Monoalphabetic Substitution Cipher Helper

cipher-helper.js

// A tool to help you create or solve monoalphabetic substitution ciphers.
// This code is left to the public domain.
//
// Run this in a Node interactive session or web browser console.
// Start by creating a machine instance with m = machine(message);
// Call m.replace() to progressively replace each letter with a new one.
// Use lowercase for your original message and uppercase for the
// substituted letters so you and the code can tell them apart.
// m.frequency will give you the fequency of each letter in the message.

function machine (code) {
    let state = code;
    let previous = state;

    return Object.freeze({
        replace (a, b) {
            previous = state;
            state = state.replace(new RegExp(a, "g"), b);
            return state;
        },

        undo () {
            [state, previous] = [previous, state];
            return state;
        },

        reset () {
            [state, previous] = [code, state];
            return state;
        },

        get frequency () {
            return [...state].reduce((freq, char) => (freq[char] = freq[char] + 1 || 1, freq), {});
        },

        get state () {
            return state;
        }
    });
}