is stored in a form that is not directly usable.  To be able
to work on the type encoding of the scheme value, the @code{SCM}
variable has to be transformed into the corresponding representation as
a @code{scm_t_bits} variable @var{y} by using the @code{SCM_UNPACK}
macro.  Once this has been done, the type of the scheme object @var{x}
can be derived from the content of the bits of the @code{scm_t_bits}
value @var{y}, in the way illustrated by the example earlier in this
chapter (@pxref{Cheaper Pairs}).  Conversely, a valid bit encoding of a
Scheme value as a @code{scm_t_bits} variable can be transformed into the
corresponding @code{SCM} value using the @code{SCM_PACK} macro.

@node Immediate objects
@subsubsection Immediate objects

A Scheme object may either be an immediate, i.e.@: carrying all necessary
information by itself, or it may contain a reference to a @dfn{cell}
with additional information on the heap.  Although in general it should
be irrelevant for user code whether an object is an immediate or not,
within Guile's own code the distinction is sometimes of importance.
Thus, the following low level macro is provided:

@deftypefn Macro int SCM_IMP (SCM @var{x})
A Scheme object is an immediate if it fulfills the @code{SCM_IMP}
predicate, otherwise it holds an encoded reference to a heap cell.  The
result of the predicate is delivered as a C style boolean value.  User
code and code that extends Guile should normally not be required to use
this macro.
@end deftypefn

@noindent
Summary:
@itemize @bullet
@item
Given a Scheme object @var{x} of unknown type, check first
with @code{SCM_IMP (@var{x})} if it is an immediate object.
@item
If so, all of the type and value information can be determined from the
@code{scm_t_bits} value that is delivered by @code{SCM_UNPACK
(@var{x})}.
@end itemize

There are a number of special values in Scheme, most of them documented
elsewhere in this manual. It's not quite the right place to put them,
but for now, here's a list of the C names given to some of these values:

@deftypefn Macro SCM SCM_EOL
The Scheme empty list object, or ``End Of List'' object, usually written
in Scheme as @code{'()}.
@end deftypefn

@deftypefn Macro SCM SCM_EOF_VAL
The Scheme end-of-file value.  It has no standard written
representation, for obvious reasons.
@end deftypefn

@deftypefn Macro SCM SCM_UNSPECIFIED
The value returned by expressions which the Scheme standard says return
an ``unspecified'' value.

This is sort of a weirdly literal way to take things, but the standard
read-eval-print loop prints nothing when the expression returns this
value, so it's not a bad idea to return this when you can't think of
anything else helpful.
@end deftypefn

@deftypefn Macro SCM SCM_UNDEFINED
The ``undefined'' value.  Its most important property is that is not
equal to any valid Scheme value.  This is put to various internal uses
by C code interacting with Guile.

For example, when you write a C function that is callable from Scheme
and which takes optional arguments, the interpreter passes
@code{SCM_UNDEFINED} for any arguments you did not receive.

We also use this to mark unbound variables.
@end deftypefn

@deftypefn Macro int SCM_UNBNDP (SCM @var{x})
Return true if @var{x} is @code{SCM_UNDEFINED}.  Note that this is not a
check to see if @var{x} is @code{SCM_UNBOUND}.  History will not be kind
to us.
@end deftypefn


@node Non-immediate objects
@subsubsection Non-immediate objects

A Scheme object of type @code{SCM} that does not fulfill the
@code{SCM_IMP} predicate holds an encoded reference to a heap cell.
This reference can be decoded to a C pointer to a heap cell using the
@code{SCM2PTR} macro.  The encoding of a pointer to a heap cell into a
@code{SCM} value is done using the @code{PTR2SCM} macro.

