/* Functions to make fuzzy comparisons between strings
   Copyright (C) 1988-1989, 1992-1993, 1995, 2001-2003, 2006, 2008
   Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program 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 General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.


   Derived from GNU diff 2.7, analyze.c et al.

   The basic idea is to consider two vectors as similar if, when
   transforming the first vector into the second vector through a
   sequence of edits (inserts and deletes of one element each),
   this sequence is short - or equivalently, if the ordered list
   of elements that are untouched by these edits is long.  For a
   good introduction to the subject, read about the "Levenshtein
   distance" in Wikipedia.

   The basic algorithm is described in:
   "An O(ND) Difference Algorithm and its Variations", Eugene Myers,
   Algorithmica Vol. 1 No. 2, 1986, pp. 251-266;
   see especially section 4.2, which describes the variation used below.

   The basic algorithm was independently discovered as described in:
   "Algorithms for Approximate String Matching", E. Ukkonen,
   Information and Control Vol. 64, 1985, pp. 100-118.

   Unless the 'find_minimal' flag is set, this code uses the TOO_EXPENSIVE
   heuristic, by Paul Eggert, to limit the cost to O(N**1.5 log N)
   at the price of producing suboptimal output for large inputs with
   many differences.  */

#include <config.h>

/* Specification.  */
#include "fstrcmp.h"

#include <string.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

#include "glthread/lock.h"
#include "glthread/tls.h"
#include "minmax.h"
#include "xalloc.h"

#ifndef uintptr_t
# define uintptr_t unsigned long
#endif


#define ELEMENT char
#define EQUAL(x,y) ((x) == (y))
#define OFFSET int
#define EXTRA_CONTEXT_FIELDS \
  /* The number of elements inserted or deleted. */ \
  int xvec_edit_count; \
  int yvec_edit_count;
#define NOTE_DELETE(ctxt, xoff) ctxt->xvec_edit_count++
#define NOTE_INSERT(ctxt, yoff) ctxt->yvec_edit_count++
/* We don't need USE_HEURISTIC, since it is unlikely in typical uses of
   fstrcmp().  */
#include "diffseq.h"


/* Because fstrcmp is typically called multiple times, attempt to minimize
   the number of memory allocations performed.  Thus, let a call reuse the
   memory already allocated by the previous call, if it is sufficient.
   To make it multithread-safe, without need for a lock that protects the
   already allocated memory, store the allocated memory per thread.  Free
   it only when the thread exits.  */

static gl_tls_key_t buffer_key;	/* TLS key for a 'int *' */
static gl_tls_key_t bufmax_key;	/* TLS key for a 'size_t' */

static void
keys_init (void)
{
  gl_tls_key_init (buffer_key, free);
  gl_tls_key_init (bufmax_key, NULL);
  /* The per-thread initial values are NULL and 0, respectively.  */
}

/* Ensure that keys_init is called once only.  */
gl_once_define(static, keys_init_once)


double
fstrcmp_bounded (const char *string1, const char *string2, double lower_bound)
{
  struct context ctxt;
  int xvec_length = strlen (string1);
  int yvec_length = strlen (string2);
  int i;

  size_t fdiag_len;
  int *buffer;
  size_t bufmax;

  /* short-circuit obvious comparisons */
  if (xvec_length == 0 || yvec_length == 0)
    return (xvec_length == 0 && yvec_length == 0 ? 1.0 : 0.0);

  if (lower_bound > 0)
    {
      /* Compute a quick upper bound.
	 Each edit is an insertion or deletion of an element, hence modifies
	 the length of the sequence by at most 1.
	 Therefore, when starting from a sequence X and ending at a sequence Y,
	 with N edits,  | yvec_length - xvec_length | <= N.  (Proof by
	 induction over N.)
	 So, at the end, we will have
	   xvec_edit_count + yvec_edit_count >= | xvec_length - yvec_length |.
	 and hence
	   result
	     = (xvec_length + yvec_length - (xvec_edit_count + yvec_edit_count))
	       / (xvec_length + yvec_length)
	     <= (xvec_length + yvec_length - | yvec_length - xvec_length |)
		/ (xvec_length + yvec_length)
	     = 2 * min (xvec_length, yvec_length) / (xvec_length + yvec_length).
       */
      volatile double upper_bound =
	(double) (2 * MIN (xvec_length, yvec_length))
	/ (xvec_length + yvec_length);

      if (upper_bound < lower_bound)
	/* Return an arbitrary value < LOWER_BOUND.  */
	return 0.0;
    }

  /* set the info for each string.  */
  ctxt.xvec = string1;
  ctxt.yvec = string2;

  /* Set TOO_EXPENSIVE to be approximate square root of input size,
     bounded below by 256.  */
  ctxt.too_expensive = 1;
  for (i = xvec_length + yvec_length;
       i != 0;
       i >>= 2)
    ctxt.too_expensive <<= 1;
  if (ctxt.too_expensive < 256)
    ctxt.too_expensive = 256;

  /* Allocate memory for fdiag and bdiag from a thread-local pool.  */
  fdiag_len = xvec_length + yvec_length + 3;
  gl_once (keys_init_once, keys_init);
  buffer = (int *) gl_tls_get (buffer_key);
  bufmax = (size_t) (uintptr_t) gl_tls_get (bufmax_key);
  if (fdiag_len > bufmax)
    {
      /* Need more memory.  */
      bufmax = 2 * bufmax;
      if (fdiag_len > bufmax)
	bufmax = fdiag_len;
      /* Calling xrealloc would be a waste: buffer's contents does not need
	 to be preserved.  */
      if (buffer != NULL)
	free (buffer);
      buffer = (int *) xnmalloc (bufmax, 2 * sizeof (int));
      gl_tls_set (buffer_key, buffer);
      gl_tls_set (bufmax_key, (void *) (uintptr_t) bufmax);
    }
  ctxt.fdiag = buffer + yvec_length + 1;
  ctxt.bdiag = ctxt.fdiag + fdiag_len;

  /* Now do the main comparison algorithm */
  ctxt.xvec_edit_count = 0;
  ctxt.yvec_edit_count = 0;
  compareseq (0, xvec_length, 0, yvec_length, 0,
	      &ctxt);

  /* The result is
	((number of chars in common) / (average length of the strings)).
     This is admittedly biased towards finding that the strings are
     similar, however it does produce meaningful results.  */
  return ((double) (xvec_length + yvec_length
		    - ctxt.yvec_edit_count - ctxt.xvec_edit_count)
	  / (xvec_length + yvec_length));
}