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

[thisismohitgupta]

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]

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]

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

[thisismohitgupta]

Thank You. That was exactly what I was looking for.

[cznic]

Alternative solution