// Copyright 2015 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package runtime
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// fastlog2 implements a fast approximation to the base 2 log of a
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// float64. This is used to compute a geometric distribution for heap
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// sampling, without introducing dependencies into package math. This
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// uses a very rough approximation using the float64 exponent and the
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// first 25 bits of the mantissa. The top 5 bits of the mantissa are
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// used to load limits from a table of constants and the rest are used
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// to scale linearly between them.
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func fastlog2(x float64) float64 {
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const fastlogScaleBits = 20
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const fastlogScaleRatio = 1.0 / (1 << fastlogScaleBits)
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xBits := float64bits(x)
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// Extract the exponent from the IEEE float64, and index a constant
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// table with the first 10 bits from the mantissa.
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xExp := int64((xBits>>52)&0x7FF) - 1023
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xManIndex := (xBits >> (52 - fastlogNumBits)) % (1 << fastlogNumBits)
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xManScale := (xBits >> (52 - fastlogNumBits - fastlogScaleBits)) % (1 << fastlogScaleBits)
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low, high := fastlog2Table[xManIndex], fastlog2Table[xManIndex+1]
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return float64(xExp) + low + (high-low)*float64(xManScale)*fastlogScaleRatio
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}
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