libgo/go/math/atan2.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 // Atan2 returns the arc tangent of y/x, using
   8 // the signs of the two to determine the quadrant
   9 // of the return value.
  10 //
  11 // Special cases are (in order):
  12 //      Atan2(y, NaN) = NaN
  13 //      Atan2(NaN, x) = NaN
  14 //      Atan2(+0, x>=0) = +0
  15 //      Atan2(-0, x>=0) = -0
  16 //      Atan2(+0, x<=-0) = +Pi
  17 //      Atan2(-0, x<=-0) = -Pi
  18 //      Atan2(y>0, 0) = +Pi/2
  19 //      Atan2(y<0, 0) = -Pi/2
  20 //      Atan2(+Inf, +Inf) = +Pi/4
  21 //      Atan2(-Inf, +Inf) = -Pi/4
  22 //      Atan2(+Inf, -Inf) = 3Pi/4
  23 //      Atan2(-Inf, -Inf) = -3Pi/4
  24 //      Atan2(y, +Inf) = 0
  25 //      Atan2(y>0, -Inf) = +Pi
  26 //      Atan2(y<0, -Inf) = -Pi
  27 //      Atan2(+Inf, x) = +Pi/2
  28 //      Atan2(-Inf, x) = -Pi/2
  29
  30 //extern atan2
  31 func libc_atan2(float64, float64) float64
  32
  33 func Atan2(y, x float64) float64 {
  34         return libc_atan2(y, x)
  35 }
  36
  37 func atan2(y, x float64) float64 {
  38         // special cases
  39         switch {
  40         case IsNaN(y) || IsNaN(x):
  41                 return NaN()
  42         case y == 0:
  43                 if x >= 0 && !Signbit(x) {
  44                         return Copysign(0, y)
  45                 }
  46                 return Copysign(Pi, y)
  47         case x == 0:
  48                 return Copysign(Pi/2, y)
  49         case IsInf(x, 0):
  50                 if IsInf(x, 1) {
  51                         switch {
  52                         case IsInf(y, 0):
  53                                 return Copysign(Pi/4, y)
  54                         default:
  55                                 return Copysign(0, y)
  56                         }
  57                 }
  58                 switch {
  59                 case IsInf(y, 0):
  60                         return Copysign(3*Pi/4, y)
  61                 default:
  62                         return Copysign(Pi, y)
  63                 }
  64         case IsInf(y, 0):
  65                 return Copysign(Pi/2, y)
  66         }
  67
  68         // Call atan and determine the quadrant.
  69         q := Atan(y / x)
  70         if x < 0 {
  71                 if q <= 0 {
  72                         return q + Pi
  73                 }
  74                 return q - Pi
  75         }
  76         return q
  77 }