Acessing unexported function `nat.string`

I want to convert a big.Int to simple base32. Not the standard base32 stuff like the RFC 4648 implemented by base32 nor zBase32 nor Crockford I want just simple normal 5-bits per character 0-9A-V character set.

I am aware of the base32 package and it does not do what I want - it builds the result in a standard base 32 number with padding and stuff I don't want. Certainly I could use it and tear off the trailing "=" characters and hack what remains but that just seems like a brutal solution.

There is a big.SetString(string, base) that can parse a base32 number in string form but there is no reverse - which is what I am really looking for, a big.GetString(base) like the Java BigInteger.toString(int base).

There is, however, a nat.string which does exactly what I want. How can I gain access to it?

Is there a way I could manually extend big to implement big.GetString(base) which trivially calls nat.string with the correct charset?

Is there a way I can reach into the unexported big.nat which big uses and call nat.string?

Is there something else I can do?

P.S. I'd also be interested in using nat.trailingZeroBits - I had to write my own because I didn't realise this was already done.

You can't access unexported functions at all. You'd need to re-write at least some portion of nat.go in order to achieve that functionality. Some of those functions look very useful, so it may be worth sending a feature request to the golang-nuts group asking for some of them to be exported in a future release.

You can however use strconv.FormatInt() to do what you require.

given a big.Int b you can do:

strconv.FormatInt(b.Int64(), 32)

Full example:

package main

import (
	"fmt"
	"math/big"
	"strconv"
)

func main() {
	i := 3286583923486565782 // Some random integer
	b := big.NewInt(int64(i))
	fmt.Println(strconv.FormatInt(b.Int64(), 32))
}

Produces:

>>2r72al99uq9cm

Playground

I don't think the go team will ever export anything in nat as that is an implementation details for big.Int. They may look kindly upon a request to export a ToBase function though from big.Int.

In the mean time here is a lightly tested naive base converter for you

func ToBase(x *big.Int, base int) string {
	if x.Sign() == 0 {
		return "0"
	}
	y := new(big.Int).Set(x)
	b := big.NewInt(int64(base))
	charset := "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
	out := make([]byte, 0, 16)
	negative := false
	if y.Sign() < 0 {
		negative = true
		y.Neg(y)
	}
	digit := new(big.Int)
	for y.Sign() != 0 {
		y.DivMod(y, b, digit)
		out = append(out, charset[digit.Int64()])
	}
	if negative {
		out = append(out, '-')
	}
	// Reverse out
	for i, j := 0, len(out)-1; i < j; i, j = i+1, j-1 {
		out[i], out[j] = out[j], out[i]
	}
	return string(out)
}

Playground link

I have posted on the go-nuts thread. They seem happy with my suggestion.

Meanwhile - my implementation of Base32 (does NOT handle sign - like nat)

// The digits
const charset string = "0123456789ABCDEFGHIJKLMNOPQRSTUV"

/*
 Convert the big to base32
*/
func Base32(n big.Int) string {
	// The bytes of the number
	bytes := n.Bytes()
	nBytes := len(bytes)
	if nBytes == 0 {
		// Special case 0.
		return "0"
	}
	// How many digits will be in the string?
	nBits := nBytes * 8
	nDigits := nBits / 5
	if nBits%5 > 0 {
		nDigits += 1
	}
	// Grow the digits into an array
	digits := make([]byte, nDigits)
	digit := nDigits - 1
	// Peel off 5 bits from the array each time.
	for b, shr := nBytes-1, uint(0); b >= 0; {
		// The next lot is there.
		shrp5 := shr + 5
		// Build my bits.
		x := (bytes[b] >> shr)
		if shrp5 > 8 && b > 0 {
			of := shrp5 - 8
			// Add in some bits from the next byte too.
			x &= (1 << (8 - shr)) - 1
			x |= bytes[b-1] << (5 - of)
		}
		// Shift 5 more.
		shr = shrp5
		// Should we step to next byte?
		if shr >= 8 {
			// Next byte
			shr -= 8
			b -= 1
		}
		x &= 0x1f
		// Build my digit
		digits[digit] = charset[x]
		digit -= 1
	}
	// Skip leading zeros.
	lz := 0
	for digits[lz] == '0' {
		lz += 1
	}
	// Return the string equivalent.
	return string(digits[lz:])
}

Comments welcome.