libgo/go/math/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 //      Ldexp(±0, exp) = ±0
  12 //      Ldexp(±Inf, exp) = ±Inf
  13 //      Ldexp(NaN, exp) = NaN
  14
  15 //extern ldexp
  16 func libc_ldexp(float64, int32) float64
  17
  18 func Ldexp(frac float64, exp int) float64 {
  19         if exp > MaxInt32 {
  20                 exp = MaxInt32
  21         } else if exp < MinInt32 {
  22                 exp = MinInt32
  23         }
  24         r := libc_ldexp(frac, int32(exp))
  25         return r
  26 }
  27
  28 func ldexp(frac float64, exp int) float64 {
  29         // special cases
  30         switch {
  31         case frac == 0:
  32                 return frac // correctly return -0
  33         case IsInf(frac, 0) || IsNaN(frac):
  34                 return frac
  35         }
  36         frac, e := normalize(frac)
  37         exp += e
  38         x := Float64bits(frac)
  39         exp += int(x>>shift)&mask - bias
  40         if exp < -1075 {
  41                 return Copysign(0, frac) // underflow
  42         }
  43         if exp > 1023 { // overflow
  44                 if frac < 0 {
  45                         return Inf(-1)
  46                 }
  47                 return Inf(1)
  48         }
  49         var m float64 = 1
  50         if exp < -1022 { // denormal
  51                 exp += 53
  52                 m = 1.0 / (1 << 53) // 2**-53
  53         }
  54         x &^= mask << shift
  55         x |= uint64(exp+bias) << shift
  56         return m * Float64frombits(x)
  57 }