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
5package 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
31func libc_atan2(float64, float64) float64
32
33func Atan2(y, x float64) float64 {
34	return libc_atan2(y, x)
35}
36
37func 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}
78