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

  let result = '';
  let runStart = 0;

  for (let i = 1; i <= input.length; i++) {
    if (i === input.length || input[i] !== input[runStart]) {
      const count = Math.abs(i - runStart - 1);
      result += `${input[runStart]}${count > 9 ? '9' : count.toString()}`;
      runStart = i;
    }
  }

  return result;
}
```