78 lines
No EOL
2.1 KiB
JavaScript
78 lines
No EOL
2.1 KiB
JavaScript
// From: https://gist.github.com/claus/2829664 */
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// Usage:
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// var buf = new Uint8Array(128);
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// var bitstream = new BitStream(buf);
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// bitstream.writeBits(12, 0xffff);
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// bitstream.seekTo(0);
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// bitstream.readBits(6); // 111111 (63)
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// bitstream.readBits(10); // 1111110000 (1008)
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// ES6'ified by mbw@geus.dk 20190605
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export class BitStream {
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constructor(uint8Array) {
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this.a = uint8Array;
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this.position = 0;
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this.bitsPending = 0;
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}
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writeBits(bits, value) {
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if (bits === 0) { return; }
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value &= (0xffffffff >>> (32 - bits));
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let bitsConsumed;
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if (this.bitsPending > 0) {
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if (this.bitsPending > bits) {
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this.a[this.position - 1] |= value << (this.bitsPending - bits);
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bitsConsumed = bits;
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this.bitsPending -= bits;
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} else if (this.bitsPending === bits) {
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this.a[this.position - 1] |= value;
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bitsConsumed = bits;
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this.bitsPending = 0;
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} else {
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this.a[this.position - 1] |= value >> (bits - this.bitsPending);
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bitsConsumed = bitsPending;
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this.bitsPending = 0;
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}
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} else {
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bitsConsumed = Math.min(8, bits);
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this.bitsPending = 8 - bitsConsumed;
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this.a[this.position++] = (value >> (bits - bitsConsumed)) << this.bitsPending;
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}
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bits -= bitsConsumed;
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if (bits > 0) {
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this.writeBits(bits, value);
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}
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}
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readBits(bits, bitBuffer) {
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if (typeof bitBuffer === "undefined") { bitBuffer = 0; }
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if (bits === 0) { return bitBuffer; }
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let partial;
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let bitsConsumed;
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if (this.bitsPending > 0) {
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const byte = this.a[this.position - 1] & (0xff >> (8 - this.bitsPending));
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bitsConsumed = Math.min(this.bitsPending, bits);
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this.bitsPending -= bitsConsumed;
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partial = byte >> this.bitsPending;
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} else {
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bitsConsumed = Math.min(8, bits);
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this.bitsPending = 8 - bitsConsumed;
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partial = this.a[this.position++] >> this.bitsPending;
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}
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bits -= bitsConsumed;
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bitBuffer = (bitBuffer << bitsConsumed) | partial;
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return (bits > 0) ? this.readBits(bits, bitBuffer) : bitBuffer;
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}
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seekTo(bitPos) {
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this.position = (bitPos / 8) | 0;
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this.bitsPending = bitPos % 8;
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if(this.bitsPending > 0) {
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this.bitsPending = 8 - this.bitsPending;
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this.position++;
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}
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}
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} |