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

  const result = [];
  let count = 1;

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

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