Initial data commit

Task/Execute-a-Markov-algorithm/Racket/execute-a-markov-algorithm-1.rkt

@@ -0,0 +1,28 @@
+#lang racket
+
+(struct ->  (A B))
+(struct ->. (A B))
+
+(define ((Markov-algorithm . rules) initial-string)
+  (let/cc stop
+    ; rewriting rules
+    (define (rewrite rule str)
+      (match rule
+        [(->  a b) (cond [(replace a str b) => apply-rules]
+                         [else str])]
+        [(->. a b) (cond [(replace a str b) => stop]
+                         [else str])]))
+    ; the cycle through rewriting rules
+    (define (apply-rules s) (foldl rewrite s rules))
+    ; the result is a fixed point of rewriting procedure
+    (fixed-point apply-rules initial-string)))
+
+;; replaces the first substring A to B in a string s
+(define (replace A s B)
+  (and (regexp-match? (regexp-quote A) s)
+       (regexp-replace (regexp-quote A) s B)))
+
+;; Finds the least fixed-point of a function
+(define (fixed-point f x0)
+  (let loop ([x x0] [fx (f x0)])
+    (if (equal? x fx) fx (loop fx (f fx)))))