src/math/log10.go

// 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

// Log10 returns the decimal logarithm of x.
// The special cases are the same as for [Log].
func Log10(x float64) float64 {
	if haveArchLog10 {
		return archLog10(x)
	}
	return log10(x)
}

func log10(x float64) float64 {
	return Log(x) * (1 / Ln10)
}

// Log2 returns the binary logarithm of x.
// The special cases are the same as for [Log].
func Log2(x float64) float64 {
	if haveArchLog2 {
		return archLog2(x)
	}
	return log2(x)
}

func log2(x float64) float64 {
	frac, exp := Frexp(x)
	// Make sure exact powers of two give an exact answer.
	// Don't depend on Log(0.5)*(1/Ln2)+exp being exactly exp-1.
	if frac == 0.5 {
		return float64(exp - 1)
	}
	// Avoid cancellation near 1. x-1 is exact for x in [0.5, 2).
	if exp == 0 || exp == 1 {
		return Log1p(x-1) * (1 / Ln2)
	}
	return Log(frac)*(1/Ln2) + float64(exp)
}