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// Copyright 2017 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 big
import (
"math"
"sync"
)
var threeOnce struct {
sync.Once
v *Float
}
func three() *Float {
threeOnce.Do(func() {
threeOnce.v = NewFloat(3.0)
})
return threeOnce.v
}
// Sqrt sets z to the rounded square root of x, and returns it.
//
// If z's precision is 0, it is changed to x's precision before the
// operation. Rounding is performed according to z's precision and
// rounding mode, but z's accuracy is not computed. Specifically, the
// result of z.Acc() is undefined.
//
// The function panics if z < 0. The value of z is undefined in that
// case.
func (z *Float) Sqrt(x *Float) *Float {
if debugFloat {
x.validate()
}
if z.prec == 0 {
z.prec = x.prec
}
if x.Sign() == -1 {
// following IEEE754-2008 (section 7.2)
panic(ErrNaN{"square root of negative operand"})
}
// handle ±0 and +∞
if x.form != finite {
z.acc = Exact
z.form = x.form
z.neg = x.neg // IEEE754-2008 requires √±0 = ±0
return z
}
// MantExp sets the argument's precision to the receiver's, and
// when z.prec > x.prec this will lower z.prec. Restore it after
// the MantExp call.
prec := z.prec
b := x.MantExp(z)
z.prec = prec
// Compute √(z·2**b) as
// √( z)·2**(½b) if b is even
// √(2z)·2**(⌊½b⌋) if b > 0 is odd
// √(½z)·2**(⌈½b⌉) if b < 0 is odd
switch b % 2 {
case 0:
// nothing to do
case 1:
z.exp++
case -1:
z.exp--
}
// 0.25 <= z < 2.0
// Solving 1/x² - z = 0 avoids Quo calls and is faster, especially
// for high precisions.
z.sqrtInverse(z)
// re-attach halved exponent
return z.SetMantExp(z, b/2)
}
// Compute √x (to z.prec precision) by solving
//
// 1/t² - x = 0
//
// for t (using Newton's method), and then inverting.
func (z *Float) sqrtInverse(x *Float) {
// let
// f(t) = 1/t² - x
// then
// g(t) = f(t)/f'(t) = -½t(1 - xt²)
// and the next guess is given by
// t2 = t - g(t) = ½t(3 - xt²)
u := newFloat(z.prec)
v := newFloat(z.prec)
three := three()
ng := func(t *Float) *Float {
u.prec = t.prec
v.prec = t.prec
u.Mul(t, t) // u = t²
u.Mul(x, u) // = xt²
v.Sub(three, u) // v = 3 - xt²
u.Mul(t, v) // u = t(3 - xt²)
u.exp-- // = ½t(3 - xt²)
return t.Set(u)
}
xf, _ := x.Float64()
sqi := newFloat(z.prec)
sqi.SetFloat64(1 / math.Sqrt(xf))
for prec := z.prec + 32; sqi.prec < prec; {
sqi.prec *= 2
sqi = ng(sqi)
}
// sqi = 1/√x
// x/√x = √x
z.Mul(x, sqi)
}
// newFloat returns a new *Float with space for twice the given
// precision.
func newFloat(prec2 uint32) *Float {
z := new(Float)
// nat.make ensures the slice length is > 0
z.mant = z.mant.make(int(prec2/_W) * 2)
return z
}
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