From: Russ Cox <rsc@golang.org>
Date: Thu, 20 Dec 2012 17:23:27 +0000 (-0500)
Subject: math: handle exponent separately in Log2
X-Git-Tag: go1.1rc2~1580
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=a3677b5f223db6ccadf26f81a751c4f8c8c0eaf9;p=gostls13.git

math: handle exponent separately in Log2

This guarantees that powers of two return exact answers.

We could do a multiprecision approximation for the
rest of the answer too, but this seems like it should be
good enough.

Fixes #4567.

R=golang-dev, iant, remyoudompheng
CC=golang-dev
https://golang.org/cl/6943074
---

diff --git a/src/pkg/math/all_test.go b/src/pkg/math/all_test.go
index cdea8035f9..0d8b10f67f 100644
--- a/src/pkg/math/all_test.go
+++ b/src/pkg/math/all_test.go
@@ -2281,6 +2281,13 @@ func TestLog2(t *testing.T) {
 			t.Errorf("Log2(%g) = %g, want %g", vflogSC[i], f, logSC[i])
 		}
 	}
+	for i := -1074; i <= 1023; i++ {
+		f := Ldexp(1, i)
+		l := Log2(f)
+		if l != float64(i) {
+			t.Errorf("Log2(2**%d) = %g, want %d", i, l, i)
+		}
+	}
 }
 
 func TestModf(t *testing.T) {
diff --git a/src/pkg/math/log10.go b/src/pkg/math/log10.go
index 67c163a490..95cfbf47ce 100644
--- a/src/pkg/math/log10.go
+++ b/src/pkg/math/log10.go
@@ -17,5 +17,6 @@ func log10(x float64) float64 {
 func Log2(x float64) float64
 
 func log2(x float64) float64 {
-	return Log(x) * (1 / Ln2)
+	frac, exp := Frexp(x)
+	return Log(frac)*(1/Ln2) + float64(exp)
 }