time-to-botec

Benchmark sampling in different programming languages
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log2.js (4205B)

      1 /**
      2 * @license Apache-2.0
      3 *
      4 * Copyright (c) 2018 The Stdlib Authors.
      5 *
      6 * Licensed under the Apache License, Version 2.0 (the "License");
      7 * you may not use this file except in compliance with the License.
      8 * You may obtain a copy of the License at
      9 *
     10 *    http://www.apache.org/licenses/LICENSE-2.0
     11 *
     12 * Unless required by applicable law or agreed to in writing, software
     13 * distributed under the License is distributed on an "AS IS" BASIS,
     14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     15 * See the License for the specific language governing permissions and
     16 * limitations under the License.
     17 *
     18 *
     19 * ## Notice
     20 *
     21 * The following copyright and license were part of the original implementation available as part of [FreeBSD]{@link https://svnweb.freebsd.org/base/release/9.3.0/lib/msun/src/e_log2.c}. The implementation follows the original, but has been modified for JavaScript.
     22 *
     23 * ```text
     24 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
     25 *
     26 * Developed at SunPro, a Sun Microsystems, Inc. business.
     27 * Permission to use, copy, modify, and distribute this
     28 * software is freely granted, provided that this notice
     29 * is preserved.
     30 * ```
     31 */
     32 
     33 'use strict';
     34 
     35 // MODULES //
     36 
     37 var getHighWord = require( '@stdlib/number/float64/base/get-high-word' );
     38 var setHighWord = require( '@stdlib/number/float64/base/set-high-word' );
     39 var setLowWord = require( '@stdlib/number/float64/base/set-low-word' );
     40 var toWords = require( '@stdlib/number/float64/base/to-words' );
     41 var isnan = require( './../../../../base/assert/is-nan' );
     42 var BIAS = require( '@stdlib/constants/float64/exponent-bias' );
     43 var NINF = require( '@stdlib/constants/float64/ninf' );
     44 var klog = require( './klog.js' );
     45 
     46 
     47 // VARIABLES //
     48 
     49 var TWO54 = 1.80143985094819840000e+16;   // 0x43500000, 0x00000000
     50 var IVLN2HI = 1.44269504072144627571e+00; // 0x3ff71547, 0x65200000
     51 var IVLN2LO = 1.67517131648865118353e-10; // 0x3de705fc, 0x2eefa200
     52 
     53 // 0x000fffff = 1048575 => 0 00000000000 11111111111111111111
     54 var HIGH_SIGNIFICAND_MASK = 0x000fffff|0; // asm type annotation
     55 
     56 // 0x7ff00000 = 2146435072 => 0 11111111111 00000000000000000000 => biased exponent: 2047 = 1023+1023 => 2^1023
     57 var HIGH_MAX_NORMAL_EXP = 0x7ff00000|0; // asm type annotation
     58 
     59 // 0x00100000 = 1048576 => 0 00000000001 00000000000000000000 => biased exponent: 1 = -1022+1023 => 2^-1022
     60 var HIGH_MIN_NORMAL_EXP = 0x00100000|0; // asm type annotation
     61 
     62 // 0x3ff00000 = 1072693248 => 0 01111111111 00000000000000000000 => biased exponent: 1023 = 0+1023 => 2^0 = 1
     63 var HIGH_BIASED_EXP_0 = 0x3ff00000|0; // asm type annotation
     64 
     65 // 0x7fffffff = 2147483647 => 0 11111111111 11111111111111111111
     66 var ABS_MASK = 0x7fffffff|0; // asm type annotation
     67 
     68 // High/low words workspace:
     69 var WORDS = [ 0|0, 0|0 ]; // WARNING: not thread safe
     70 
     71 
     72 // MAIN //
     73 
     74 /**
     75 * Evaluates the binary logarithm (base two).
     76 *
     77 * @param {NonNegativeNumber} x - input value
     78 * @returns {number} function value
     79 *
     80 * @example
     81 * var v = log2( 4.0 );
     82 * // returns 2.0
     83 *
     84 * @example
     85 * var v = log2( 8.0 );
     86 * // returns 3.0
     87 *
     88 * @example
     89 * var v = log2( 0.0 );
     90 * // returns -Infinity
     91 *
     92 * @example
     93 * var v = log2( Infinity );
     94 * // returns Infinity
     95 *
     96 * @example
     97 * var v = log2( NaN );
     98 * // returns NaN
     99 *
    100 * @example
    101 * var v = log2( -4.0 );
    102 * // returns NaN
    103 */
    104 function log2( x ) {
    105 	var hi;
    106 	var lo;
    107 	var hx;
    108 	var lx;
    109 	var f;
    110 	var i;
    111 	var k;
    112 
    113 	if ( isnan( x ) || x < 0.0 ) {
    114 		return NaN;
    115 	}
    116 	toWords( WORDS, x );
    117 	hx = WORDS[ 0 ];
    118 	lx = WORDS[ 1 ];
    119 	k = 0|0; // asm type annotation
    120 	if ( hx < HIGH_MIN_NORMAL_EXP ) {
    121 		// Case: x < 2**-1022
    122 		if ( ( (hx&ABS_MASK) | lx ) === 0 ) {
    123 			return NINF;
    124 		}
    125 		k -= 54|0; // asm type annotation
    126 
    127 		// Subnormal number, scale up x:
    128 		x *= TWO54;
    129 		hx = getHighWord( x );
    130 	}
    131 	if ( hx >= HIGH_MAX_NORMAL_EXP ) {
    132 		return x + x;
    133 	}
    134 	k += ( (hx>>20) - BIAS )|0; // asm type annotation
    135 	hx &= HIGH_SIGNIFICAND_MASK;
    136 	i = ( ( hx+0x95f64 ) & 0x100000 )|0; // asm type annotation
    137 
    138 	// Normalize x or x/2...
    139 	x = setHighWord( x, hx|(i^HIGH_BIASED_EXP_0) );
    140 	k += (i>>20)|0; // asm type annotation
    141 	f = klog( x );
    142 	x -= 1;
    143 	hi = setLowWord( x, 0 );
    144 	lo = x - hi;
    145 	return ( (x+f)*IVLN2LO ) + ( (lo+f)*IVLN2HI ) + ( hi*IVLN2HI ) + k;
    146 }
    147 
    148 
    149 // EXPORTS //
    150 
    151 module.exports = log2;