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math: speed up Gamma(+Inf)

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Add special case for Gamma(+∞) which speeds it up:

benchmark            old ns/op     new ns/op     delta
BenchmarkGamma-4     14.5          7.44          -48.69%

The documentation for math.Gamma already specifies it as a special
case:

        Gamma(+Inf) = +Inf

The original C code that has been used as the reference implementation
(as mentioned in the comments in gamma.go) also treats Gamma(+∞) as a
special case:

if( x == INFINITY )
        return(x);

Change-Id: Idac36e19192b440475aec0796faa2d2c7f8abe0b
Reviewed-on: https://go-review.googlesource.com/31370
Reviewed-by: Robert Griesemer <gri@golang.org>
This commit is contained in:
Mohit Agarwal 2016-10-18 14:00:30 +05:30 committed by Robert Griesemer
parent 97b04152bc
commit 7eed848a17
1 changed files with 2 additions and 0 deletions
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2
src/math/gamma.go
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@ -133,6 +133,8 @@ func Gamma(x float64) float64 {
switch { switch {
case isNegInt(x) || IsInf(x, -1) || IsNaN(x): case isNegInt(x) || IsInf(x, -1) || IsNaN(x):
return NaN() return NaN()
case IsInf(x, 1):
return Inf(1)
case x == 0: case x == 0:
if Signbit(x) { if Signbit(x) {
return Inf(-1) return Inf(-1)