rochars / ieee754-buffer

 * Copyright (c) 2018-2019 Rafael da Silva Rocha.

 * Copyright (c) 2013 DeNA Co., Ltd.

 * Copyright (c) 2010, Linden Research, Inc

 *

 * Permission is hereby granted, free of charge, to any person obtaining

 * a copy of this software and associated documentation files (the

 * "Software"), to deal in the Software without restriction, including

 * without limitation the rights to use, copy, modify, merge, publish,

 * distribute, sublicense, and/or sell copies of the Software, and to

 * permit persons to whom the Software is furnished to do so, subject to

 * the following conditions:

 *

 * The above copyright notice and this permission notice shall be

 * included in all copies or substantial portions of the Software.

 *

 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,

 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF

 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND

 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE

 * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION

 * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION

 * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

 *

 */

/**

 * @fileoverview Encode and decode IEEE 754 floating point numbers.

 * @see https://github.com/rochars/ieee754-buffer

 * @see https://bitbucket.org/lindenlab/llsd/raw/7d2646cd3f9b4c806e73aebc4b32bd81e4047fdc/js/typedarray.js

 * @see https://github.com/kazuho/ieee754.js/blob/master/ieee754.js

 */

/** 

 * @module ieee754-buffer

 */

/**

 * A class to encode and decode IEEE 754 floating-point numbers.

 */

export class IEEE754Buffer {

  /**

   * Pack a IEEE 754 floating point number.

   * @param {number} ebits The exponent bits.

   * @param {number} fbits The fraction bits.

   */

    this.ebits = ebits;

    this.fbits = fbits;

    this.bias = (1 << (ebits - 1)) - 1;

    this.numBytes = Math.ceil((ebits + fbits) / 8);

    this.biasP2 = Math.pow(2, this.bias + 1);

    this.ebitsFbits = (ebits + fbits);

    this.fbias = Math.pow(2, -(8 * this.numBytes - 1 - ebits));

  }

  /**

   * Pack a IEEE 754 floating point number.

   * @param {!Uint8Array|!Array<number>} buffer The buffer.

   * @param {number} num The number.

   * @param {number} index The index to write on the buffer.

   * @return {number} The next index to write on the buffer.

   */

  pack(buffer, num, index) {

    // Round overflows

    if (Math.abs(num) > this.biasP2 - (this.ebitsFbits * 2)) {

      num = num < 0 ? -Infinity : Infinity;

    }

    /**

     * sign, need this to handle negative zero

     * @see http://cwestblog.com/2014/02/25/javascript-testing-for-negative-zero/

     * @type {number}

     */

    let sign = (((num = +num) || 1 / num) < 0) ? 1 : num < 0 ? 1 : 0;

    num = Math.abs(num);

    /** @type {number} */

    let exp = Math.min(Math.floor(Math.log(num) / Math.LN2), 1023);

    /** @type {number} */

    let fraction = this.roundToEven(num / Math.pow(2, exp) * Math.pow(2, this.fbits));

    // NaN

    if (num !== num) {

      fraction = Math.pow(2, this.fbits - 1);

      exp = (1 << this.ebits) - 1;

    // Number

    } else if (num !== 0) {

      if (num >= Math.pow(2, 1 - this.bias)) {

        if (fraction / Math.pow(2, this.fbits) >= 2) {

          exp = exp + 1;

          fraction = 1;

        }

        // Overflow

        if (exp > this.bias) {

          exp = (1 << this.ebits) - 1;

          fraction = 0;

        } else {

          exp = exp + this.bias;

          fraction = this.roundToEven(fraction) - Math.pow(2, this.fbits);

        }

      } else {

        fraction = this.roundToEven(num / Math.pow(2, 1 - this.bias - this.fbits));

        exp = 0;

      } 

    }

    return this.packFloatBits_(buffer, index, sign, exp, fraction);

  }

  /**

   * Unpack a IEEE 754 floating point number.

   * Derived from IEEE754 by DeNA Co., Ltd., MIT License. 

   * Adapted to handle NaN. Should port the solution to the original repo.

   * @param {!Uint8Array|!Array<number>} buffer The buffer.

   * @param {number} index The index to read from the buffer.

   * @return {number} The floating point number.

   */

  /**

   * Pack a IEEE754 from its sign, exponent and fraction bits

   * and place it in a byte buffer.

   * @param {!Uint8Array|!Array<number>} buffer The byte buffer to write to.

   * @param {number} index The buffer index to write.

   * @param {number} sign The sign.

   * @param {number} exp the exponent.

   * @param {number} fraction The fraction.

   * @return {number}

   * @private

   */

  packFloatBits_(buffer, index, sign, exp, fraction) {

    /** @type {!Array<number>} */

    let bits = [];

    // the sign

    bits.push(sign);

    // the exponent

    for (let i = this.ebits; i > 0; i -= 1) {

      bits[i] = (exp % 2 ? 1 : 0);

      exp = Math.floor(exp / 2);

    }

    // the fraction

    let len = bits.length;

    for (let i = this.fbits; i > 0; i -= 1) {

      bits[len + i] = (fraction % 2 ? 1 : 0);

      fraction = Math.floor(fraction / 2);

    }

    // pack as bytes

    /** @type {string} */

    let str = bits.join('');

    /** @type {number} */

    let numBytes = this.numBytes + index - 1;

    /** @type {number} */

    let k = index;

    while (numBytes >= index) {

      buffer[numBytes] = parseInt(str.substring(0, 8), 2);

      str = str.substring(8);

      numBytes--;

      k++;

    }

    return k;

  }

  roundToEven(n) {

    var w = Math.floor(n), f = n - w;

    if (f < 0.5) {

      return w;

    }

    if (f > 0.5) {

      return w + 1;

    }

    return w % 2 ? w + 1 : w;

  }

}