Add Huffman Encoding in Racket

Author: lazyprop

contents/huffman_encoding/code/racket/huffman.rkt

@@ -0,0 +1,90 @@
+#lang racket
+
+(require data/heap)
+
+(struct branch (freq left right))
+(struct leaf (sym freq))
+
+(define (weight node)
+ (if (branch? node)
+ (branch-freq node)
+ (leaf-freq node)))
+
+(define (make-huffman-tree leaves)
+ (define (combine x y)
+ (branch (+ (weight x) (weight y)) x y))
+
+ (define hp (make-heap
+ (lambda (x y)
+ (<= (weight x) (weight y)))))
+
+ (heap-add-all! hp leaves)
+
+ (define (merge)
+ (if (= (heap-count hp) 1)
+; if there's only one node in the heap then return that element
+ (heap-min hp)
+
+; else merge the two smallest nodes
+ (let ((a (heap-min hp)))
+; get the smallest node and remove it
+ (heap-remove-min! hp)
+
+ (let ((b (heap-min hp)))
+; get the second smallest node and remove it
+ (heap-remove-min! hp)
+
+; add the combined node to the heap
+ (heap-add! hp (combine a b))
+ (merge)))))
+ (merge))
+
+(define (make-codebook tree)
+ (define ht (make-hash))
+
+ (define (dfs node path)
+ (if (leaf? node)
+; if node is a leaf then add the symbol to the codebook
+ (hash-set! ht (leaf-sym node) path)
+ (begin (dfs (branch-left node) (append path (list 0)))
+ (dfs (branch-right node) (append path (list 1))))))
+
+ (dfs tree null)
+ ht)
+
+(define (encode codebook msg)
+ (if (null? msg)
+ null
+ (append (hash-ref codebook (car msg))
+ (encode codebook (cdr msg)))))
+
+(define (decode tree msg)
+ (define (dfs node bits)
+ (if (leaf? node)
+ (cons (leaf-sym node) (decode tree bits))
+ (if (= (car bits) 0)
+ (dfs (branch-left node) (cdr bits))
+ (dfs (branch-right node) (cdr bits)))))
+
+ (if (null? msg)
+ null
+ (dfs tree msg)))
+
+(define (generate-leaves msg)
+ (define ht (make-hash))
+ (for ([ch msg])
+ (hash-update! ht ch add1 (lambda () 0)))
+ (for/list ([(k v) (in-hash ht)])
+ (leaf k v)))
+
+; test
+(define message (string->list "bibbity bobbity"))
+(define leaves (generate-leaves message))
+(define huffman-tree (make-huffman-tree leaves))
+(define codebook (make-codebook huffman-tree))
+
+(define encoded (encode codebook message))
+(define decoded (list->string (decode huffman-tree encoded)))
+
+(println encoded)
+(println decoded)

contents/huffman_encoding/huffman_encoding.md

@@ -93,6 +93,8 @@ Whether you use a stack or straight-up recursion also depends on the language, b
 [import, lang:"asm-x64"](code/asm-x64/huffman.s)
 {% sample lang="scala" %}
 [import, lang:"scala"](code/scala/huffman_encoding.scala)
+{% sample lang="racket" %}
+[import, lang:"lisp"](code/racket/huffman.rkt)
 {% sample lang="scratch" %}
 The code snippet was taken from this [scratch project](https://scratch.mit.edu/projects/389604255/)
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