```ts
/**
 * Encodes input 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');
  }

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

  let output = '';
  let count = 1;

  for (let i = 1; i < input.length; i++) {
    if (input[i] === input[i - 1]) {
      count++;
    } else {
      output += `${input[i - 1]}${count}`;
      count = 1;
    }
  }

  output += `${input[input.length - 1]}${count}`;

  return output;
}
```