/*
 * 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.
   */
  constructor(ebits, fbits) {
    /**
     * @type {number}
     * @private
     */
    this.ebits = ebits;
    /**
     * @type {number}
     * @private
     */
    this.fbits = fbits;
    /**
     * @type {number}
     * @private
     */
    this.bias = (1 << (ebits - 1)) - 1;
    /**
     * @type {number}
     * @private
     */
    this.numBytes = Math.ceil((ebits + fbits) / 8);
    /**
     * @type {number}
     * @private
     */
    this.biasP2 = Math.pow(2, this.bias + 1);
    /**
     * @type {number}
     * @private
     */
    this.ebitsFbits = (ebits + fbits);
    /**
     * @type {number}
     * @private
     */
    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.
   * @throws {TypeError} If input is not a number.
   */
  pack(buffer, num, index) {
    // Only numbers can be packed
    if (typeof num != 'number') {
      throw new TypeError();
    }
    // 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 = 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 = roundToEven(fraction) - Math.pow(2, this.fbits);
        }
      } else {
        fraction = 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.
   */
  unpack(buffer, index) {
    /** @type {number} */
    let eMax = (1 << this.ebits) - 1;
    /** @type {number} */
    let significand;
    /** @type {string} */
    let leftBits = "";
    for (let i = this.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, this.ebits), 2);
    leftBits = leftBits.substring(this.ebits);
    if (exponent == eMax) {
      if (parseInt(leftBits, 2) !== 0) {
        return NaN;
      }
      return sign * Infinity;  
    } else if (exponent === 0) {
      exponent += 1;
      significand = parseInt(leftBits, 2);
    } else {
      significand = parseInt("1" + leftBits, 2);
    }
    return sign * significand * this.fbias * Math.pow(2, exponent - this.bias);
  }

  /**
   * 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;
  }
}

/**
 * Round a number to its nearest even value.
 * @param {number} n The number.
 * @return {number}
 * @private
 */
function roundToEven(n) {
  /** @type {number} */
  let w = Math.floor(n);
  let f = n - w;
  if (f < 0.5) {
    return w;
  }
  if (f > 0.5) {
    return w + 1;
  }
  return w % 2 ? w + 1 : w;
}