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

  if (input === '') {
    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.toString();
      count = 1;
    }
  }

  // Add the last run
  encoded += input[input.length - 1] + count.toString();

  return encoded;
}
```