// 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

func isOddInt( float64) bool {
	,  := Modf()
	return  == 0 && int64()&1 == 1
}

// Special cases taken from FreeBSD's /usr/src/lib/msun/src/e_pow.c
// updated by IEEE Std. 754-2008 "Section 9.2.1 Special values".

// Pow returns x**y, the base-x exponential of y.
//
// Special cases are (in order):
//	Pow(x, ±0) = 1 for any x
//	Pow(1, y) = 1 for any y
//	Pow(x, 1) = x for any x
//	Pow(NaN, y) = NaN
//	Pow(x, NaN) = NaN
//	Pow(±0, y) = ±Inf for y an odd integer < 0
//	Pow(±0, -Inf) = +Inf
//	Pow(±0, +Inf) = +0
//	Pow(±0, y) = +Inf for finite y < 0 and not an odd integer
//	Pow(±0, y) = ±0 for y an odd integer > 0
//	Pow(±0, y) = +0 for finite y > 0 and not an odd integer
//	Pow(-1, ±Inf) = 1
//	Pow(x, +Inf) = +Inf for |x| > 1
//	Pow(x, -Inf) = +0 for |x| > 1
//	Pow(x, +Inf) = +0 for |x| < 1
//	Pow(x, -Inf) = +Inf for |x| < 1
//	Pow(+Inf, y) = +Inf for y > 0
//	Pow(+Inf, y) = +0 for y < 0
//	Pow(-Inf, y) = Pow(-0, -y)
//	Pow(x, y) = NaN for finite x < 0 and finite non-integer y
func (,  float64) float64 {
	if haveArchPow {
		return archPow(, )
	}
	return pow(, )
}

func pow(,  float64) float64 {
	switch {
	case  == 0 ||  == 1:
		return 1
	case  == 1:
		return 
	case IsNaN() || IsNaN():
		return NaN()
	case  == 0:
		switch {
		case  < 0:
			if isOddInt() {
				return Copysign(Inf(1), )
			}
			return Inf(1)
		case  > 0:
			if isOddInt() {
				return 
			}
			return 0
		}
	case IsInf(, 0):
		switch {
		case  == -1:
			return 1
		case (Abs() < 1) == IsInf(, 1):
			return 0
		default:
			return Inf(1)
		}
	case IsInf(, 0):
		if IsInf(, -1) {
			return Pow(1/, -) // Pow(-0, -y)
		}
		switch {
		case  < 0:
			return 0
		case  > 0:
			return Inf(1)
		}
	case  == 0.5:
		return Sqrt()
	case  == -0.5:
		return 1 / Sqrt()
	}

	,  := Modf(Abs())
	if  != 0 &&  < 0 {
		return NaN()
	}
	if  >= 1<<63 {
		// yi is a large even int that will lead to overflow (or underflow to 0)
		// for all x except -1 (x == 1 was handled earlier)
		switch {
		case  == -1:
			return 1
		case (Abs() < 1) == ( > 0):
			return 0
		default:
			return Inf(1)
		}
	}

	// ans = a1 * 2**ae (= 1 for now).
	 := 1.0
	 := 0

	// ans *= x**yf
	if  != 0 {
		if  > 0.5 {
			--
			++
		}
		 = Exp( * Log())
	}

	// ans *= x**yi
	// by multiplying in successive squarings
	// of x according to bits of yi.
	// accumulate powers of two into exp.
	,  := Frexp()
	for  := int64();  != 0;  >>= 1 {
		if  < -1<<12 || 1<<12 <  {
			// catch xe before it overflows the left shift below
			// Since i !=0 it has at least one bit still set, so ae will accumulate xe
			// on at least one more iteration, ae += xe is a lower bound on ae
			// the lower bound on ae exceeds the size of a float64 exp
			// so the final call to Ldexp will produce under/overflow (0/Inf)
			 += 
			break
		}
		if &1 == 1 {
			 *= 
			 += 
		}
		 *= 
		 <<= 1
		if  < .5 {
			 += 
			--
		}
	}

	// ans = a1*2**ae
	// if y < 0 { ans = 1 / ans }
	// but in the opposite order
	if  < 0 {
		 = 1 / 
		 = -
	}
	return Ldexp(, )
}