github.com/c12o16h1/go/src@v0.0.0-20200114212001-5a151c0f00ed/math/ldexp.go (about)

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package math
     6  
     7  // Ldexp is the inverse of Frexp.
     8  // It returns frac × 2**exp.
     9  //
    10  // Special cases are:
    11  //	Ldexp(±0, exp) = ±0
    12  //	Ldexp(±Inf, exp) = ±Inf
    13  //	Ldexp(NaN, exp) = NaN
    14  func Ldexp(frac float64, exp int) float64
    15  
    16  func ldexp(frac float64, exp int) float64 {
    17  	// special cases
    18  	switch {
    19  	case frac == 0:
    20  		return frac // correctly return -0
    21  	case IsInf(frac, 0) || IsNaN(frac):
    22  		return frac
    23  	}
    24  	frac, e := normalize(frac)
    25  	exp += e
    26  	x := Float64bits(frac)
    27  	exp += int(x>>shift)&mask - bias
    28  	if exp < -1075 {
    29  		return Copysign(0, frac) // underflow
    30  	}
    31  	if exp > 1023 { // overflow
    32  		if frac < 0 {
    33  			return Inf(-1)
    34  		}
    35  		return Inf(1)
    36  	}
    37  	var m float64 = 1
    38  	if exp < -1022 { // denormal
    39  		exp += 53
    40  		m = 1.0 / (1 << 53) // 2**-53
    41  	}
    42  	x &^= mask << shift
    43  	x |= uint64(exp+bias) << shift
    44  	return m * Float64frombits(x)
    45  }