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// Copyright 2010 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

// The original C code and the the comment below are from

// FreeBSD's /usr/src/lib/msun/src/e_remainder.c and came

// with this notice.  The go code is a simplified version of

// the original C.

//

// ====================================================

// Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.

//

// Developed at SunPro, a Sun Microsystems, Inc. business.

// Permission to use, copy, modify, and distribute this

// software is freely granted, provided that this notice

// is preserved.

// ====================================================

//

// __ieee754_remainder(x,y)

// Return :

//      returns  x REM y  =  x - [x/y]*y  as if in infinite

//      precision arithmetic, where [x/y] is the (infinite bit)

//      integer nearest x/y (in half way cases, choose the even one).

// Method :

//      Based on Mod() returning  x - [x/y]chopped * y  exactly.

// Remainder returns the IEEE 754 floating-point remainder of x/y.

//

// Special cases are:

//      Remainder(±Inf, y) = NaN

//      Remainder(NaN, y) = NaN

//      Remainder(x, 0) = NaN

//      Remainder(x, ±Inf) = x

//      Remainder(x, NaN) = NaN

func Remainder(x, y float64) float64 {

        return remainder(x, y)

}

func remainder(x, y float64) float64 {

        const (

                Tiny    = 4.45014771701440276618e-308 // 0x0020000000000000

                HalfMax = MaxFloat64 / 2

        )

        // special cases

        switch {

        case IsNaN(x) || IsNaN(y) || IsInf(x, 0) || y == 0:

                return NaN()

        case IsInf(y, 0):

                return x

        }

        sign := false

        if x < 0 {

                x = -x

                sign = true

        }

        if y < 0 {

                y = -y

        }

        if x == y {

                return 0

        }

        if y <= HalfMax {

                x = Mod(x, y+y) // now x < 2y

        }

        if y < Tiny {

                if x+x > y {

                        x -= y

                        if x+x >= y {

                                x -= y

                        }

                }

        } else {

                yHalf := 0.5 * y

                if x > yHalf {

                        x -= y

                        if x >= yHalf {

                                x -= y

                        }

                }

        }

        if sign {

                x = -x

        }

        return x

}