math: Add Round function (ties away from zero)

This function avoids subtle faults found in many ad-hoc implementations,
and is simple enough to be inlined by the compiler.

Fixes #20100

Change-Id: Ib320254e9b1f1f798c6ef906b116f63bc29e8d08
Reviewed-on: https://go-review.googlesource.com/43652Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/all_test.go

@@ -529,6 +529,18 @@ var remainder = []float64{
 	8.734595415957246977711748e-01,
 	1.314075231424398637614104e+00,
 }
+var round = []float64{
+	5,
+	8,
+	Copysign(0, -1),
+	-5,
+	10,
+	3,
+	5,
+	3,
+	2,
+	-9,
+}
 var signbit = []bool{
 	false,
 	false,
@@ -1755,6 +1767,20 @@ var pow10SC = []float64{
 	Inf(1),   // pow10(MaxInt32)
 }
 
+var vfroundSC = [][2]float64{
+	{0, 0},
+	{1.390671161567e-309, 0}, // denormal
+	{0.49999999999999994, 0}, // 0.5-epsilon
+	{0.5, 1},
+	{0.5000000000000001, 1}, // 0.5+epsilon
+	{-1.5, -2},
+	{NaN(), NaN()},
+	{Inf(1), Inf(1)},
+	{2251799813685249.5, 2251799813685250}, // 1 bit fraction
+	{4503599627370495.5, 4503599627370496}, // 1 bit fraction, rounding to 0 bit fraction
+	{4503599627370497, 4503599627370497},   // large integer
+}
+
 var vfsignbitSC = []float64{
 	Inf(-1),
 	Copysign(0, -1),
@@ -2713,6 +2739,19 @@ func TestRemainder(t *testing.T) {
 	}
 }
 
+func TestRound(t *testing.T) {
+	for i := 0; i < len(vf); i++ {
+		if f := Round(vf[i]); !alike(round[i], f) {
+			t.Errorf("Round(%g) = %g, want %g", vf[i], f, round[i])
+		}
+	}
+	for i := 0; i < len(vfroundSC); i++ {
+		if f := Round(vfroundSC[i][0]); !alike(vfroundSC[i][1], f) {
+			t.Errorf("Round(%g) = %g, want %g", vfroundSC[i][0], f, vfroundSC[i][1])
+		}
+	}
+}
+
 func TestSignbit(t *testing.T) {
 	for i := 0; i < len(vf); i++ {
 		if f := Signbit(vf[i]); signbit[i] != f {
@@ -3360,6 +3399,16 @@ func BenchmarkPow10Neg(b *testing.B) {
 	GlobalF = x
 }
 
+var roundNeg = float64(-2.5)
+
+func BenchmarkRound(b *testing.B) {
+	x := 0.0
+	for i := 0; i < b.N; i++ {
+		x = Round(roundNeg)
+	}
+	GlobalF = x
+}
+
 func BenchmarkRemainder(b *testing.B) {
 	x := 0.0
 	for i := 0; i < b.N; i++ {

src/math/floor.go

-// Copyright 2009-2010 The Go Authors. All rights reserved.
+// 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.
 
@@ -54,3 +54,46 @@ func trunc(x float64) float64 {
 	d, _ := Modf(x)
 	return d
 }
+
+// Round returns the nearest integer, rounding half away from zero.
+//
+// Special cases are:
+//	Round(±0) = ±0
+//	Round(±Inf) = ±Inf
+//	Round(NaN) = NaN
+func Round(x float64) float64 {
+	// Round is a faster implementation of:
+	//
+	// func Round(x float64) float64 {
+	//   t := Trunc(x)
+	//   if Abs(x-t) >= 0.5 {
+	//     return t + Copysign(1, x)
+	//   }
+	//   return t
+	// }
+	const (
+		signMask = 1 << 63
+		fracMask = 1<<shift - 1
+		half     = 1 << (shift - 1)
+		one      = bias << shift
+	)
+
+	bits := Float64bits(x)
+	e := uint(bits>>shift) & mask
+	if e < bias {
+		// Round abs(x) < 1 including denormals.
+		bits &= signMask // +-0
+		if e == bias-1 {
+			bits |= one // +-1
+		}
+	} else if e < bias+shift {
+		// Round any abs(x) >= 1 containing a fractional component [0,1).
+		//
+		// Numbers with larger exponents are returned unchanged since they
+		// must be either an integer, infinity, or NaN.
+		e -= bias
+		bits += half >> e
+		bits &^= fracMask >> e
+	}
+	return Float64frombits(bits)
+}