Run-Length Encoding in JavaScript

Introduction

Run-length encoding (RLE) is a lossless data compression method that replaces sequences of repeated values with a single value and a count of how many times it appears consecutively. For example, ‘AAAA’ becomes ‘A4’.

Caveat

Run-Length Encoding (RLE) is most efficient on data that contains many consecutive repeated values (runs). For data with little repetition, using RLE may actually increase the file size rather than reduce it.

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Implementation

function runLengthEncode(input) {
  if (input.length === 0) return '';

  let encoded = '';
  let count = 1;

  for (let i = 1; i <= input.length; i++) {
    if (input[i] === input[i - 1]) {
      count++;
    } else {
      encoded += input[i - 1] + count;
      count = 1;
    }
  }

  return encoded;
}

function runLengthDecode(encoded) {
  let decoded = '';
  let i = 0;

  while (i < encoded.length) {
    const char = encoded[i];
    i++;

    let countStr = ''; // Capture the full number
    while (i < encoded.length && /\d/.test(encoded[i])) {
      countStr += encoded[i];
      i++;
    }

    const count = parseInt(countStr, 10);
    decoded += char.repeat(count);
  }

  return decoded;
}

Example Usage

const input = 'aaabbbcccaaa';
const encoded = runLengthEncode(input);

console.log('Encoded:', encoded); // => Encoded: a3b3c3a3

const decoded = runLengthDecode(encoded);

console.log('Decoded:', decoded); // => Decoded: aaabbbcccaaa
console.log(input === decoded); // => true