mirror of https://bitbucket.org/ausocean/av.git
97 lines
2.3 KiB
JavaScript
97 lines
2.3 KiB
JavaScript
const indexTable = [
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-1, -1, -1, -1, 2, 4, 6, 8,
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-1, -1, -1, -1, 2, 4, 6, 8
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]
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const stepTable = [
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7, 8, 9, 10, 11, 12, 13, 14,
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16, 17, 19, 21, 23, 25, 28, 31,
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34, 37, 41, 45, 50, 55, 60, 66,
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73, 80, 88, 97, 107, 118, 130, 143,
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157, 173, 190, 209, 230, 253, 279, 307,
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337, 371, 408, 449, 494, 544, 598, 658,
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724, 796, 876, 963, 1060, 1166, 1282, 1411,
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1552, 1707, 1878, 2066, 2272, 2499, 2749, 3024,
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3327, 3660, 4026, 4428, 4871, 5358, 5894, 6484,
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7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
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15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794,
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32767
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]
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const byteDepth = 2, // We are working with 16-bit samples. TODO(Trek): make configurable.
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initSamps = 2, // Number of samples used to initialise the encoder.
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initBytes = initSamps * byteDepth,
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headBytes = 4, // Number of bytes in the header of ADPCM.
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samplesPerEnc = 2, // Number of sample encoded at a time eg. 2 16-bit samples get encoded into 1 byte.
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bytesPerEnc = samplesPerEnc * byteDepth,
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compFact = 4; // In general ADPCM compresses by a factor of 4.
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let est = 0, // Estimation of sample based on quantised ADPCM nibble.
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idx = 0, // Index to step used for estimation.
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step = 0;
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function decodeSample(nibble) {
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let diff = 0;
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if ((nibble & 4) != 0) {
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diff += step;
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}
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if ((nibble & 2) != 0) {
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diff += step >> 1;
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}
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if ((nibble & 1) != 0) {
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diff += step >> 2;
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}
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diff += step >> 3;
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if ((nibble & 8) != 0) {
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diff = -diff;
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}
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est += diff;
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idx += indexTable[nibble];
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if (idx < 0) {
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idx = 0;
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} else if (idx > stepTable.length - 1) {
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idx = stepTable.length - 1;
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}
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step = stepTable[idx];
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result = est;
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return result;
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}
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function decode(b) {
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// Initialize Decoder with first 4 bytes of b.
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est = bytesToInt16(b[0], b[1]);
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idx = b[byteDepth];
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step = stepTable[idx];
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var result = b.slice(0, 2);
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for (var i = headBytes; i < b.length - b[3]; i++) {
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var twoNibs = b[i];
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var nib2 = twoNibs >> 4;
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var nib1 = (nib2 << 4) ^ twoNibs
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var sample1 = int16ToBytes(decodeSample(nib1))
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result.push(...sample1)
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var sample2 = int16ToBytes(decodeSample(nib2))
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result.push(...sample2)
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}
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if (b[3] == 1) {
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var padNib = b[b.length - 1]
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var sample = int16ToBytes(decodeSample(padNib))
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result.push(...sample)
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}
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return result;
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}
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function int16ToBytes(num) {
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return [(num & 0x00ff), (num & 0xff00) >> 8];
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}
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function bytesToInt16(b) {
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return (b[0] | (b[1] << 8))
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} |