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

   1 /** @file
   2  * Defines Damerau-Levenshtein 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_dl(const ed_elem *src, ed_size slen,
  14                          const ed_elem *tgt, ed_size tlen)
  15 {
  16         /* Optimization: Avoid malloc when distance matrix can fit on the stack.
  17          */
  18         ed_dist stackdist[ED_STACK_DIST_VALS];
  19
  20         /* Lowrance-Wagner distance matrix, in row-major order. */
  21         size_t matsize = ((size_t)slen + 2) * (tlen + 2);
  22         ed_dist *distmem = matsize <= ED_STACK_DIST_VALS ? stackdist :
  23             malloc(matsize * sizeof(ed_dist));
  24         ed_dist *dist = distmem;
  25
  26 #ifdef ED_HASH_ON_STACK
  27         ed_size lasttgt[ED_HASH_MAX + 1] = { 0 };
  28 #else
  29         ed_size *lasttgt = calloc(ED_HASH_MAX + 1, sizeof(ed_size));
  30 #endif
  31
  32         /* Upper bound on distance between strings. */
  33         ed_dist maxdist = 0;
  34
  35 #ifdef ED_DEL_COST_CONST
  36         maxdist += (ed_dist)slen *ED_DEL_COST();
  37 #else
  38         /* Lower-triangular matrix of deletion costs.
  39          * delcost[i2, i1] is cost to delete src[i1..i2-1].
  40          * delcost[i, i] is 0. */
  41         ed_dist *delcost = malloc(ED_TMAT_SIZE(slen + 1) * sizeof(ed_dist));
  42         ed_dist *delcostitr = delcost;
  43         ed_dist *delcostprevitr = delcost;
  44         *delcostitr++ = 0;
  45         for (ed_size i2 = 1; i2 <= slen; ++i2) {
  46                 ed_dist costi2 = ED_DEL_COST(src[i2 - 1]);
  47                 for (ed_size i1 = 0; i1 < i2; ++i1) {
  48                         *delcostitr++ = *delcostprevitr++ + costi2;
  49                 }
  50                 *delcostitr++ = 0;
  51         }
  52         maxdist += delcost[ED_TMAT_IND(slen, 0)];
  53 #endif
  54
  55 #ifdef ED_INS_COST_CONST
  56         maxdist += (ed_dist)tlen *ED_INS_COST();
  57 #else
  58         /* Lower-triangular matrix of insertion costs.
  59          * inscost[j2, j1] is cost to insert tgt[j1..j2-1].
  60          * inscost[j, j] is 0. */
  61         ed_dist *inscost = malloc(ED_TMAT_SIZE(tlen + 1) * sizeof(ed_dist));
  62         ed_dist *inscostitr = inscost;
  63         ed_dist *inscostprevitr = inscost;
  64         *inscostitr++ = 0;
  65         for (ed_size j2 = 1; j2 <= tlen; ++j2) {
  66                 ed_dist costj2 = ED_INS_COST(tgt[j2 - 1]);
  67                 for (ed_size j1 = 0; j1 < j2; ++j1) {
  68                         *inscostitr++ = *inscostprevitr++ + costj2;
  69                 }
  70                 *inscostitr++ = 0;
  71         }
  72         maxdist += inscost[ED_TMAT_IND(tlen, 0)];
  73 #endif
  74
  75         /* Initialize first row with maximal cost */
  76         for (ed_size i = 0; i < slen + 2; ++i) {
  77                 dist[i] = maxdist;
  78         }
  79
  80         /* Position dist to match other algorithms.  dist[-1] will be maxdist */
  81         dist += slen + 3;
  82
  83         /* Initialize row with cost to delete src[0..i-1] */
  84         dist[-1] = maxdist;
  85         dist[0] = 0;
  86         for (ed_size i = 1; i <= slen; ++i) {
  87                 dist[i] = dist[i - 1] + ED_DEL_COST(src[i - 1]);
  88         }
  89
  90         for (ed_size j = 1; j <= tlen; ++j) {
  91                 /* Largest y < i such that src[y] = tgt[j] */
  92                 ed_size lastsrc = 0;
  93                 ed_dist *prevdist = dist;
  94                 dist += slen + 2;
  95                 dist[-1] = maxdist;
  96                 dist[0] = prevdist[0] + ED_INS_COST(tgt[j - 1]);
  97
  98                 /* Loop invariant: dist[i] is the edit distance between first j
  99                  * elements of tgt and first i elements of src.
