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(define-module (transform 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)))
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