fix bug in levenshtein distance & add tests

vlib/strings/similarity.v

@@ -4,6 +4,9 @@ module strings
 // the distance between between two strings (lower is closer)
 pub fn levenshtein_distance(a, b string) int {
 	mut f := [0].repeat(b.len + 1)
+	for j in 0..f.len {
+		f[j] = j
+	}
 	for ca in a {
 		mut j := 1
 		mut fj1 := f[0]

vlib/strings/similarity_test.v

@@ -0,0 +1,13 @@
+import strings
+
+fn test_levenshtein_distance() {
+	assert strings.levenshtein_distance('', '') == 0
+	assert strings.levenshtein_distance('one', 'one') == 0
+	assert strings.levenshtein_distance('', 'two') == 3
+	assert strings.levenshtein_distance('three', '') == 5
+	assert strings.levenshtein_distance('bananna', '') == 7
+	assert strings.levenshtein_distance('cats', 'hats') == 1
+	assert strings.levenshtein_distance('hugs', 'shrugs') == 2
+	assert strings.levenshtein_distance('broom', 'shroom') == 2
+	assert strings.levenshtein_distance('flomax', 'volmax') == 3
+}