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/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001,2002,2003,2004,2005,2006,2007,2008,2009,2010
* Free Software Foundation, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public License
* as published by the Free Software Foundation; either version 3 of
* the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
* 02110-1301 USA
*/
#ifdef HAVE_CONFIG_H
# include <config.h>
#endif
#include "libguile/_scm.h"
#include "libguile/eq.h"
#include "libguile/validate.h"
#include "libguile/list.h"
#include "libguile/vectors.h"
#include "libguile/srcprop.h"
#include "libguile/trees.h"
#include <stdarg.h>
/* scm_copy_tree creates deep copies of pairs and vectors, but not of any other
* data types.
*
* To avoid infinite recursion due to cyclic structures, the hare-and-tortoise
* pattern is used to detect cycles. In fact, the pattern is used in two
* dimensions, vertical (indicated in the code by the variable names 'hare'
* and 'tortoise') and horizontal ('rabbit' and 'turtle'). In both
* dimensions, the hare/rabbit will take two steps when the tortoise/turtle
* takes one.
*
* The vertical dimension corresponds to recursive calls to function
* copy_tree: This happens when descending into vector elements, into cars of
* lists and into the cdr of an improper list. In this dimension, the
* tortoise follows the hare by using the processor stack: Every stack frame
* will hold an instance of struct t_trace. These instances are connected in
* a way that represents the trace of the hare, which thus can be followed by
* the tortoise. The tortoise will always point to struct t_trace instances
* relating to SCM objects that have already been copied. Thus, a cycle is
* detected if the tortoise and the hare point to the same object,
*
* The horizontal dimension is within one execution of copy_tree, when the
* function cdr's along the pairs of a list. This is the standard
* hare-and-tortoise implementation, found several times in guile. */
struct t_trace {
struct t_trace *trace; /* These pointers form a trace along the stack. */
SCM obj; /* The object handled at the respective stack frame.*/
};
static SCM
copy_tree (struct t_trace *const hare,
struct t_trace *tortoise,
unsigned int tortoise_delay);
SCM_DEFINE (scm_copy_tree, "copy-tree", 1, 0, 0,
(SCM obj),
"Recursively copy the data tree that is bound to @var{obj}, and return a\n"
"the new data structure. @code{copy-tree} recurses down the\n"
"contents of both pairs and vectors (since both cons cells and vector\n"
"cells may point to arbitrary objects), and stops recursing when it hits\n"
"any other object.")
#define FUNC_NAME s_scm_copy_tree
{
/* Prepare the trace along the stack. */
struct t_trace trace;
trace.obj = obj;
/* In function copy_tree, if the tortoise makes its step, it will do this
* before the hare has the chance to move. Thus, we have to make sure that
* the very first step of the tortoise will not happen after the hare has
* really made two steps. This is achieved by passing '2' as the initial
* delay for the tortoise. NOTE: Since cycles are unlikely, giving the hare
* a bigger advantage may improve performance slightly. */
return copy_tree (&trace, &trace, 2);
}
#undef FUNC_NAME
static SCM
copy_tree (struct t_trace *const hare,
struct t_trace *tortoise,
unsigned int tortoise_delay)
#define FUNC_NAME s_scm_copy_tree
{
if (!scm_is_pair (hare->obj) && !scm_is_vector (hare->obj))
{
return hare->obj;
}
else
{
/* Prepare the trace along the stack. */
struct t_trace new_hare;
hare->trace = &new_hare;
/* The tortoise will make its step after the delay has elapsed. Note
* that in contrast to the typical hare-and-tortoise pattern, the step
* of the tortoise happens before the hare takes its steps. This is, in
* principle, no problem, except for the start of the algorithm: Then,
* it has to be made sure that the hare actually gets its advantage of
* two steps. */
if (tortoise_delay == 0)
{
tortoise_delay = 1;
tortoise = tortoise->trace;
if (SCM_UNLIKELY (scm_is_eq (hare->obj, tortoise->obj)))
scm_wrong_type_arg_msg (FUNC_NAME, 1, hare->obj,
"expected non-circular data structure");
}
else
{
--tortoise_delay;
}
if (scm_is_vector (hare->obj))
{
size_t length = SCM_SIMPLE_VECTOR_LENGTH (hare->obj);
SCM new_vector = scm_c_make_vector (length, SCM_UNSPECIFIED);
/* Each vector element is copied by recursing into copy_tree, having
* the tortoise follow the hare into the depths of the stack. */
unsigned long int i;
for (i = 0; i < length; ++i)
{
SCM new_element;
new_hare.obj = SCM_SIMPLE_VECTOR_REF (hare->obj, i);
new_element = copy_tree (&new_hare, tortoise, tortoise_delay);
SCM_SIMPLE_VECTOR_SET (new_vector, i, new_element);
}
return new_vector;
}
else /* scm_is_pair (hare->obj) */
{
SCM result;
SCM tail;
SCM rabbit = hare->obj;
SCM turtle = hare->obj;
SCM copy;
/* The first pair of the list is treated specially, in order to
* preserve a potential source code position. */
result = tail = scm_cons_source (rabbit, SCM_EOL, SCM_EOL);
new_hare.obj = SCM_CAR (rabbit);
copy = copy_tree (&new_hare, tortoise, tortoise_delay);
SCM_SETCAR (tail, copy);
/* The remaining pairs of the list are copied by, horizontally,
* having the turtle follow the rabbit, and, vertically, having the
* tortoise follow the hare into the depths of the stack. */
rabbit = SCM_CDR (rabbit);
while (scm_is_pair (rabbit))
{
new_hare.obj = SCM_CAR (rabbit);
copy = copy_tree (&new_hare, tortoise, tortoise_delay);
SCM_SETCDR (tail, scm_cons (copy, SCM_UNDEFINED));
tail = SCM_CDR (tail);
rabbit = SCM_CDR (rabbit);
if (scm_is_pair (rabbit))
{
new_hare.obj = SCM_CAR (rabbit);
copy = copy_tree (&new_hare, tortoise, tortoise_delay);
SCM_SETCDR (tail, scm_cons (copy, SCM_UNDEFINED));
tail = SCM_CDR (tail);
rabbit = SCM_CDR (rabbit);
turtle = SCM_CDR (turtle);
if (SCM_UNLIKELY (scm_is_eq (rabbit, turtle)))
scm_wrong_type_arg_msg (FUNC_NAME, 1, rabbit,
"expected non-circular data structure");
}
}
/* We have to recurse into copy_tree again for the last cdr, in
* order to handle the situation that it holds a vector. */
new_hare.obj = rabbit;
copy = copy_tree (&new_hare, tortoise, tortoise_delay);
SCM_SETCDR (tail, copy);
return result;
}
}
}
#undef FUNC_NAME
void
scm_init_trees ()
{
#include "libguile/trees.x"
}
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