// Copyright 2015 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 runtime

// fastlog2 implements a fast approximation to the base 2 log of a
// float64. This is used to compute a geometric distribution for heap
// sampling, without introducing dependencies into package math. This
// uses a very rough approximation using the float64 exponent and the
// first 25 bits of the mantissa. The top 5 bits of the mantissa are
// used to load limits from a table of constants and the rest are used
// to scale linearly between them.
func ( float64) float64 {
	const  = 20
	const  = 1.0 / (1 << )

	 := float64bits()
	// Extract the exponent from the IEEE float64, and index a constant
	// table with the first 10 bits from the mantissa.
	 := int64((>>52)&0x7FF) - 1023
	 := ( >> (52 - fastlogNumBits)) % (1 << fastlogNumBits)
	 := ( >> (52 - fastlogNumBits - )) % (1 << )

	,  := fastlog2Table[], fastlog2Table[+1]
	return float64() +  + (-)*float64()*
}