edit_distance: calculate edit distance between strings

[ccan] / ccan / edit_distance / edit_distance_rdl.c

diff --git a/ccan/edit_distance/edit_distance_rdl.c b/ccan/edit_distance/edit_distance_rdl.c
new file mode 100644 (file)
index 0000000..76ba3ce
--- /dev/null
+++ b/ccan/edit_distance/edit_distance_rdl.c
@@ -0,0 +1,98 @@
+/** @file
+ * Defines Restricted Damerau-Levenshtein distance functions.
+ *
+ * @copyright 2016 Kevin Locke <kevin@kevinlocke.name>
+ *            MIT license - see LICENSE file for details
+ */
+#include <stdlib.h>            /* free, malloc */
+
+#include "edit_distance.h"
+#include "edit_distance-params.h"
+#include "edit_distance-private.h"
+
+ed_dist edit_distance_rdl(const ed_elem *src, ed_size slen,
+                         const ed_elem *tgt, ed_size tlen)
+{
+       /* Optimization: Avoid malloc when required rows of distance matrix can
+        * fit on the stack.
+        */
+       ed_dist stackdist[ED_STACK_ELEMS];
+
+       /* Two rows of the Wagner-Fischer distance matrix. */
+       ed_dist *distmem, *dist, *prevdist;
+       if (slen < ED_STACK_ELEMS / 2) {
+               distmem = stackdist;
+               dist = distmem;
+               prevdist = distmem + slen + 1;
+       } else {
+               distmem = malloc((slen + 1) * sizeof(ed_dist) * 2);
+               dist = distmem;
+               prevdist = distmem + slen + 1;
+       }
+
+       /* Initialize row with cost to delete src[0..i-1] */
+       dist[0] = 0;
+       for (ed_size i = 1; i <= slen; ++i) {
+               dist[i] = dist[i - 1] + ED_DEL_COST(src[i - 1]);
+       }
+
+       for (ed_size j = 1; j <= tlen; ++j) {
+               /* Value for dist[j-2][i-1] (two rows up, one col left). */
+               /* Note: dist[0] is not initialized when j == 1, var unused. */
+               ed_dist diagdist1 = prevdist[0];
+               /* Value for dist[j-2][i-2] (two rows up, two cols left).
+                * Initialization value only used to placate GCC. */
+               ed_dist diagdist2 = 0;
+
+               ED_SWAP(dist, prevdist, ed_dist *);
+
+               dist[0] = prevdist[0] + ED_INS_COST(tgt[j - 1]);
+
+               /* Loop invariant: dist[i] is the edit distance between first j
+                * elements of tgt and first i elements of src.
+                */
+               for (ed_size i = 1; i <= slen; ++i) {
+                       ed_dist nextdiagdist = dist[i];
+
+                       if (ED_ELEM_EQUAL(src[i - 1], tgt[j - 1])) {
+                               /* Same as tgt upto j-2, src upto i-2. */
+                               dist[i] = prevdist[i - 1];
+                       } else {
+                               /* Insertion is tgt upto j-2, src upto i-1
+                                * + insert tgt[j-1] */
+                               ed_dist insdist =
+                                   prevdist[i] + ED_INS_COST(tgt[j - 1]);
+
+                               /* Deletion is tgt upto j-1, src upto i-2
+                                * + delete src[i-1] */
+                               ed_dist deldist =
+                                   dist[i - 1] + ED_DEL_COST(src[i - 1]);
+
+                               /* Substitution is tgt upto j-2, src upto i-2
+                                * + substitute tgt[j-1] for src[i-1] */
+                               ed_dist subdist = prevdist[i - 1] +
+                                   ED_SUB_COST(src[i - 1], tgt[j - 1]);
+
+                               /* Use best distance available */
+                               dist[i] = ED_MIN3(insdist, deldist, subdist);
+
+                               if (j > 1 && i > 1 &&
+                                   ED_ELEM_EQUAL(src[i - 2], tgt[j - 1]) &&
+                                   ED_ELEM_EQUAL(src[i - 1], tgt[j - 2])) {
+                                       ed_dist tradist = diagdist2 +
+                                           ED_TRA_COST(src[j - 2], src[j - 1]);
+                                       dist[i] = ED_MIN2(dist[i], tradist);
+                               }
+                       }
+
+                       diagdist2 = diagdist1;
+                       diagdist1 = nextdiagdist;
+               }
+       }
+
+       ed_dist total = dist[slen];
+       if (distmem != stackdist) {
+               free(distmem);
+       }
+       return total;
+}