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

  const encodedParts = [];
  let count = 1;

  for (let i = 0; i < input.length; i++) {
    if (input[i] === input[i + 1]) {
      count++;
    } else {
      encodedParts.push(input[i] + count.toString());
      count = 1;
    }
  }

  return encodedParts.join("");
}
```