https://forum.lowyat.net/topic/4149756

combo2.go

package main

import (
	"fmt"
	"os"
)

var bitCounts = []uint8{}

func init() {
	bitCounts = make([]uint8, 256, 256)
	for n := 1; n <= 255; n++ {
		count := uint8(0)
		for k := uint8(n); k > 0; k >>= 1 {
			if k&1 == 1 {
				count++
			}
		}
		bitCounts[n] = count
	}
}

func prettyPrint(size uint8, num uint64) {
	for ; size > 0; size-- {
		if num&1 == 1 {
			fmt.Print("B")
		} else {
			fmt.Print("A")
		}
		num >>= 1
	}
	fmt.Println("")
}

func listCombos(numA, numB uint8) {
	size := uint8(numA + numB)
	if size >= 64 {
		fmt.Println("Sorry, too lazy to bignum this for now")
		return
	}
	n := uint64(1 << size)
	for i := uint64(0); i < n; i++ {
		bits := uint8(0)
		for j := i; j > 0; j >>= 8 {
			bits = bits + bitCounts[j&0xff]
		}
		if bits == numB {
			prettyPrint(size, i)
		}
	}
}

func countAB(ab string) (uint8, uint8) {
	var a, b uint8
	for i := 0; i < len(ab); i++ {
		switch ab[i] {
		case 'A':
			a++
		case 'B':
			b++
		}
	}
	return a, b
}

func main() {
	input := ""
	if len(os.Args) > 1 {
		input = os.Args[1]
	} else {
		fmt.Println("Usage: combo2 AAB\nWhere AAB is the string of the combinations you're after")
		return
	}
	a, b := countAB(input)
	listCombos(a, b)
}