```ts
/**
 * Encodes a string using run-length encoding.
 */
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') {
    throw new Error('Input must be a string');
  }
  const length = input.length;
  if (length === 0) return '';
  let result = '';
  let i = 0;
  while (i < length) {
    const currentChar = input[i];
    let count = 1;
    while (i + count < length && input[i + count] === currentChar) {
      count++;
    }
    result += currentChar + count.toString();
    i += count;
  }
  return result;
}
```