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

  const result: string[] = [];
  let i = 0;
  while (i < input.length) {
    const char = input[i];
    let count = 1;
    while (i + count < input.length && input[i + count] === char) {
      count++;
    }
    result.push(`${char}${count}`);
    i += count;
  }
  return result.join('');
}
```