src/pkg/math/sin.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package math

/*
	Floating-point sine and cosine.

	Coefficients are #5077 from Hart & Cheney. (18.80D)
*/

func sinus(x float64, quad int) float64 {
	const (
		P0 = .1357884097877375669092680e8
		P1 = -.4942908100902844161158627e7
		P2 = .4401030535375266501944918e6
		P3 = -.1384727249982452873054457e5
		P4 = .1459688406665768722226959e3
		Q0 = .8644558652922534429915149e7
		Q1 = .4081792252343299749395779e6
		Q2 = .9463096101538208180571257e4
		Q3 = .1326534908786136358911494e3
	)
	if x < 0 {
		x = -x
		quad = quad + 2
	}
	x = x * (2 / Pi) /* underflow? */
	var y float64
	if x > 32764 {
		var e float64
		e, y = Modf(x)
		e = e + float64(quad)
		f, _ := Modf(0.25 * e)
		quad = int(e - 4*f)
	} else {
		k := int32(x)
		y = x - float64(k)
		quad = (quad + int(k)) & 3
	}

	if quad&1 != 0 {
		y = 1 - y
	}
	if quad > 1 {
		y = -y
	}

	yy := y * y
	temp1 := ((((P4*yy+P3)*yy+P2)*yy+P1)*yy + P0) * y
	temp2 := ((((yy+Q3)*yy+Q2)*yy+Q1)*yy + Q0)
	return temp1 / temp2
}

// Cos returns the cosine of x.
func Cos(x float64) float64 {
	if x < 0 {
		x = -x
	}
	return sinus(x, 1)
}

// Sin returns the sine of x.
func Sin(x float64) float64 { return sinus(x, 0) }