Created
December 28, 2014 16:10
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var alg = require("algorithms"), | |
sort = alg.mergeSort; | |
/* | |
* Huffman coding. | |
* | |
* @api public | |
*/ | |
var huffman = function(alphabet){ | |
if(!(this instanceof huffman)) | |
return new huffman(alphabet); | |
var syms = [], | |
symtbl = {}; | |
for(var sym in alphabet){ | |
symtbl[sym] = null; | |
syms.push({ obj: [sym], key: alphabet[sym] }); | |
} | |
var cmp = function(a, b){ return a.key >= b.key }, | |
h = sort(syms, cmp); | |
while(h.length !== 2){ | |
var fmin = h.pop(), | |
smin = h.pop(); | |
var meta = smin.obj.concat(fmin.obj), | |
key = fmin.key + smin.key; | |
h.push({ obj: meta, key: key }); | |
h = sort(h, cmp); | |
} | |
var rsyms = h[0]["obj"], | |
lsyms = h[1]["obj"], | |
isone = false, | |
split = function(array, base){ | |
if(array.length === 1){ | |
if(isone) symtbl[array[0]] = base + "0"; | |
else symtbl[array[0]] = base + "1"; | |
}else if(array.length === 2){ | |
var sym = array.shift(); | |
symtbl[sym] = base + "0"; | |
var sym = array.shift(); | |
symtbl[sym] = base + "1"; | |
}else{ | |
var sym = array.shift(); | |
symtbl[sym] = base + "0"; | |
split(array, base + "1"); | |
} | |
isone = !isone; | |
return; | |
}; | |
if(lsyms.length === 1){ | |
symtbl[lsyms[0]] = "0"; | |
split(rsyms, "1"); | |
}else{ | |
split(lsyms, "0"); | |
split(rsyms, "1"); | |
} | |
var luptbl = {}; | |
for(var i in symtbl){ | |
luptbl[symtbl[i]] = i; | |
} | |
this.symtbl = symtbl; | |
this.luptbl = luptbl; | |
} | |
/* | |
* Encode a string. | |
* | |
* @returns {String} encoded string | |
* | |
* @api public | |
*/ | |
huffman.prototype.encode = function(str){ | |
var enc = []; | |
for(var i = 0; i < str.length; i++){ | |
var code = this.symtbl[str[i]]; | |
if(code !== undefined){ | |
enc = enc.concat(code); | |
} | |
} | |
return enc.join(""); | |
} | |
/* | |
* Decode a buffer. | |
* | |
* @returns {String} decoded string | |
* | |
* @api public | |
*/ | |
huffman.prototype.decode = function(str){ | |
var holder = "", | |
out = "" | |
luptbl = this.luptbl; | |
for(var i = 0; i < str.length; i++){ | |
var letter = str[i]; | |
holder += letter; | |
var decode = luptbl[holder]; | |
if(decode){ | |
out += decode; | |
holder = ""; | |
} | |
} | |
return out; | |
} | |
// Exports | |
module.exports = huffman; |
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