pow10gen.go

     1  // Copyright 2025 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  //go:build ignore
     6  
     7  package main
     8  
     9  import (
    10  	"bytes"
    11  	"fmt"
    12  	"go/format"
    13  	"log"
    14  	"math/big"
    15  	"os"
    16  )
    17  
    18  const (
    19  	minExp = -348
    20  	maxExp = 347
    21  )
    22  
    23  func main() {
    24  	log.SetPrefix("pow10gen: ")
    25  	log.SetFlags(0)
    26  
    27  	var (
    28  		one = big.NewInt(1)
    29  		ten = big.NewInt(10)
    30  
    31  		b1p64  = new(big.Int).Lsh(one, 64)
    32  		b1p128 = new(big.Int).Lsh(one, 128)
    33  
    34  		r2     = big.NewRat(2, 1)
    35  		r1p128 = new(big.Rat).SetInt(b1p128)
    36  	)
    37  
    38  	var out bytes.Buffer
    39  	fmt.Fprintf(&out, top, minExp, maxExp)
    40  	for e := int64(minExp); e <= maxExp; e++ {
    41  		var r *big.Rat
    42  		if e >= 0 {
    43  			r = new(big.Rat).SetInt(new(big.Int).Exp(ten, big.NewInt(e), nil))
    44  		} else {
    45  			r = new(big.Rat).SetFrac(one, new(big.Int).Exp(ten, big.NewInt(-e), nil))
    46  		}
    47  		be := 0
    48  		for r.Cmp(r1p128) < 0 {
    49  			r.Mul(r, r2)
    50  			be++
    51  		}
    52  		for r.Cmp(r1p128) >= 0 {
    53  			r.Quo(r, r2)
    54  			be--
    55  		}
    56  		d := new(big.Int).Div(r.Num(), r.Denom())
    57  		hi, lo := new(big.Int).DivMod(d, b1p64, new(big.Int))
    58  		fmt.Fprintf(&out, "\t{%#016x, %#016x}, // 1e%d * 2**%d\n", hi.Uint64(), lo.Uint64(), e, be)
    59  	}
    60  	fmt.Fprintf(&out, "}\n")
    61  
    62  	src, err := format.Source(out.Bytes())
    63  	if err != nil {
    64  		log.Fatal(err)
    65  	}
    66  
    67  	if err := os.WriteFile("pow10tab.go", src, 0666); err != nil {
    68  		log.Fatal(err)
    69  	}
    70  }
    71  
    72  var top = `// Copyright 2025 The Go Authors. All rights reserved.
    73  // Use of this source code is governed by a BSD-style
    74  // license that can be found in the LICENSE file.
    75  
    76  // Code generated by: go run pow10gen.go. DO NOT EDIT.
    77  //
    78  //go:generate go run pow10gen.go
    79  
    80  package strconv
    81  
    82  const (
    83  	pow10Min = %d
    84  	pow10Max = %d
    85  )
    86  
    87  
    88  // pow10Tab holds 128-bit mantissas of powers of 10.
    89  // The values are scaled so the high bit is always set; there is no "implicit leading 1 bit".
    90  var pow10Tab = [...]uint128{
    91  `
    92
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