```ts
/**
 * Encodes a string using run-length encoding.
 * 
 * @param input - The string to encode.
 * @returns The encoded string.
 */
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') {
    throw new Error('Input must be a string');
  }

  return input.split('').reduce((acc, char, i) => {
    const nextChar = input[i + 1];
    if (char === nextChar) {
      return acc;
    } else {
      return acc + char + (i - acc.lastIndexOf(char) + 1);
    }
  }, '');
}
```