rochars / imaadpcm

 * imaadpcm

 * JavaScript IMA ADPCM codec.

 * Copyright (c) 2018 Rafael da Silva Rocha.

 * https://github.com/rochars/imaadpcm

 *

 * References:

 * http://www.cs.columbia.edu/~hgs/audio/dvi/

 * https://github.com/acida/pyima

 * https://wiki.multimedia.cx/index.php/IMA_ADPCM

 * 

 */

/** @private */

const INDEX_TABLE = [

    -1, -1, -1, -1, 2, 4, 6, 8,

    -1, -1, -1, -1, 2, 4, 6, 8];

/** @private */

const STEP_TABLE = [

    7, 8, 9, 10, 11, 12, 13, 14,

    16, 17, 19, 21, 23, 25, 28, 31,

    34, 37, 41, 45, 50, 55, 60, 66,

    73, 80, 88, 97, 107, 118, 130, 143,

    157, 173, 190, 209, 230, 253, 279, 307,

    337, 371, 408, 449, 494, 544, 598, 658,

    724, 796, 876, 963, 1060, 1166, 1282, 1411,

    1552, 1707, 1878, 2066, 2272, 2499, 2749, 3024,

    3327, 3660, 4026, 4428, 4871, 5358, 5894, 6484,

    7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,

    15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794,

    32767];

/** @private */

var encoderPredicted_ = 0;

/** @private */

var encoderIndex_ = 0;

/** @private */

var encoderStep_ = 7;

/** @private */

var decoderPredicted_ = 0;

/** @private */

var decoderIndex_ = 0;

/** @private */

var decoderStep_ = 7;

/**

 * Sign a 16-bit integer.

 * @return {number}

 * @private

 */

function sign_(number) {

    return number > 32768 ? number - 65536 : number;

}

/**

 * Compress a 16-bit PCM sample into a 4-bit ADPCM sample.

 * @param {number} sample The sample.

 * @return {number}

 * @private

 */

function encodeSample_(sample) {

    let delta = sample - encoderPredicted_;

    let value = 0;

    if (delta >= 0) {

        value = 0;

    }

    else {

        value = 8;

        delta = -delta;

    }

    let step = STEP_TABLE[encoderIndex_];

    let diff = step >> 3;

    if (delta > step) {

        value |= 4;

        delta -= step;

        diff += step;

    }

    step >>= 1;

    if (delta > step) {

        value |= 2;

        delta -= step;

        diff += step;

    }

    step >>= 1;

    if (delta > step) {

        value |= 1;

        diff += step;

    }

        encoderPredicted_ -= diff;

    }

    else {

        encoderPredicted_ += diff;

    }

    if (encoderPredicted_ < -0x8000) {

        encoderPredicted_ = -0x8000;

    } else if (encoderPredicted_ > 0x7fff) {

        encoderPredicted_ = 0x7fff;

    }

    encoderIndex_ += INDEX_TABLE[value & 7];

    if (encoderIndex_ < 0) {

        encoderIndex_ = 0;

    } else if (encoderIndex_ > 88) {

        encoderIndex_ = 88;

    }

    return value;

}

/**

 * Decode a 4-bit ADPCM sample into a 16-bit PCM sample.

 * @param {number} nibble A 4-bit adpcm sample.

 * @return {number}

 * @private

 */

/**

 * Return the head of a ADPCM sample block.

 * @param {number} sample The first sample of the block.

 * @return {!Array<number>}

 */

function blockHead_(sample) {

    encodeSample_(sample);

    let adpcmSamples = [];

    adpcmSamples.push(sample & 0xFF);

    adpcmSamples.push((sample >> 8) & 0xFF);

    adpcmSamples.push(encoderIndex_);

    adpcmSamples.push(0);

    return adpcmSamples;

}

/**

 * Encode a block of 505 16-bit samples as 4-bit ADPCM samples.

 * @param {!Array<number>} block A sample block of 505 samples.

 * @return {!Array<number>}

 */

function encodeBlock(block) {

    let adpcmSamples = blockHead_(block[0]);

    for (let i=3; i<block.length; i+=2) {

        let sample2 = encodeSample_(block[i]);

        let sample = encodeSample_(block[i + 1]);

        adpcmSamples.push((sample << 4) | sample2);

    }

    while (adpcmSamples.length < 256) {

        adpcmSamples.push(0);

    }

    return adpcmSamples;

}

/**

 * Decode a block of 256 ADPCM samples into 16-bit PCM samples.

 * @param {!Array<number>} block A adpcm sample block of 256 samples.

 * @return {!Array<number>}

 */

function decodeBlock(block) {

    decoderPredicted_ = sign_((block[1] << 8) | block[0]);

    decoderIndex_ = block[2];

    decoderStep_ = STEP_TABLE[decoderIndex_];

    let result = [

            decoderPredicted_,

            sign_((block[3] << 8) | block[2])

        ];

    for (let i=4; i<block.length; i++) {

        let original_sample = block[i];

        let second_sample = original_sample >> 4;

        let first_sample = (second_sample << 4) ^ original_sample;

        result.push(decodeSample_(first_sample));

        result.push(decodeSample_(second_sample));

    }

    return result;

}

/**

 * Encode 16-bit PCM samples into 4-bit IMA ADPCM samples.

 * @param {!Array<number>} samples A array of samples.

 * @return {!Array<number>}

 */

function encode(samples) {

    let adpcmSamples = [];

    let block = [];

    for (let i=0; i<samples.length; i++) {

        block.push(samples[i]);

        if ((i % 505 == 0 && i != 0) || i == samples.length - 1) {

            adpcmSamples = adpcmSamples.concat(encodeBlock(block));

            block = [];

        }

    }

    return adpcmSamples;

}

/**

 * Decode IMA ADPCM samples into 16-bit PCM samples.

 * @param {!Array<number>} adpcmSamples A array of ADPCM samples.

 * @param {number} blockAlign The block size.

 * @return {!Array<number>}

 */

function decode(adpcmSamples, blockAlign=256) {

    let samples = [];

    let block = [];

    for (let i=0; i<adpcmSamples.length; i++) {

        if (i % blockAlign == 0 && i != 0) {            

            samples = samples.concat(decodeBlock(block));

            block = [];

        }

        block.push(adpcmSamples[i]);

    }

    return samples;

}

module.exports.encode = encode;

module.exports.decode = decode;

module.exports.encodeBlock = encodeBlock;

module.exports.decodeBlock = decodeBlock;