```ts
/** Run-length encodes a string. Each maximal run of repeated characters is replaced with that character followed by its count. */
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') {
    throw new Error('Input must be a string');
  }

  let encoded = '';
  let i = 0;

  while (i < input.length) {
    const char = input[i];
    let count = 1;
    while (i + 1 < input.length && input[i + 1] === char) {
      i += 1;
      count += 1;
    }
    encoded += `${char}${count}`;
    i += 1;
  }

  return encoded;
}
```