traceur/src/runtime/polyfills/fround.js

ES6 to ES5 compiler

// Copyright 2015 Traceur Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Based on code from:
// https://github.com/inexorabletash/polyfill/blob/master/typedarray.js
// Used with permission:
// https://github.com/google/traceur-compiler/issues/1703

var $isFinite = isFinite;
var $isNaN = isNaN;

var {
  LN2,
  abs,
  floor,
  log,
  min,
  pow,
} = Math;

function packIEEE754(v, ebits, fbits) {

  var bias = (1 << (ebits - 1)) - 1,
      s, e, f, ln,
      i, bits, str, bytes;

  function roundToEven(n) {
    var w = floor(n), f = n - w;
    if (f < 0.5)
      return w;
    if (f > 0.5)
      return w + 1;
    return w % 2 ? w + 1 : w;
  }

  // Compute sign, exponent, fraction
  if (v !== v) {
    // NaN
    // http://dev.w3.org/2006/webapi/WebIDL/#es-type-mapping
    e = (1 << ebits) - 1; f = pow(2, fbits - 1); s = 0;
  } else if (v === Infinity || v === -Infinity) {
    e = (1 << ebits) - 1; f = 0; s = (v < 0) ? 1 : 0;
  } else if (v === 0) {
    e = 0; f = 0; s = (1 / v === -Infinity) ? 1 : 0;
  } else {
    s = v < 0;
    v = abs(v);

    if (v >= pow(2, 1 - bias)) {
      e = min(floor(log(v) / LN2), 1023);
      f = roundToEven(v / pow(2, e) * pow(2, fbits));
      if (f / pow(2, fbits) >= 2) {
        e = e + 1;
        f = 1;
      }
      if (e > bias) {
        // Overflow
        e = (1 << ebits) - 1;
        f = 0;
      } else {
        // Normalized
        e = e + bias;
        f = f - pow(2, fbits);
      }
    } else {
      // Denormalized
      e = 0;
      f = roundToEven(v / pow(2, 1 - bias - fbits));
    }
  }

  // Pack sign, exponent, fraction
  bits = [];
  for (i = fbits; i; i -= 1) { bits.push(f % 2 ? 1 : 0); f = floor(f / 2); }
  for (i = ebits; i; i -= 1) { bits.push(e % 2 ? 1 : 0); e = floor(e / 2); }
  bits.push(s ? 1 : 0);
  bits.reverse();
  str = bits.join('');

  // Bits to bytes
  bytes = [];
  while (str.length) {
    bytes.push(parseInt(str.substring(0, 8), 2));
    str = str.substring(8);
  }
  return bytes;
}

function unpackIEEE754(bytes, ebits, fbits) {
  // Bytes to bits
  var bits = [], i, j, b, str,
      bias, s, e, f;

  for (i = bytes.length; i; i -= 1) {
    b = bytes[i - 1];
    for (j = 8; j; j -= 1) {
      bits.push(b % 2 ? 1 : 0); b = b >> 1;
    }
  }
  bits.reverse();
  str = bits.join('');

  // Unpack sign, exponent, fraction
  bias = (1 << (ebits - 1)) - 1;
  s = parseInt(str.substring(0, 1), 2) ? -1 : 1;
  e = parseInt(str.substring(1, 1 + ebits), 2);
  f = parseInt(str.substring(1 + ebits), 2);

  // Produce number
  if (e === (1 << ebits) - 1) {
    return f !== 0 ? NaN : s * Infinity;
  } else if (e > 0) {
    // Normalized
    return s * pow(2, e - bias) * (1 + f / pow(2, fbits));
  } else if (f !== 0) {
    // Denormalized
    return s * pow(2, -(bias - 1)) * (f / pow(2, fbits));
  } else {
    return s < 0 ? -0 : 0;
  }
}

function unpackF32(b) {
  return unpackIEEE754(b, 8, 23);
}

function packF32(v) {
  return packIEEE754(v, 8, 23);
}

export function fround(x) {
  if (x === 0 || !$isFinite(x) || $isNaN(x)) {
    return x;
  }
  return unpackF32(packF32(Number(x)));
}