```typescript
/**
 * Encodes a string by replacing every maximal run of one repeated character with that character followed by its count.
 *
 * @param input The 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.');
    }

    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;
            currentChar = char;
            currentCount = 1;
        }
    }

    return result;
}
```