```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 parts: string[] = [];
  let currentChar = input[0];
  let count = 1;

  for (let i = 1; i < input.length; i++) {
    const ch = input[i];
    if (ch === currentChar) {
      count++;
    } else {
      parts.push(currentChar + String(count));
      currentChar = ch;
      count = 1;
    }
  }

  parts.push(currentChar + String(count));
  return parts.join('');
}
```
