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@claus
Created May 29, 2012 17:37
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A simple Uint8Array based bitstream JavaScript class
// Usage:
// var buf = new Uint8Array(128);
// var bitstream = new BitStream(buf);
// bitstream.writeBits(12, 0xffff);
// bitstream.seekTo(0);
// bitstream.readBits(6); // 111111
// bitstream.readBits(10); // 1111110000
window.BitStream = function(uint8Array) {
this.a = uint8Array;
this.position = 0;
this.bitsPending = 0;
};
window.BitStream.prototype = {
writeBits: function(bits, value) {
if (bits == 0) { return; }
value &= (0xffffffff >>> (32 - bits));
var bitsConsumed;
if (this.bitsPending > 0) {
if (this.bitsPending > bits) {
this.a[this.position - 1] |= value << (this.bitsPending - bits);
bitsConsumed = bits;
this.bitsPending -= bits;
} else if (this.bitsPending == bits) {
this.a[this.position - 1] |= value;
bitsConsumed = bits;
this.bitsPending = 0;
} else {
this.a[this.position - 1] |= value >> (bits - this.bitsPending);
bitsConsumed = bitsPending;
this.bitsPending = 0;
}
} else {
bitsConsumed = Math.min(8, bits);
this.bitsPending = 8 - bitsConsumed;
this.a[this.position++] = (value >> (bits - bitsConsumed)) << this.bitsPending;
}
bits -= bitsConsumed;
if (bits > 0) {
this.writeBits(bits, value);
}
},
readBits: function(bits, bitBuffer) {
if (typeof bitBuffer == "undefined") { bitBuffer = 0; }
if (bits == 0) { return bitBuffer; }
var partial;
var bitsConsumed;
if (this.bitsPending > 0) {
var byte = this.a[this.position - 1] & (0xff >> (8 - this.bitsPending));
bitsConsumed = Math.min(this.bitsPending, bits);
this.bitsPending -= bitsConsumed;
partial = byte >> this.bitsPending;
} else {
bitsConsumed = Math.min(8, bits);
this.bitsPending = 8 - bitsConsumed;
partial = this.a[this.position++] >> this.bitsPending;
}
bits -= bitsConsumed;
bitBuffer = (bitBuffer << bitsConsumed) | partial;
return (bits > 0) ? this.readBits(bits, bitBuffer) : bitBuffer;
},
seekTo: function(bitPos) {
this.position = (bitPos / 8) | 0;
this.bitsPending = bitPos % 8;
if(this.bitsPending > 0) {
this.bitsPending = 8 - this.bitsPending;
this.position++;
}
}
};
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