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

  let encoded = '';
  let count = 1;

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

  return encoded;
}
```