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Merge pull request #105 from mzgoddard/loop-optimizations

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ADPCM Loop Optimizations
This commit is contained in:
Eric Rosenbaum 2018-10-23 16:25:15 -04:00 committed by GitHub
commit 0ee5fb95c3
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GPG key ID: 4AEE18F83AFDEB23
1 changed files with 63 additions and 26 deletions
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89
src/ADPCMSoundDecoder.js
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@ -23,6 +23,36 @@ const INDEX_TABLE = [
-1, -1, -1, -1, 2, 4, 6, 8
];
let _deltaTable = null;
/**
* Build a table of deltas from the 89 possible steps and 16 codes.
* @return {Array<number>} computed delta values
*/
const deltaTable = function () {
if (_deltaTable === null) {
const NUM_STEPS = STEP_TABLE.length;
const NUM_INDICES = INDEX_TABLE.length;
_deltaTable = new Array(NUM_STEPS * NUM_INDICES).fill(0);
let i = 0;
for (let index = 0; index < NUM_STEPS; index++) {
for (let code = 0; code < NUM_INDICES; code++) {
const step = STEP_TABLE[index];
let delta = 0;
if (code & 4) delta += step;
if (code & 2) delta += step >> 1;
if (code & 1) delta += step >> 2;
delta += step >> 3;
_deltaTable[i++] = (code & 8) ? -delta : delta;
}
}
}
return _deltaTable;
};
/**
* Decode wav audio files that have been compressed with the ADPCM format.
* This is necessary because, while web browsers have native decoders for many audio
@ -156,7 +186,6 @@ class ADPCMSoundDecoder {
*/
imaDecompress (compressedData, blockSize, out) {
let sample;
let step;
let code;
let delta;
let index = 0;
@ -168,40 +197,48 @@ class ADPCMSoundDecoder {
compressedData.position = 0;
const size = out.length;
const samplesAfterBlockHeader = (blockSize - 4) * 2;
const DELTA_TABLE = deltaTable();
let i = 0;
while (i < size) {
if (((compressedData.position % blockSize) === 0) && (lastByte < 0)) { // read block header
if (compressedData.getBytesAvailable() === 0) break;
sample = compressedData.readInt16();
index = compressedData.readUint8();
compressedData.position++; // skip extra header byte
if (index > 88) index = 88;
out[i++] = sample / 32768;
} else {
// read block header
sample = compressedData.readInt16();
index = compressedData.readUint8();
compressedData.position++; // skip extra header byte
if (index > 88) index = 88;
out[i++] = sample / 32768;
const blockLength = Math.min(samplesAfterBlockHeader, size - i);
const blockStart = i;
while (i - blockStart < blockLength) {
// read 4-bit code and compute delta from previous sample
if (lastByte < 0) {
if (compressedData.getBytesAvailable() === 0) break;
lastByte = compressedData.readUint8();
code = lastByte & 0xF;
} else {
code = (lastByte >> 4) & 0xF;
lastByte = -1;
}
step = STEP_TABLE[index];
delta = 0;
if (code & 4) delta += step;
if (code & 2) delta += step >> 1;
if (code & 1) delta += step >> 2;
delta += step >> 3;
lastByte = compressedData.readUint8();
code = lastByte & 0xF;
delta = DELTA_TABLE[index * 16 + code];
// compute next index
index += INDEX_TABLE[code];
if (index > 88) index = 88;
if (index < 0) index = 0;
else if (index < 0) index = 0;
// compute and output sample
sample += (code & 8) ? -delta : delta;
sample += delta;
if (sample > 32767) sample = 32767;
if (sample < -32768) sample = -32768;
else if (sample < -32768) sample = -32768;
out[i++] = sample / 32768;
// use 4-bit code from lastByte and compute delta from previous
// sample
code = (lastByte >> 4) & 0xF;
delta = DELTA_TABLE[index * 16 + code];
// compute next index
index += INDEX_TABLE[code];
if (index > 88) index = 88;
else if (index < 0) index = 0;
// compute and output sample
sample += delta;
if (sample > 32767) sample = 32767;
else if (sample < -32768) sample = -32768;
out[i++] = sample / 32768;
}
}