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[/] [openrisc/] [trunk/] [gnu-dev/] [or1k-gcc/] [libgo/] [go/] [math/] [pow.go] - Blame information for rev 833

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Line No. Rev Author Line
1 747 jeremybenn 
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package math
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func isOddInt(x float64) bool {
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        xi, xf := Modf(x)
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        return xf == 0 && int64(xi)&1 == 1
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}
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// Special cases taken from FreeBSD's /usr/src/lib/msun/src/e_pow.c
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// updated by IEEE Std. 754-2008 "Section 9.2.1 Special values".
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// Pow returns x**y, the base-x exponential of y.
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//
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// Special cases are (in order):
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//      Pow(x, ±0) = 1 for any x
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//      Pow(1, y) = 1 for any y
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//      Pow(x, 1) = x for any x
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//      Pow(NaN, y) = NaN
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//      Pow(x, NaN) = NaN
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//      Pow(±0, y) = ±Inf for y an odd integer < 0
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//      Pow(±0, -Inf) = +Inf
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//      Pow(±0, +Inf) = +0
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//      Pow(±0, y) = +Inf for finite y < 0 and not an odd integer
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//      Pow(±0, y) = ±0 for y an odd integer > 0
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//      Pow(±0, y) = +0 for finite y > 0 and not an odd integer
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//      Pow(-1, ±Inf) = 1
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//      Pow(x, +Inf) = +Inf for |x| > 1
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//      Pow(x, -Inf) = +0 for |x| > 1
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//      Pow(x, +Inf) = +0 for |x| < 1
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//      Pow(x, -Inf) = +Inf for |x| < 1
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//      Pow(+Inf, y) = +Inf for y > 0
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//      Pow(+Inf, y) = +0 for y < 0
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//      Pow(-Inf, y) = Pow(-0, -y)
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//      Pow(x, y) = NaN for finite x < 0 and finite non-integer y
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func Pow(x, y float64) float64 {
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        switch {
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        case y == 0 || x == 1:
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                return 1
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        case y == 1:
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                return x
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        case y == 0.5:
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                return Sqrt(x)
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        case y == -0.5:
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                return 1 / Sqrt(x)
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        case IsNaN(x) || IsNaN(y):
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                return NaN()
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        case x == 0:
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                switch {
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                case y < 0:
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                        if isOddInt(y) {
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                                return Copysign(Inf(1), x)
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                        }
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                        return Inf(1)
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                case y > 0:
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                        if isOddInt(y) {
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                                return x
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                        }
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                        return 0
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                }
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        case IsInf(y, 0):
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                switch {
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                case x == -1:
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                        return 1
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                case (Abs(x) < 1) == IsInf(y, 1):
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                        return 0
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                default:
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                        return Inf(1)
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                }
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        case IsInf(x, 0):
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                if IsInf(x, -1) {
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                        return Pow(1/x, -y) // Pow(-0, -y)
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                }
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                switch {
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                case y < 0:
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                        return 0
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                case y > 0:
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                        return Inf(1)
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                }
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        }
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        absy := y
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        flip := false
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        if absy < 0 {
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                absy = -absy
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                flip = true
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        }
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        yi, yf := Modf(absy)
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        if yf != 0 && x < 0 {
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                return NaN()
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        }
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        if yi >= 1<<63 {
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                return Exp(y * Log(x))
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        }
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        // ans = a1 * 2**ae (= 1 for now).
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        a1 := 1.0
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        ae := 0
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        // ans *= x**yf
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        if yf != 0 {
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                if yf > 0.5 {
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                        yf--
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                        yi++
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                }
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                a1 = Exp(yf * Log(x))
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        }
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        // ans *= x**yi
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        // by multiplying in successive squarings
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        // of x according to bits of yi.
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        // accumulate powers of two into exp.
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        x1, xe := Frexp(x)
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        for i := int64(yi); i != 0; i >>= 1 {
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                if i&1 == 1 {
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                        a1 *= x1
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                        ae += xe
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                }
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                x1 *= x1
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                xe <<= 1
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                if x1 < .5 {
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                        x1 += x1
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                        xe--
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                }
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        }
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        // ans = a1*2**ae
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        // if flip { ans = 1 / ans }
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        // but in the opposite order
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        if flip {
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                a1 = 1 / a1
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                ae = -ae
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        }
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        return Ldexp(a1, ae)
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}

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