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

  if (input === '') {
    return '';
  }

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

  for (let i = 1; i <= input.length; i++) {
    if (i < input.length && input[i] === currentChar) {
      count++;
    } else {
      result += currentChar + count.toString();
      if (i < input.length) {
        currentChar = input[i];
        count = 1;
      }
    }
  }

  return result;
}
```