ldexp.go

     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  //
    12  //	Ldexp(±0, exp) = ±0
    13  //	Ldexp(±Inf, exp) = ±Inf
    14  //	Ldexp(NaN, exp) = NaN
    15  func Ldexp(frac float64, exp int) float64 {
    16  	if haveArchLdexp {
    17  		return archLdexp(frac, exp)
    18  	}
    19  	return ldexp(frac, exp)
    20  }
    21  
    22  func ldexp(frac float64, exp int) float64 {
    23  	// special cases
    24  	switch {
    25  	case frac == 0:
    26  		return frac // correctly return -0
    27  	case IsInf(frac, 0) || IsNaN(frac):
    28  		return frac
    29  	}
    30  	frac, e := normalize(frac)
    31  	exp += e
    32  	x := Float64bits(frac)
    33  	exp += int(x>>shift)&mask - bias
    34  	if exp < -1075 {
    35  		return Copysign(0, frac) // underflow
    36  	}
    37  	if exp > 1023 { // overflow
    38  		if frac < 0 {
    39  			return Inf(-1)
    40  		}
    41  		return Inf(1)
    42  	}
    43  	var m float64 = 1
    44  	if exp < -1022 { // denormal
    45  		exp += 53
    46  		m = 1.0 / (1 << 53) // 2**-53
    47  	}
    48  	x &^= mask << shift
    49  	x |= uint64(exp+bias) << shift
    50  	return m * Float64frombits(x)
    51  }
    52
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