Squirrel hexToInteger function

/**
 * Convert hex string to an integer
 * Credits: https://forums.electricimp.com/discussion/comment/2877/#Comment_2877
 * @param hex  The hex to convert.
 * @return int
 */
function hexToInteger (hex) {
    local result = 0;
    local shift = hex.len() * 4;

    // For each digit..
    for (local d = 0; d < hex.len(); d++) {
        local digit;

        // Convert from ASCII Hex to integer
        if (hex[d] >= 0x61)
            digit = hex[d] - 0x57;
        else if (hex[d] >= 0x41)
             digit = hex[d] - 0x37;
        else
             digit = hex[d] - 0x30;

        // Accumulate digit
        shift -= 4;
        result += digit << shift;
    }

    return result;
}