```ts

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

    if (input.length === 0) {
        return '';
    }

    let result = '';
    let count = 1;

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

    // Add the last run
    result += input[input.length - 1] + count.toString();

    return result;
}
```