ldexp.mx raw

   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