from typing import Optional


def casrn_checksum(value: Optional[int]) -> int:
    """Computes the checksum from an integer.

    Checksum is the:
     - Reverse of the original integer
     - Each digit multiplied by the index
     - Summed
     - Mod by 10

    789 => (9 * 1) + (8 * 2) + (7 * 3) => 46 % 10 => 6

    Args:
        value (int): A CAS-RN.

    Returns:
        The CAS-RN checksum integer.

    """
    return sum([i * int(x) for i, x in enumerate(str(value)[::-1], 1)]) % 10