math: special cases for Modf, Frexp and Ldexp; added Modf_386

Also moved Modf from bits.go into modf.go and added timing tests.

R=rsc
CC=golang-dev
http://codereview.appspot.com/202042

Committer: Russ Cox <rsc@golang.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/src/pkg/math/modf.go b/src/pkg/math/modf.go
new file mode 100644
index 000000000..ae0c7c887
--- /dev/null
+++ b/src/pkg/math/modf.go
@@ -0,0 +1,33 @@
+// 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
+
+// Modf returns integer and fractional floating-point numbers
+// that sum to f.  Both values have the same sign as f.
+//
+// Special cases are:
+//	Modf(+Inf) = +Inf, NaN
+//	Modf(-Inf) = -Inf, NaN
+//	Modf(NaN) = NaN, NaN
+func Modf(f float64) (int float64, frac float64) {
+	if f < 1 {
+		if f < 0 {
+			int, frac = Modf(-f)
+			return -int, -frac
+		}
+		return 0, f
+	}
+
+	x := Float64bits(f)
+	e := uint(x>>shift)&mask - bias
+
+	// Keep the top 11+e bits, the integer part; clear the rest.
+	if e < 64-11 {
+		x &^= 1<<(64-11-e) - 1
+	}
+	int = Float64frombits(x)
+	frac = f - int
+	return
+}