```typescript
/**
 * Encodes a string using run-length encoding.
 *
 * Replaces every maximal run of one repeated character with that character followed by the run length,
 * in order: "aaabbc" encodes to "a3b2c1". A run of length one still carries its count, so "abc"
 * encodes to "a1b1c1". The empty string encodes to the empty string.
 *
 * @param input The input string to encode
 * @throws {Error} If the input is not a string
 */
export function runLengthEncode(input: string): string {
    if (typeof input !== 'string') {
        throw new Error('Input must be a string');
    }

    let result = '';
    let currentChar = input[0];
    let currentCount = 1;

    for (let i = 1; i <= input.length; i++) {
        const char = input[i];

        if (char === currentChar) {
            currentCount++;
        } else {
            result += currentChar + (currentCount > 9 ? currentCount.toString() : currentCount + '');
            currentChar = char;
            currentCount = 1;
        }
    }

    return result;
}
```