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Source file src/runtime/mkfastlog2table.go

Documentation: runtime

  // 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.
  
  // +build ignore
  
  // fastlog2Table contains log2 approximations for 5 binary digits.
  // This is used to implement fastlog2, which is used for heap sampling.
  
  package main
  
  import (
  	"bytes"
  	"fmt"
  	"io/ioutil"
  	"log"
  	"math"
  )
  
  func main() {
  	var buf bytes.Buffer
  
  	fmt.Fprintln(&buf, "// AUTO-GENERATED by mkfastlog2table.go")
  	fmt.Fprintln(&buf, "// Run go generate from src/runtime to update.")
  	fmt.Fprintln(&buf, "// See mkfastlog2table.go for comments.")
  	fmt.Fprintln(&buf)
  	fmt.Fprintln(&buf, "package runtime")
  	fmt.Fprintln(&buf)
  	fmt.Fprintln(&buf, "const fastlogNumBits =", fastlogNumBits)
  	fmt.Fprintln(&buf)
  
  	fmt.Fprintln(&buf, "var fastlog2Table = [1<<fastlogNumBits + 1]float64{")
  	table := computeTable()
  	for _, t := range table {
  		fmt.Fprintf(&buf, "\t%v,\n", t)
  	}
  	fmt.Fprintln(&buf, "}")
  
  	if err := ioutil.WriteFile("fastlog2table.go", buf.Bytes(), 0644); err != nil {
  		log.Fatalln(err)
  	}
  }
  
  const fastlogNumBits = 5
  
  func computeTable() []float64 {
  	fastlog2Table := make([]float64, 1<<fastlogNumBits+1)
  	for i := 0; i <= (1 << fastlogNumBits); i++ {
  		fastlog2Table[i] = math.Log2(1.0 + float64(i)/(1<<fastlogNumBits))
  	}
  	return fastlog2Table
  }
  

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