```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++) {
    if (i < input.length && input[i] === input[i - 1]) {
      count++;
    } else {
      result += input[i - 1] + count.toString();
      count = 1;
    }
  }
  
  return result.slice(0, result.length - 1); // Remove the trailing '1' if it exists
}
```