ShannonEntropy

"use strict"
test()

function test() {
    let testMessage = "abrakadabra";
    let alphabet = GetAlphabetWithFrequency(testMessage);
    let HX = CalculateShannonEntropy(alphabet);
    console.log(HX);
}

function GetAlphabetWithFrequency(message) {
    let alphabet = {
        length: 0,
    }
    for (let i = 0; i < message.length; i++) {
        let letter = String(message[i]);
        if ([letter] in alphabet) alphabet[letter] += 1;
        else {
            alphabet[letter] = 1;
            alphabet.length += 1;
        }
    }
    return alphabet;
}


function CalculateShannonEntropy(alphabetWithFrequency) {
    let HX = 0;
    let n = alphabetWithFrequency.length;
    for (let letter in alphabetWithFrequency) {
        if ([letter] != "length") {
            let p_i = alphabetWithFrequency[letter] / n;
            HX += p_i * getBaseLog(n, p_i);
        }
    }
    return -HX;
}


function getBaseLog(x, y) {
    return Math.log(y) / Math.log(x);
}