Initial data commit

Task/LZW-compression/Racket/lzw-compression.rkt

@@ -0,0 +1,57 @@
+#lang racket
+; utilities
+(define-syntax def (make-rename-transformer #'define))
+(define (dict-ref d w) (hash-ref d w #f))
+(define (append-char w c)  (string-append w (string c)))
+(define (append-first w s) (append-char w (string-ref s 0)))
+
+;; Compress a string with LZW
+(define (compress uncompressed)
+  (def d (make-hash))
+  (def (dict-add d w) (hash-set! d w (hash-count d)))
+  ; build initial dictionary
+  (for ([i (in-range 256)])
+    (def s (string (integer->char i)))
+    (hash-set! d s s))
+  ; compress the string
+  (def result '())
+  (def (emit! i) (set! result (cons i result)))
+  (def w "")
+  (for ([c uncompressed])
+    (define wc (append-char w c))
+    (cond
+      [(dict-ref d wc) (set! w wc)]
+      [else            (emit! (dict-ref d w))
+                       (dict-add d wc)
+                       (set! w (string c))]))
+  (emit! (dict-ref d w))
+  (reverse result))
+
+;; Decompress a LZW compressed string
+(define (decompress compressed)
+  (def d (make-hash))
+  (def (dict-add! w) (hash-set! d (hash-count d) w))
+  ; build initial dictionary
+  (for ([i (in-range 256)])
+    (def s (string (integer->char i)))
+    (hash-set! d s s))
+  ; decompress the list
+  (def w (first compressed))
+  (apply string-append
+         w
+         (for/list ([k (rest compressed)])
+           (def entry
+             (or (dict-ref d k)
+                 (if (eqv? k (hash-count d))
+                     (append-first w w)
+                     (error 'lzq-decompress "faulty input"))))
+           (dict-add! (append-first w entry))
+           (set! w entry)
+           entry)))
+
+(def uncompressed "TOBEORNOTTOBEORTOBEORNOT")
+(displayln uncompressed)
+(def compressed (compress uncompressed))
+(displayln compressed)
+(def decompressed (decompress compressed))
+(displayln decompressed)