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Algorithms Problem Solving: Decode the Message

Decode the Message

This post is part of the Algorithms Problem Solving series.

Problem description

This is the Decode the Message problem. The description looks like this:

You are given the strings key and message, which represent a cipher key and a secret message, respectively. The steps to decode message are as follows:

  1. Use the first appearance of all 26 lowercase English letters in key as the order of the substitution table.
  2. Align the substitution table with the regular English alphabet.
  3. Each letter in message is then substituted using the table.
  4. Spaces ' ' are transformed to themselves.
  • For example, given key = "**hap**p**y** **bo**y" (actual key would have at least one instance of each letter in the alphabet), we have the partial substitution table of ('h' -> 'a', 'a' -> 'b', 'p' -> 'c', 'y' -> 'd', 'b' -> 'e', 'o' -> 'f').

Return the decoded message.

Examples

Example 1

Input: key = "the quick brown fox jumps over the lazy dog", message = "vkbs bs t suepuv"
Output: "this is a secret"
Explanation: The diagram above shows the substitution table.
It is obtained by taking the first appearance of each letter in "thequickbrownfoxjumps over thelazydog".

Example 2

Input: key = "eljuxhpwnyrdgtqkviszcfmabo", message = "zwx hnfx lqantp mnoeius ycgk vcnjrdb"
Output: "the five boxing wizards jump quickly"
Explanation: The diagram above shows the substitution table.
It is obtained by taking the first appearance of each letter in "eljuxhpwnyrdgtqkviszcfmabo".

Solution

Adding the tests first:

import { decodeMessage } from '../decode-the-message';
import { describe, expect, it } from 'vitest';

describe('decodeMessage', () => {
  it('example 1', () => {
    expect(
      decodeMessage(
        'eljuxhpwnyrdgtqkviszcfmabo',
        'zwx hnfx lqantp mnoeius ycgk vcnjrdb',
      ),
    ).toEqual('the five boxing wizards jump quickly');
  });

  it('example 2', () => {
    expect(
      decodeMessage(
        'the quick brown fox jumps over the lazy dog',
        'vkbs bs t suepuv',
      ),
    ).toEqual('this is a secret');
  });
});

With the tests done, it's time to start writing our algorithm.

The algorithm is pretty simple. The idea behind it is to use a hashmap data structure to map each character in the key to the alphabet and use it to unravel the secret message.

let hashmap = new Map();

We will also have the alphabet value and a counter:

let alphabet = 'abcdefghijklmnopqrstuvwxyz';
let counter = 0;

We'll use the counter as the index to access the alphabet. For the first character in the key , it will access the index 0 and the increment the counter for the following character.

We also need to handle the space character. We do it by setting a space character in the hashmap.

hashmap.set(' ', ' ');

Now we need to continue building the hashmap based on the characters of the key.

for (let char of key) {
  if (!hashmap.has(char)) {
    hashmap.set(char, alphabet[counter]);
    counter++;
  }
}

For each character in the key, we need to ask for the hashmap if it has the character. If it doesn't, we should add this character in the hashmap.

Using the counter with the alphabet, we get the "current character" and set it as the value to the char key.

Now we need to increment the counter for the following characters. Also, if the hashmap already has the character, it shouldn't do anything else.

Then we can just loop over the message, use the hashmap to get the character and push it to a result array.

let result = [];

for (let char of message) {
  result.push(hashmap.get(char));
}

After this we can just return a string based on the result array using the join method.

return result.join('');

Complete solution

function decodeMessage(key, message) {
  let hashmap = new Map();
  let alphabet = 'abcdefghijklmnopqrstuvwxyz';
  let counter = 0;

  hashmap.set(' ', ' ');

  for (let char of key) {
    if (!hashmap.has(char)) {
      hashmap.set(char, alphabet[counter]);
      counter++;
    }
  }

  let result = [];

  for (let char of message) {
    result.push(hashmap.get(char));
  }

  return result.join('');
}

Resources

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