package main

import (
    "fmt"
    "math"
    "strings"
)

func main(){
    fmt.Println(H("1223334444"))
}

func H(data string) (entropy float64) {
    if data == "" {
        return 0
    }
    for i := 0; i < 256; i++ {
        px := float64(strings.Count(data, string(byte(i)))) / float64(len(data))
        if px > 0 {
	    entropy += -px * math.Log2(px)
	}
    }
    return entropy
}