1// Copyright 2015 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// +build ignore
6
7// fastlog2Table contains log2 approximations for 5 binary digits.
8// This is used to implement fastlog2, which is used for heap sampling.
9
10package main
11
12import (
13	"bytes"
14	"fmt"
15	"io/ioutil"
16	"log"
17	"math"
18)
19
20func main() {
21	var buf bytes.Buffer
22
23	fmt.Fprintln(&buf, "// Code generated by mkfastlog2table.go; DO NOT EDIT.")
24	fmt.Fprintln(&buf, "// Run go generate from src/runtime to update.")
25	fmt.Fprintln(&buf, "// See mkfastlog2table.go for comments.")
26	fmt.Fprintln(&buf)
27	fmt.Fprintln(&buf, "package runtime")
28	fmt.Fprintln(&buf)
29	fmt.Fprintln(&buf, "const fastlogNumBits =", fastlogNumBits)
30	fmt.Fprintln(&buf)
31
32	fmt.Fprintln(&buf, "var fastlog2Table = [1<<fastlogNumBits + 1]float64{")
33	table := computeTable()
34	for _, t := range table {
35		fmt.Fprintf(&buf, "\t%v,\n", t)
36	}
37	fmt.Fprintln(&buf, "}")
38
39	if err := ioutil.WriteFile("fastlog2table.go", buf.Bytes(), 0644); err != nil {
40		log.Fatalln(err)
41	}
42}
43
44const fastlogNumBits = 5
45
46func computeTable() []float64 {
47	fastlog2Table := make([]float64, 1<<fastlogNumBits+1)
48	for i := 0; i <= (1 << fastlogNumBits); i++ {
49		fastlog2Table[i] = math.Log2(1.0 + float64(i)/(1<<fastlogNumBits))
50	}
51	return fastlog2Table
52}
53