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

  if (input.length === 0) return '';

  let result = '';
  let currentChar = input[0];
  let count = 1;

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

  result += currentChar + count;
  return result;
}
```