@c (FIXME:: this name should be changed)
@deftypefn Macro {scm_t_cell *} SCM2PTR (SCM @var{x})
Extract and return the heap cell pointer from a non-immediate @code{SCM}
object @var{x}.
@end deftypefn

@c (FIXME:: this name should be changed)
@deftypefn Macro SCM PTR2SCM (scm_t_cell * @var{x})
Return a @code{SCM} value that encodes a reference to the heap cell
pointer @var{x}.
@end deftypefn

Note that it is also possible to transform a non-immediate @code{SCM}
value by using @code{SCM_UNPACK} into a @code{scm_t_bits} variable.
However, the result of @code{SCM_UNPACK} may not be used as a pointer to
a @code{scm_t_cell}: only @code{SCM2PTR} is guaranteed to transform a
@code{SCM} object into a valid pointer to a heap cell.  Also, it is not
allowed to apply @code{PTR2SCM} to anything that is not a valid pointer
to a heap cell.

@noindent
Summary:  
@itemize @bullet
@item
Only use @code{SCM2PTR} on @code{SCM} values for which @code{SCM_IMP} is
false!
@item
Don't use @code{(scm_t_cell *) SCM_UNPACK (@var{x})}!  Use @code{SCM2PTR
(@var{x})} instead!
@item
Don't use @code{PTR2SCM} for anything but a cell pointer!
@end itemize

@node Allocating Cells
@subsubsection Allocating Cells

Guile provides both ordinary cells with two slots, and double cells
with four slots.  The following two function are the most primitive
way to allocate such cells.

If the caller intends to use it as a header for some other type, she
must pass an appropriate magic value in @var{word_0}, to mark it as a
member of that type, and pass whatever value as @var{word_1}, etc that
the type expects.  You should generally not need these functions,
unless you are implementing a new datatype, and thoroughly understand
the code in @code{<libguile/tags.h>}.

If you just want to allocate pairs, use @code{scm_cons}.

@deftypefn Function SCM scm_cell (scm_t_bits word_0, scm_t_bits word_1)
Allocate a new cell, initialize the two slots with @var{word_0} and
@var{word_1}, and return it.

Note that @var{word_0} and @var{word_1} are of type @code{scm_t_bits}.
If you want to pass a @code{SCM} object, you need to use
@code{SCM_UNPACK}.
@end deftypefn

@deftypefn Function SCM scm_double_cell (scm_t_bits word_0, scm_t_bits word_1, scm_t_bits word_2, scm_t_bits word_3)
Like @code{scm_cell}, but allocates a double cell with four
slots.
@end deftypefn

@node Heap Cell Type Information
@subsubsection Heap Cell Type Information

Heap cells contain a number of entries, each of which is either a scheme
object of type @code{SCM} or a raw C value of type @code{scm_t_bits}.
Which of the cell entries contain Scheme objects and which contain raw C
values is determined by the first entry of the cell, which holds the
cell type information.

@deftypefn Macro scm_t_bits SCM_CELL_TYPE (SCM @var{x})
For a non-immediate Scheme object @var{x}, deliver the content of the
first entry of the heap cell referenced by @var{x}.  This value holds
the information about the cell type.
@end deftypefn

@deftypefn Macro void SCM_SET_CELL_TYPE (SCM @var{x}, scm_t_bits @var{t})
For a non-immediate Scheme object @var{x}, write the value @var{t} into
the first entry of the heap cell referenced by @var{x}.  The value
@var{t} must hold a valid cell type.
@end deftypefn


@node Accessing Cell Entries
@subsubsection Accessing Cell Entries

For a non-immediate Scheme object @var{x}, the object type can be
determined by reading the cell type entry using the @code{SCM_CELL_TYPE}
macro.  For each different type of cell it is known which cell entries
hold Scheme objects and which cell entries hold raw C data.  To access
the different cell entries appropriately, the following macros are
provided.

