floor.go

package round

import "math"

const (
	uvnan    = 0x7FF8000000000001
	uvinf    = 0x7FF0000000000000
	uvneginf = 0xFFF0000000000000
	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 {
	const (
		signMask = 1 << 63
		fracMask = (1 << shift) - 1
		halfMask = 1 << (shift - 1)
		one      = bias << shift
	)

	bits := math.Float64bits(x)
	e := uint(bits>>shift) & mask
	switch {
	case e < bias:
		// Round abs(x)<1 including denormals.
		bits &= signMask // +-0
		if e == bias-1 {
			bits |= one // +-1
		}
	case 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 += halfMask >> e
		bits &^= fracMask >> e
	}
	return math.Float64frombits(bits)
}