gcc/spellcheck.c

/* Find near-matches for strings.
   Copyright (C) 2015-2016 Free Software Foundation, Inc.

This file is part of GCC.

GCC 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, or (at your option) any later
version.

GCC 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 GCC; see the file COPYING3.  If not see
<http://www.gnu.org/licenses/>.  */

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "tree.h"
#include "spellcheck.h"

/* The Levenshtein distance is an "edit-distance": the minimal
   number of one-character insertions, removals or substitutions
   that are needed to change one string into another.

   This implementation uses the Wagner-Fischer algorithm.  */

edit_distance_t
levenshtein_distance (const char *s, int len_s,
		      const char *t, int len_t)
{
  const bool debug = false;

  if (debug)
    {
      printf ("s: \"%s\" (len_s=%i)\n", s, len_s);
      printf ("t: \"%s\" (len_t=%i)\n", t, len_t);
    }

  if (len_s == 0)
    return len_t;
  if (len_t == 0)
    return len_s;

  /* We effectively build a matrix where each (i, j) contains the
     Levenshtein distance between the prefix strings s[0:j]
     and t[0:i].
     Rather than actually build an (len_t + 1) * (len_s + 1) matrix,
     we simply keep track of the last row, v0 and a new row, v1,
     which avoids an (len_t + 1) * (len_s + 1) allocation and memory accesses
     in favor of two (len_s + 1) allocations.  These could potentially be
     statically-allocated if we impose a maximum length on the
     strings of interest.  */
  edit_distance_t *v0 = new edit_distance_t[len_s + 1];
  edit_distance_t *v1 = new edit_distance_t[len_s + 1];

  /* The first row is for the case of an empty target string, which
     we can reach by deleting every character in the source string.  */
  for (int i = 0; i < len_s + 1; i++)
    v0[i] = i;

  /* Build successive rows.  */
  for (int i = 0; i < len_t; i++)
    {
      if (debug)
	{
	  printf ("i:%i v0 = ", i);
	  for (int j = 0; j < len_s + 1; j++)
	    printf ("%i ", v0[j]);
	  printf ("\n");
	}

      /* The initial column is for the case of an empty source string; we
	 can reach prefixes of the target string of length i
	 by inserting i characters.  */
      v1[0] = i + 1;

      /* Build the rest of the row by considering neighbors to
	 the north, west and northwest.  */
      for (int j = 0; j < len_s; j++)
	{
	  edit_distance_t cost = (s[j] == t[i] ? 0 : 1);
	  edit_distance_t deletion     = v1[j] + 1;
	  edit_distance_t insertion    = v0[j + 1] + 1;
	  edit_distance_t substitution = v0[j] + cost;
	  edit_distance_t cheapest = MIN (deletion, insertion);
	  cheapest = MIN (cheapest, substitution);
	  v1[j + 1] = cheapest;
	}

      /* Prepare to move on to next row.  */
      for (int j = 0; j < len_s + 1; j++)
	v0[j] = v1[j];
    }

  if (debug)
    {
      printf ("final v1 = ");
      for (int j = 0; j < len_s + 1; j++)
	printf ("%i ", v1[j]);
      printf ("\n");
    }

  edit_distance_t result = v1[len_s];
  delete[] v0;
  delete[] v1;
  return result;
}

/* Calculate Levenshtein distance between two nil-terminated strings.  */

edit_distance_t
levenshtein_distance (const char *s, const char *t)
{
  return levenshtein_distance (s, strlen (s), t, strlen (t));
}

/* Given TARGET, a non-NULL string, and CANDIDATES, a non-NULL ptr to
   an autovec of non-NULL strings, determine which element within
   CANDIDATES has the lowest edit distance to TARGET.  If there are
   multiple elements with the same minimal distance, the first in the
   vector wins.

   If more than half of the letters were misspelled, the suggestion is
   likely to be meaningless, so return NULL for this case.  */

const char *
find_closest_string (const char *target,
		     const auto_vec<const char *> *candidates)
{
  gcc_assert (target);
  gcc_assert (candidates);

  int i;
  const char *candidate;
  const char *best_candidate = NULL;
  edit_distance_t best_distance = MAX_EDIT_DISTANCE;
  size_t len_target = strlen (target);
  FOR_EACH_VEC_ELT (*candidates, i, candidate)
    {
      gcc_assert (candidate);
      edit_distance_t dist
	= levenshtein_distance (target, len_target,
				candidate, strlen (candidate));
      if (dist < best_distance)
	{
	  best_distance = dist;
	  best_candidate = candidate;
	}
    }

  /* If more than half of the letters were misspelled, the suggestion is
     likely to be meaningless.  */
  if (best_candidate)
    {
      unsigned int cutoff = MAX (len_target, strlen (best_candidate)) / 2;
      if (best_distance > cutoff)
	return NULL;
    }

  return best_candidate;
}