Skip to content

Instantly share code, notes, and snippets.

@panzi
Created October 4, 2014 04:43
Show Gist options
  • Save panzi/56bbd884e00a5cc548dc to your computer and use it in GitHub Desktop.
Save panzi/56bbd884e00a5cc548dc to your computer and use it in GitHub Desktop.
JavaScript bit field class with Base64 serialization.
var BitField = (function () {
"use strict";
function BitField (bits) {
if (Array.isArray(bits)) {
return BitField.fromJSON(bits);
}
else if (typeof bits === "string") {
return BitField.fromBase64(bits);
}
else if (bits instanceof BitField) {
if (window.Uint8Array) {
this.values = new Uint8Array(bits.values);
}
else {
this.values = new Array(bits.values.length);
for (var i = 0; i < bits.values.length; ++ i) {
this.values[i] = bits.values[i];
}
}
}
else if (typeof bits === "number") {
var cells = Math.ceil(bits/8);
if (window.Uint8Array) {
this.values = new Uint8Array(cells);
}
else {
this.values = new Array(cells);
for (var i = 0; i < cells; ++ i) {
this.values[i] = 0;
}
}
}
else {
throw new TypeError("illegal bits argument: "+bits);
}
}
BitField.prototype = {
get: function (index) {
var i = (index / 8) | 0;
var bit = index % 8;
return (this.values[i] & (1 << bit)) !== 0;
},
set: function (index, value) {
var i = (index / 8) | 0;
var bit = index % 8;
if (value) {
this.values[i] |= 1 << bit;
}
else {
this.values[i] &= ~(1 << bit);
}
},
// base64 code is from https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding
toBase64: function () {
var aBytes = this.values;
var nMod3 = 2;
var sB64Enc = "";
for (var nLen = aBytes.length, nUint24 = 0, nIdx = 0; nIdx < nLen; nIdx++) {
nMod3 = nIdx % 3;
nUint24 |= aBytes[nIdx] << (16 >>> nMod3 & 24);
if (nMod3 === 2 || aBytes.length - nIdx === 1) {
sB64Enc += String.fromCharCode(
uint6ToB64(nUint24 >>> 18 & 63),
uint6ToB64(nUint24 >>> 12 & 63),
uint6ToB64(nUint24 >>> 6 & 63),
uint6ToB64(nUint24 & 63));
nUint24 = 0;
}
}
return sB64Enc.substr(0, sB64Enc.length - 2 + nMod3) +
(nMod3 === 2 ? '' : nMod3 === 1 ? '=' : '==');
},
toJSON: function () {
var json = new Array(this.values.length*8);
for (var i = 0; i < json.length; ++ i) {
json[i] = this.get(i)?1:0;
}
return json;
},
toString: function () {
return this.toBase64();
}
};
BitField.fromBase64 = function (sBase64) {
var sB64Enc = sBase64.replace(/[^A-Za-z0-9\+\/]/g, "");
var nInLen = sB64Enc.length;
var nOutLen = nInLen * 3 + 1 >> 2;
var bitField = new BitField(nOutLen*8);
var taBytes = bitField.values;
for (var nMod3, nMod4, nUint24 = 0, nOutIdx = 0, nInIdx = 0; nInIdx < nInLen; nInIdx++) {
nMod4 = nInIdx & 3;
nUint24 |= b64ToUint6(sB64Enc.charCodeAt(nInIdx)) << 18 - 6 * nMod4;
if (nMod4 === 3 || nInLen - nInIdx === 1) {
for (nMod3 = 0; nMod3 < 3 && nOutIdx < nOutLen; nMod3++, nOutIdx++) {
taBytes[nOutIdx] = nUint24 >>> (16 >>> nMod3 & 24) & 255;
}
nUint24 = 0;
}
}
return bitField;
};
BitField.fromJSON = function (json) {
var bitfield = new BitField(json.length);
for (var i = 0; i < json.length; ++ i) {
if (json[i]) {
bitfield.set(i, true);
}
}
return bitfield;
};
function b64ToUint6 (nChr) {
return (
nChr > 64 && nChr < 91 ? nChr - 65 :
nChr > 96 && nChr < 123 ? nChr - 71 :
nChr > 47 && nChr < 58 ? nChr + 4 :
nChr === 43 ? 62 :
nChr === 47 ? 63 :
0);
}
function uint6ToB64 (nUint6) {
return (
nUint6 < 26 ? nUint6 + 65 :
nUint6 < 52 ? nUint6 + 71 :
nUint6 < 62 ? nUint6 - 4 :
nUint6 === 62 ? 43 :
nUint6 === 63 ? 47 :
65);
}
return BitField;
})();
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment