```ts
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') throw new Error('Input must be a string');

  const encoded = [];
  let count = 1;

  for (let i = 0; i < input.length; i++) {
    if (input[i] === input[i - 1]) {
      count++;
    } else {
      encoded.push(input[i - 1], count.toString());
      count = 1;
    }

    // Handle the last run
    if (i === input.length - 1) {
      encoded.push(input[i - 1], count.toString());
    }
  }

  return encoded.join('');
}
```