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Source file src/math/remainder.go

Documentation: math

     1  // Copyright 2010 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  // The original C code and the comment below are from
     8  // FreeBSD's /usr/src/lib/msun/src/e_remainder.c and came
     9  // with this notice. The go code is a simplified version of
    10  // the original C.
    11  //
    12  // ====================================================
    13  // Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
    14  //
    15  // Developed at SunPro, a Sun Microsystems, Inc. business.
    16  // Permission to use, copy, modify, and distribute this
    17  // software is freely granted, provided that this notice
    18  // is preserved.
    19  // ====================================================
    20  //
    21  // __ieee754_remainder(x,y)
    22  // Return :
    23  //      returns  x REM y  =  x - [x/y]*y  as if in infinite
    24  //      precision arithmetic, where [x/y] is the (infinite bit)
    25  //      integer nearest x/y (in half way cases, choose the even one).
    26  // Method :
    27  //      Based on Mod() returning  x - [x/y]chopped * y  exactly.
    28  
    29  // Remainder returns the IEEE 754 floating-point remainder of x/y.
    30  //
    31  // Special cases are:
    32  //	Remainder(±Inf, y) = NaN
    33  //	Remainder(NaN, y) = NaN
    34  //	Remainder(x, 0) = NaN
    35  //	Remainder(x, ±Inf) = x
    36  //	Remainder(x, NaN) = NaN
    37  func Remainder(x, y float64) float64
    38  
    39  func remainder(x, y float64) float64 {
    40  	const (
    41  		Tiny    = 4.45014771701440276618e-308 // 0x0020000000000000
    42  		HalfMax = MaxFloat64 / 2
    43  	)
    44  	// special cases
    45  	switch {
    46  	case IsNaN(x) || IsNaN(y) || IsInf(x, 0) || y == 0:
    47  		return NaN()
    48  	case IsInf(y, 0):
    49  		return x
    50  	}
    51  	sign := false
    52  	if x < 0 {
    53  		x = -x
    54  		sign = true
    55  	}
    56  	if y < 0 {
    57  		y = -y
    58  	}
    59  	if x == y {
    60  		return 0
    61  	}
    62  	if y <= HalfMax {
    63  		x = Mod(x, y+y) // now x < 2y
    64  	}
    65  	if y < Tiny {
    66  		if x+x > y {
    67  			x -= y
    68  			if x+x >= y {
    69  				x -= y
    70  			}
    71  		}
    72  	} else {
    73  		yHalf := 0.5 * y
    74  		if x > yHalf {
    75  			x -= y
    76  			if x >= yHalf {
    77  				x -= y
    78  			}
    79  		}
    80  	}
    81  	if sign {
    82  		x = -x
    83  	}
    84  	return x
    85  }
    86  

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