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package float

import "math"

// ISO 80000-1:2012
// q =  \sgn(y) \left\lfloor \left| y \right| + 0.5 \right\rfloor
//   = -\sgn(y) \left\lceil -\left| y \right| - 0.5 \right\rceil

// RoundN a float value to n decimal places
func RoundN(x float64, n int) float64 {
	pow := math.Pow(10, float64(n))
	return Round(x*pow) / pow
}

// Round a float value
// https://www.cockroachlabs.com/blog/rounding-implementations-in-go/
func Round(x float64) float64 {
	const (
		mask     = 0x7FF
		shift    = 64 - 11 - 1
		bias     = 1023
		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).
		e -= bias
		bits += halfMask >> e
		bits &^= fracMask >> e
	}
	return math.Float64frombits(bits)
}