rochars / byte-data

 * read-bytes: Function to read data from bytes.

 * Copyright (c) 2017 Rafael da Silva Rocha.

 * https://github.com/rochars/byte-data

 */

let helpers = require("../src/helpers.js");

const floats = require("../src/floats.js");

const intBits = require("int-bits");

/**

 * Read a group of bytes by turning it to bits.

 * Useful for 40 & 48-bit, but underperform.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @param {number} numBytes The number of bytes

 *      (1 for 8-bit, 2 for 16-bit, etc).

 * @return {number}

 */

function readBytesAsBits(bytes, i, numBytes) {

    let j = numBytes-1;

    let bits = "";

    while (j >= 0) {

        bits += helpers.bytePadding(bytes[j + i].toString(2), 2);

        j--;

    }

    return parseInt(bits, 2);

}

/**

 * Read 1 8-bit int from from bytes.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @return {number}

 */

function read8Bit(bytes, i) {

    return bytes[i];

}

/**

 * Read 1 16-bit int from from bytes.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @return {number}

 */

function read16Bit(bytes, i) {

    return bytes[1 + i] << 8 | bytes[i];

}

/**

 * Read 1 16-bit float from from bytes.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @return {number}

 */

/**

 * Read 1 24-bit int from from bytes.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @return {number}

 */

function read24Bit(bytes, i) {

    return bytes[2 + i] << 16 |

        bytes[1 + i] << 8 |

        bytes[i];

}

/**

 * Read 1 32-bit int from from bytes.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @return {number}

 */

function read32Bit(bytes, i) {

    return (bytes[3 + i] << 24 |

        bytes[2 + i] << 16 |

        bytes[1 + i] << 8 |

        bytes[i]) >>> 0;

}

/**

 * Read 1 32-bit float from from bytes.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @return {number}

 */

function read32BitFloat(bytes, i) {

    return intBits.pack(read32Bit(bytes, i));

}

/**

 * Read 1 40-bit int from from bytes.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @return {number}

 */

function read40Bit(bytes, i) {

    return readBytesAsBits(bytes, i, 5);

}

/**

 * Read 1 48-bit int from bytes.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @return {number}

 */

function read48Bit(bytes, i) {

    return readBytesAsBits(bytes, i, 6);

}

/**

 * Read 1 64-bit double from bytes.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @return {number}

 */

function read64BitFloat(bytes, i) {

    return floats.decodeFloat64(bytes.slice(i,i+8));

}

/**

 * Read 1 char from bytes.

 * @param {!Array<number>|Uint8Array} bytes An array of bytes.

 * @param {number} i The index to read.

 * @return {string}

 */

function readChar(bytes, i, type) {

    let chrs = "";

    let j = 0;

    let len = type.bits / 8;

    while(j < len) {

        chrs += String.fromCharCode(bytes[i+j]);

        j++;

    }

    return chrs;

}

module.exports.readChar = readChar;

module.exports.read8Bit = read8Bit;

module.exports.read16Bit = read16Bit;

module.exports.read16BitFloat = read16BitFloat;

module.exports.read24Bit = read24Bit;

module.exports.read32Bit = read32Bit;

module.exports.read32BitFloat = read32BitFloat;

module.exports.read40Bit = read40Bit;

module.exports.read48Bit = read48Bit;

module.exports.read64BitFloat = read64BitFloat;