A Go data structure for recording the counts of each character in a string and sorting it by character frequency, and then alphabetically in the case of a tie.

counter.go

package main

import (
	"fmt"
	"sort"
)

type letter struct {
	char  rune
	count int
}

type letters []letter

func (l letters) Less(i, j int) bool {
	switch fd := l[i].count - l[j].count; {
	case fd < 0:
		return false
	case fd > 0:
		return true
	}
	return l[i].char < l[j].char
}

func (l letters) Len() int { return len(l) }

func (l letters) Swap(i, j int) {
	l[i], l[j] = l[j], l[i]
}

func (l letters) Print() {
	for _, letter := range l {
		fmt.Printf("%s : %d ", string(letter.char), letter.count)
	}
}

func countChars(s string) {
	counter := make(map[rune]int)
	for _, c := range s {
		if c != ' ' {
			counter[c] += 1
		}
	}
	lettersSlice := letters{}
	for char, freq := range counter {
		lettersSlice = append(lettersSlice, letter{char, freq})
	}
	sort.Sort(lettersSlice)
	lettersSlice.Print()
}

func main() {
	countChars("hello world")
}