| // 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. |

| // 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 fastlog2(x float64) float64 { |

| const fastlogScaleBits = 20 |

| const fastlogScaleRatio = 1.0 / (1 << fastlogScaleBits) |

| // Extract the exponent from the IEEE float64, and index a constant |

| // table with the first 10 bits from the mantissa. |

| xExp := int64((xBits>>52)&0x7FF) - 1023 |

| xManIndex := (xBits >> (52 - fastlogNumBits)) % (1 << fastlogNumBits) |

| xManScale := (xBits >> (52 - fastlogNumBits - fastlogScaleBits)) % (1 << fastlogScaleBits) |

| low, high := fastlog2Table[xManIndex], fastlog2Table[xManIndex+1] |

| return float64(xExp) + low + (high-low)*float64(xManScale)*fastlogScaleRatio |