tpl: Add math.Ceil, Floor, and Round

Ceil and Floor are frontends for the stdlib math functions. The Round
implementation is essentially the same thing except that the Go stdlib
doesn't include a Round implementation in a stable release yet.  I've
included the Round function slated for Go 1.10.

Fixes #3883

tpl/math/round.go

@@ -0,0 +1,61 @@
+// 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.
+
+// According to https://github.com/golang/go/issues/20100, the Go stdlib will
+// include math.Round beginning with Go 1.10.
+//
+// The following implementation was taken from https://golang.org/cl/43652.
+
+package math
+
+import "math"
+
+const (
+	mask  = 0x7FF
+	shift = 64 - 11 - 1
+	bias  = 1023
+)
+
+// 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 := math.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 math.Float64frombits(bits)
+}