```typescript
/**
 * Encodes a string by replacing every maximal run of one repeated character with that character followed by the run length.
 *
 * @param input The input string to be encoded.
 * @returns The encoded string.
 */
export function runLengthEncode(input: string): string {
    if (typeof input !== 'string') {
        throw new Error('Input must be a string.');
    }

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

    for (let i = 1; i <= input.length; i++) {
        if (i < input.length && input[i] !== currentChar) {
            result.push(currentChar + currentCount.toString());
            currentChar = input[i];
            currentCount = 1;
        } else {
            currentCount++;
        }
    }

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