CountFunc and SplitNFunc for byte arrays in golang

main.go

package main

import (
	"bytes"
	"fmt"
)

func CountFunc(s []byte, f func(rune) bool) int {
	n := 0
	for i := 0; i < len(s); i++ {
	    if f(rune(s[i])) {
		n++
	    }
	}
	return n
}

func SplitNFunc(s []byte, f func(rune) bool, n int) [][]byte {
	if n == 0 {
		return nil
	}
	if n < 0 {
		n = CountFunc(s, f) + 1
	}

	arr := make([][]byte, n)
	n--
	i := 0
	for i < n {
		m := bytes.IndexFunc(s, f)
		if m < 0 {
			break
		}
		arr[i] = s[: m+1 : m+1]
		s = s[m+1:]
		i++
	}
	arr[i] = s
	return arr[:i+1]
}

func main() {
	f := func (r rune) bool {
		return '0' <= r && r <= '9'
	}
	fmt.Printf("%q\n", SplitNFunc([]byte("a1b2c3d"), f,  -1))
}