```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 result = '';
  let count = 1;

  for (let i = 1; i < input.length; i++) {
    const currentChar = input[i];
    const prevChar = input[i - 1];

    if (currentChar === prevChar) {
      count++;
    } else {
      result += prevChar + count.toString();
      count = 1;
    }
  }

  // Append the last run
  result += input[input.length - 1] + count.toString();

  return result;
}
```