Added string-replace and string-index implemented in #lang whalesong

whalesong/selfhost/selfhost-strings.rkt

@@ -0,0 +1,51 @@
+#lang whalesong (require "selfhost-lang.rkt" whalesong/lang/for)
+
+(provide string-replace) ; (string-replace k r s) replace all occurences of k in s with r
+
+; string-index : string string [integer] -> integer
+;   return the index of the first occurence of k in the string s
+;   whose index is from or greater
+(define (string-index k s [from 0])
+  (define kn (string-length k))
+  (define sn (string-length s))
+  (and (<= (+ from kn) sn)
+       (for/or ([i (in-range from (- sn kn -1))])
+         (and (for/and ([j (in-range i (+ i kn))] [l (in-range kn)])
+                (char=? (string-ref s j) (string-ref k l)))
+              i))))
+
+; a new string is returned where occurences of the string k (the key)
+; are replaced with the string r (the replacement) in the string s.
+(define (string-replace k r s)
+  (define kn (string-length k))
+  (define sn (string-length s))
+  (let loop ([start 0] [from 0] [chunks '()])
+    (define i (string-index k s from))
+    (displayln (list 'loop start from chunks i))
+    (cond
+      [i    
+       (define new-start (+ i kn))
+       (loop new-start
+             new-start
+             (cons r (cons (substring s start i) chunks)))]
+      [(empty? chunks) 
+       (string-copy s)]
+      [else
+       (apply string-append
+         (reverse (cons (if (<= start sn)
+                            (substring s start)
+                            "")
+                        chunks)))])))
+
+; Test must evaluate to #t
+#;(and (= (string-index "bar" "foobarbazbar")
+        (string-index "bar" "foobarbazbar" 1)
+        (string-index "bar" "foobarbazbar" 2)
+        (string-index "bar" "foobarbazbar" 3) 
+        3)
+     (= (string-index "bar" "foobarbazbar" 4) 9)
+     (equal? (string-index "bar" "foobarbazbar" 10) #f))
+
+; (string-replace "foo" "_" "abfoocdfoooo")
+
+