 100                  *
 101                  * Loop invariant: lasttgt[ED_HASH_ELEM(c)] holds the largest
 102                  * x < j such that tgt[x-1] = c or 0 if no such x exists.
 103                  */
 104                 for (ed_size i = 1; i <= slen; ++i) {
 105                         ed_size i1 = lastsrc;
 106                         ed_size j1 = lasttgt[ED_HASH_ELEM(src[i - 1])];
 107
 108                         if (ED_ELEM_EQUAL(src[i - 1], tgt[j - 1])) {
 109                                 /* Same as tgt upto j-2, src upto i-2. */
 110                                 dist[i] = prevdist[i - 1];
 111                                 lastsrc = i;
 112                         } else {
 113                                 /* Insertion is tgt upto j-2, src upto i-1
 114                                  * + insert tgt[j-1] */
 115                                 ed_dist insdist =
 116                                     prevdist[i] + ED_INS_COST(tgt[j - 1]);
 117
 118                                 /* Deletion is tgt upto j-1, src upto i-2
 119                                  * + delete src[i-1] */
 120                                 ed_dist deldist =
 121                                     dist[i - 1] + ED_DEL_COST(src[i - 1]);
 122
 123                                 /* Substitution is tgt upto j-2, src upto i-2
 124                                  * + substitute tgt[j-1] for src[i-1] */
 125                                 ed_dist subdist = prevdist[i - 1] +
 126                                     ED_SUB_COST(src[i - 1], tgt[j - 1]);
 127
 128                                 /* Use best distance available */
 129                                 dist[i] = ED_MIN3(insdist, deldist, subdist);
 130
 131                                 ed_dist swpdist =
 132                                     distmem[(size_t)j1 * (slen + 2) + i1];
 133 #ifdef ED_INS_COST_CONST
 134                                 swpdist +=
 135                                     (ed_dist)(j - j1 - 1) * ED_INS_COST();
 136 #else
 137                                 swpdist += inscost[ED_TMAT_IND(j - 1, j1)];
 138 #endif
 139 #ifdef ED_TRA_COST_CONST
 140                                 swpdist += ED_TRA_COST(,);
 141 #else
 142                                 if (i1 > 0) {
 143                                         swpdist +=
 144                                             ED_TRA_COST(src[i1 - 1],
 145                                                         src[i - 1]);
 146                                 }
 147 #endif
 148 #ifdef ED_DEL_COST_CONST
 149                                 swpdist +=
 150                                     (ed_dist)(i - i1 - 1) * ED_DEL_COST();
 151 #else
 152                                 swpdist += delcost[ED_TMAT_IND(i - 1, i1)];
 153 #endif
 154
 155                                 dist[i] = ED_MIN2(dist[i], swpdist);
 156                         }
 157                 }
 158
 159                 lasttgt[ED_HASH_ELEM(tgt[j - 1])] = j;
 160         }
 161
 162 #ifndef ED_HASH_ON_STACK
 163         free(lasttgt);
 164 #endif
 165
 166 #ifndef ED_DEL_COST_CONST
 167         free(delcost);
 168 #endif
 169
 170 #ifndef ED_INS_COST_CONST
 171         free(inscost);
 172 #endif
 173
 174         ed_dist total = dist[slen];
 175         if (distmem != stackdist) {
 176                 free(distmem);
 177         }
 178         return total;
 179 }