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);

}

const BitReader = {

    /**

     * 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}

     */

    "read8Bit": function (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}

     */

    "read16Bit": function (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}

     */

    "read16BitFloat": function (bytes, i) {

        return floats.decodeFloat16(bytes.slice(i,i+2));

    },

    /**

     * 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}

     */

    "read24Bit": function (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.

    },

    /**

     * 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}

     */

    "read32BitFloat": function (bytes, i) {

        return intBits.pack(BitReader["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}

     */

    "read40Bit": function (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}

     */

    "read48Bit": function (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}

     */

    "read64BitFloat": function (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}

     */

    "readChar": function (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 = BitReader;