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math/big: cannot calculate log16 of a 256 bit integer #28300

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thisismohitgupta opened this issue Oct 21, 2018 · 4 comments
Closed

math/big: cannot calculate log16 of a 256 bit integer #28300

thisismohitgupta opened this issue Oct 21, 2018 · 4 comments
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@thisismohitgupta
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thisismohitgupta commented Oct 21, 2018 •
edited

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

What did you expect to see?

how it works in python...

import math 
a = 0xc6d86e5a2cb4bc532361c2d4940f0b1a0138066e25d65c1c530d080b11f8ca24
a>>89940344608680314083397671686667731393131665861770496634981932531495305005604L
math.log(a)/math.log(16.0)

63.908875905794794

What did you see instead?

does not work for Big Ints

s := "c6d86e5a2cb4bc532361c2d4940f0b1a0138066e25d65c1c530d080b11f8ca24" // Hex value
i := new(big.Int)
i.SetString(s, 16)
// how to get the log with base 16 for a math/big Int variable.
@ALTree
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ALTree commented Oct 21, 2018 •
edited

There's no Log for big.Float in the standard library. You'll have to use an external library. For example, using mine (ALTree/bigfloat):

package main

import (
	"fmt"
	"math/big"

	"github.com/ALTree/bigfloat"
)

func main() {
	x := new(big.Float).SetPrec(256)
	x.SetString("89940344608680314083397671686667731393131665861770496634981932531495305005604")

	log16 := bigfloat.Log(big.NewFloat(16.0).SetPrec(256))
	logX := bigfloat.Log(x)

	res := new(big.Float).SetPrec(256)
	fmt.Printf("%g\n", res.Quo(logX, log16))
}

$ go run test.go
63.908875905794799149011030723455843229394283193466612998787786375106246936971

@ALTree
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ALTree commented Oct 21, 2018

(There was a proposal to integrate a few additional operations (Log, Pow) into the standard library, but it was rejected: #14102).

@ALTree ALTree changed the title Cannot calculate log16 of a 256 bit Integer in Golang math/big: cannot calculate log16 of a 256 bit integer Oct 21, 2018
@thisismohitgupta
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Thank You. That was exactly what I was looking for.

@ALTree ALTree closed this as completed Oct 21, 2018
@cznic
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cznic commented Oct 21, 2018

Alternative solution

@golang golang locked and limited conversation to collaborators Oct 21, 2019
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@cznic @ALTree @gopherbot @thisismohitgupta