Source file src/pkg/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	func Atan2(y, x float64) float64
    30	
    31	func atan2(y, x float64) float64 {
    32		// special cases
    33		switch {
    34		case IsNaN(y) || IsNaN(x):
    35			return NaN()
    36		case y == 0:
    37			if x >= 0 && !Signbit(x) {
    38				return Copysign(0, y)
    39			}
    40			return Copysign(Pi, y)
    41		case x == 0:
    42			return Copysign(Pi/2, y)
    43		case IsInf(x, 0):
    44			if IsInf(x, 1) {
    45				switch {
    46				case IsInf(y, 0):
    47					return Copysign(Pi/4, y)
    48				default:
    49					return Copysign(0, y)
    50				}
    51			}
    52			switch {
    53			case IsInf(y, 0):
    54				return Copysign(3*Pi/4, y)
    55			default:
    56				return Copysign(Pi, y)
    57			}
    58		case IsInf(y, 0):
    59			return Copysign(Pi/2, y)
    60		}
    61	
    62		// Call atan and determine the quadrant.
    63		q := Atan(y / x)
    64		if x < 0 {
    65			if q <= 0 {
    66				return q + Pi
    67			}
    68			return q - Pi
    69		}
    70		return q
    71	}