```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;
  const length = input.length;

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

  return result;
}
```