libgo/go/math/log10.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 // Log10 returns the decimal logarithm of x.
   8 // The special cases are the same as for Log.
   9
  10 //extern log10
  11 func libc_log10(float64) float64
  12
  13 func Log10(x float64) float64 {
  14         return libc_log10(x)
  15 }
  16
  17 func log10(x float64) float64 {
  18         return Log(x) * (1 / Ln10)
  19 }
  20
  21 // Log2 returns the binary logarithm of x.
  22 // The special cases are the same as for Log.
  23
  24 func Log2(x float64) float64 {
  25         return log2(x)
  26 }
  27
  28 func log2(x float64) float64 {
  29         frac, exp := Frexp(x)
  30         // Make sure exact powers of two give an exact answer.
  31         // Don't depend on Log(0.5)*(1/Ln2)+exp being exactly exp-1.
  32         if frac == 0.5 {
  33                 return float64(exp - 1)
  34         }
  35         return Log(frac)*(1/Ln2) + float64(exp)
  36 }