Move string similarity functions to separate module.

* dump.scm: Use (dump string-similarity).
(trigrams, jaccard-index, jaccard-string-similarity,
jaccard-string-similar?): Move to ...
* dump/string-similarity.scm: ... here.

1 files changed, 37 insertions, 0 deletions

diff --git a/dump/string-similarity.scm b/dump/string-similarity.scm
new file mode 100644
index 0000000..4bcdf7c
--- /dev/null
+++ b/dump/string-similarity.scm
@@ -0,0 +1,37 @@
+(define-module (dump string-similarity)
+  #:use-module (srfi srfi-1)
+  #:export (jaccard-string-similar?))
+
+(define (trigrams str)
+  "Return all trigrams in STR."
+  (if (< (string-length str) 3)
+      '()
+      (map (lambda (start)
+             (substring str start (+ start 3)))
+           (iota (- (string-length str) 2)))))
+
+(define (jaccard-index set1 set2)
+  "Return the Jaccard similarity coefficient between lists SET1 and
+SET2. Similarity between null sets is defined to be 0."
+  (if (and (null? set1)
+           (null? set2))
+      0
+      (let ((length-of-intersection (length (lset-intersection equal? set1 set2))))
+        (exact->inexact
+         (/ length-of-intersection
+            (- (+ (length set1) (length set2))
+               length-of-intersection))))))
+
+(define (jaccard-string-similarity str1 str2)
+  "Return the trigram similarity between strings STR1 and STR2 as
+defined by the Jaccard index."
+  (jaccard-index (trigrams (string-downcase str1))
+                 (trigrams (string-downcase str2))))
+
+(define (jaccard-string-similar? str1 str2)
+  "Return #t if STR1 and STR2 have a trigram similarity greater than
+0.8. Else, return #f. The Jaccard index is used as the similarity
+metric."
+  (let ((similarity-threshold 0.8))
+    (> (jaccard-string-similarity str1 str2)
+       similarity-threshold)))