```ts
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') throw new Error('Argument must be a string');
  
  let encoded = '';
  let count = 1;

  for (let i = 1; i <= input.length; i++) {
    if (input[i] === input[i - 1]) {
      count++;
    } else {
      const char = input[i - 1];
      encoded += `${char}${count > 9 ? 'x' : count.toString()}`;
      count = 1;
    }
  }

  return encoded;
}
```