rochars / byte-data

 * Copyright (c) 2017-2018 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 Functions to pack and unpack IEEE 754 floating point numbers.

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

 */

/**

 * Pack a IEEE754 floating point number.

 * Derived from typedarray.js by Linden Research, MIT License.

 * Adapted to round overflows and underflows to Infinity and -Infinity.

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

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

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

 * @param {number} num The number.

 * @param {number} ebits The number of bits of the exponent.

 * @param {number} fbits The number of bits of the fraction.

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

 */

export function pack(buffer, index, num, ebits, fbits) {

  /** @type {number} */

  let bias = (1 << (ebits - 1)) - 1;

  // Round overflows and underflows

  if (Math.abs(num) > Math.pow(2, bias + 1) - ((ebits + fbits) * 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 = Math.round(num / Math.pow(2, exp) * Math.pow(2, fbits));

  // NaN

  if (num !== num) {

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

    exp = (1 << ebits) - 1;

  // Number

  } else if (num !== 0) {

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

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

        exp = exp + 1;

        fraction = 1;

      }

      // Overflow

      if (exp > bias) {

        exp = (1 << ebits) - 1;

        fraction = 0;

      } else {

        exp = exp + bias;

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

      }

    } else {

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

      exp = 0;

    } 

  }

  return packFloatBits_(buffer, index, ebits, fbits, sign, exp, fraction);

}

/**

 * Unpack a IEEE754 floating point number.

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

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

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

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

 * @param {number} index the start index to read.

 * @param {number} ebits The number of bits of the exponent.

 * @param {number} fbits The number of bits of the fraction.

 */

export function unpack(buffer, index, ebits, fbits) {

  let exponentBias = (1 << (ebits - 1)) - 1;

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

  /** @type {number} */

  let eMax = (1 << ebits) - 1;

  /** @type {number} */

  let bias = Math.pow(2, -(8 * numBytes - 1 - ebits));

  /** @type {number} */

  let significand;

  /** @type {string} */

  let leftBits = "";

  for (let i = numBytes - 1; i >= 0 ; i--) {

    /** @type {string} */

    let t = buffer[i + index].toString(2);

    leftBits += "00000000".substring(t.length) + t;

  }

  /** @type {number} */

  let sign = leftBits.charAt(0) == "1" ? -1 : 1;

  leftBits = leftBits.substring(1);

  /** @type {number} */

  let exponent = parseInt(leftBits.substring(0, ebits), 2);

  leftBits = leftBits.substring(ebits);

  if (exponent == eMax) {

    if (parseInt(leftBits, 2) !== 0) {

      return NaN;

    }

    return sign * Infinity;  

    exponent += 1;

    significand = parseInt(leftBits, 2);

  } else {

    significand = parseInt("1" + leftBits, 2);

  }

  return sign * significand * bias * Math.pow(2, exponent - exponentBias);

}

/**

 * 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} ebits The number of bits of the exponent.

 * @param {number} fbits The number of bits of the fraction.

 * @param {number} sign The sign.

 * @param {number} exp the exponent.

 * @param {number} fraction The fraction.

 * @return {number}

 * @private

 */