@deftypefn Macro scm_t_bits SCM_CELL_WORD (SCM @var{x}, unsigned int @var{n})
Deliver the cell entry @var{n} of the heap cell referenced by the
non-immediate Scheme object @var{x} as raw data.  It is illegal, to
access cell entries that hold Scheme objects by using these macros.  For
convenience, the following macros are also provided.
@itemize @bullet
@item
SCM_CELL_WORD_0 (@var{x}) @result{} SCM_CELL_WORD (@var{x}, 0)
@item
SCM_CELL_WORD_1 (@var{x}) @result{} SCM_CELL_WORD (@var{x}, 1)
@item
@dots{}
@item
SCM_CELL_WORD_@var{n} (@var{x}) @result{} SCM_CELL_WORD (@var{x}, @var{n})
@end itemize
@end deftypefn

@deftypefn Macro SCM SCM_CELL_OBJECT (SCM @var{x}, unsigned int @var{n})
Deliver the cell entry @var{n} of the heap cell referenced by the
non-immediate Scheme object @var{x} as a Scheme object.  It is illegal,
to access cell entries that do not hold Scheme objects by using these
macros.  For convenience, the following macros are also provided.
@itemize @bullet
@item
SCM_CELL_OBJECT_0 (@var{x}) @result{} SCM_CELL_OBJECT (@var{x}, 0)
@item
SCM_CELL_OBJECT_1 (@var{x}) @result{} SCM_CELL_OBJECT (@var{x}, 1)
@item
@dots{}
@item
SCM_CELL_OBJECT_@var{n} (@var{x}) @result{} SCM_CELL_OBJECT (@var{x},
@var{n})
@end itemize
@end deftypefn

@deftypefn Macro void SCM_SET_CELL_WORD (SCM @var{x}, unsigned int @var{n}, scm_t_bits @var{w})
Write the raw C value @var{w} into entry number @var{n} of the heap cell
referenced by the non-immediate Scheme value @var{x}.  Values that are
written into cells this way may only be read from the cells using the
@code{SCM_CELL_WORD} macros or, in case cell entry 0 is written, using
the @code{SCM_CELL_TYPE} macro.  For the special case of cell entry 0 it
has to be made sure that @var{w} contains a cell type information which
does not describe a Scheme object.  For convenience, the following
macros are also provided.
@itemize @bullet
@item
SCM_SET_CELL_WORD_0 (@var{x}, @var{w}) @result{} SCM_SET_CELL_WORD
(@var{x}, 0, @var{w})
@item
SCM_SET_CELL_WORD_1 (@var{x}, @var{w}) @result{} SCM_SET_CELL_WORD
(@var{x}, 1, @var{w})
@item
@dots{}
@item
SCM_SET_CELL_WORD_@var{n} (@var{x}, @var{w}) @result{} SCM_SET_CELL_WORD
(@var{x}, @var{n}, @var{w})
@end itemize
@end deftypefn

@deftypefn Macro void SCM_SET_CELL_OBJECT (SCM @var{x}, unsigned int @var{n}, SCM @var{o})
Write the Scheme object @var{o} into entry number @var{n} of the heap
cell referenced by the non-immediate Scheme value @var{x}.  Values that
are written into cells this way may only be read from the cells using
the @code{SCM_CELL_OBJECT} macros or, in case cell entry 0 is written,
using the @code{SCM_CELL_TYPE} macro.  For the special case of cell
entry 0 the writing of a Scheme object into this cell is only allowed
if the cell forms a Scheme pair.  For convenience, the following macros
are also provided.
@itemize @bullet
@item
SCM_SET_CELL_OBJECT_0 (@var{x}, @var{o}) @result{} SCM_SET_CELL_OBJECT
(@var{x}, 0, @var{o})
@item
SCM_SET_CELL_OBJECT_1 (@var{x}, @var{o}) @result{} SCM_SET_CELL_OBJECT
(@var{x}, 1, @var{o})
@item
@dots{}
@item
SCM_SET_CELL_OBJECT_@var{n} (@var{x}, @var{o}) @result{}
SCM_SET_CELL_OBJECT (@var{x}, @var{n}, @var{o})
@end itemize
@end deftypefn

@noindent
Summary:
@itemize @bullet
@item
For a non-immediate Scheme object @var{x} of unknown type, get the type
information by using @code{SCM_CELL_TYPE (@var{x})}.
@item
As soon as the cell type information is available, only use the
appropriate access methods to read and write data to the different cell
entries.
@end itemize


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