```ts
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') {
    throw new Error('Input must be a string');
  }

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

  let encoded = '';
  let currentChar = input[0];
  let count = 1;

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

  // Append the last run
  encoded += `${currentChar}${count}`;

  return encoded;
}
```