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

    const result = [];
    let count = 1;

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

    return result.join('');
}
```