git.ozlabs.org Git - ccan/blob - ccan/edit_distance/edit_distance_lcs.c

   1 /** @file
   2  * Defines Longest Common Subsequence distance functions.
   3  *
   4  * @copyright 2016 Kevin Locke <kevin@kevinlocke.name>
   5  *            MIT license - see LICENSE file for details
   6  */
   7 #include <stdlib.h>             /* free, malloc */
   8
   9 #include "edit_distance.h"
  10 #include "edit_distance-params.h"
  11 #include "edit_distance-private.h"
  12
  13 ed_dist edit_distance_lcs(const ed_elem *src, ed_size slen,
  14                           const ed_elem *tgt, ed_size tlen)
  15 {
  16         /* Optimization: Avoid malloc when row of distance matrix can fit on
  17          * the stack.
  18          */
  19         ed_dist stackdist[ED_STACK_ELEMS];
  20
  21         /* One row of the Wagner-Fischer distance matrix. */
  22         ed_dist *dist = slen < ED_STACK_ELEMS ? stackdist :
  23             malloc((slen + 1) * sizeof(ed_dist));
  24
  25         /* Initialize row with cost to delete src[0..i-1] */
  26         dist[0] = 0;
  27         for (ed_size i = 1; i <= slen; ++i) {
  28                 dist[i] = dist[i - 1] + ED_DEL_COST(src[i - 1]);
  29         }
  30
  31         for (ed_size j = 1; j <= tlen; ++j) {
  32                 /* Value for dist[j-1][i-1] (one row up, one col left). */
  33                 ed_dist diagdist = dist[0];
  34                 dist[0] = dist[0] + ED_INS_COST(tgt[j - 1]);
  35
  36                 /* Loop invariant: dist[i] is the edit distance between first j
  37                  * elements of tgt and first i elements of src.
  38                  */
  39                 for (ed_size i = 1; i <= slen; ++i) {
  40                         ed_dist nextdiagdist = dist[i];
  41
  42                         if (ED_ELEM_EQUAL(src[i - 1], tgt[j - 1])) {
  43                                 /* Same as tgt upto j-2, src upto i-2. */
  44                                 dist[i] = diagdist;
  45                         } else {
  46                                 /* Insertion is tgt upto j-2, src upto i-1
  47                                  * + insert tgt[j-1] */
  48                                 ed_dist insdist =
  49                                     dist[i] + ED_INS_COST(tgt[j - 1]);
  50
  51                                 /* Deletion is tgt upto j-1, src upto i-2
  52                                  * + delete src[i-1] */
  53                                 ed_dist deldist =
  54                                     dist[i - 1] + ED_DEL_COST(src[i - 1]);
  55
  56                                 /* Use best distance available */
  57                                 dist[i] = ED_MIN2(insdist, deldist);
  58                         }
  59
  60                         diagdist = nextdiagdist;
  61                 }
  62         }
  63
  64         ed_dist total = dist[slen];
  65         if (dist != stackdist) {
  66                 free(dist);
  67         }
  68         return total;
  69 }