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slightly simpler math.Pow per gri's suggestion

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R=gri
DELTA=28  (2 added, 9 deleted, 17 changed)
OCL=19707
CL=19707
This commit is contained in:
Russ Cox 2008-11-20 11:56:48 -08:00
parent 5014da7cb0
commit 2e7e76073a
1 changed files with 19 additions and 26 deletions
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45
src/lib/math/pow.go
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@ -6,7 +6,7 @@ package math
import "math"
// x^y: exponentation
// x^y: exponentiation
export func Pow(x, y float64) float64 {
// TODO: x or y NaN, ±Inf, maybe ±0.
switch {
@ -38,7 +38,9 @@ export func Pow(x, y float64) float64 {
return Exp(y * Log(x));
}
ans := float64(1);
// ans = a1 * 2^ae (= 1 for now).
a1 := float64(1);
ae := 0;
// ans *= x^yf
if yf != 0 {
@ -46,42 +48,33 @@ export func Pow(x, y float64) float64 {
yf--;
yi++;
}
ans = Exp(yf * Log(x));
a1 = Exp(yf * Log(x));
}
// ans *= x^yi
// by multiplying in successive squarings
// of x according to bits of yi.
// accumulate powers of two into exp.
// will still have to do ans *= 2^exp later.
x1, xe := sys.frexp(x);
exp := 0;
if i := int64(yi); i != 0 {
for {
if i&1 == 1 {
ans *= x1;
exp += xe;
}
i >>= 1;
if i == 0 {
break;
}
x1 *= x1;
xe <<= 1;
if x1 < .5 {
x1 += x1;
xe--;
}
for i := int64(yi); i != 0; i >>= 1 {
if i&1 == 1 {
a1 *= x1;
ae += xe;
}
x1 *= x1;
xe <<= 1;
if x1 < .5 {
x1 += x1;
xe--;
}
}
// ans *= 2^exp
// ans = a1*2^ae
// if flip { ans = 1 / ans }
// but in the opposite order
if flip {
ans = 1 / ans;
exp = -exp;
a1 = 1 / a1;
ae = -ae;
}
return sys.ldexp(ans, exp);
return sys.ldexp(a1, ae);
}