// Copyright (c) 2007 David Pinch (www.thoughtproject.com/Snippets/Mod10/).
// Free for any use including commercial and closed source.
//
// Do not use this code on production systems until you have fully tested it.
// The MOD10 algorithm is commonly used in financial and e-commerce systems.
// You downloaded this code from the Internet.  Be careful, OK?

using System;

namespace Thought.Mathematics // change to your desired namespace
{

    public static class Mod10
    {

        /// <summary>
        ///     Validates a string of digits with the MOD10 algorithm.
        /// </summary>
        /// <returns>
        ///     True if digits pass the checksum test.
        /// </returns>
        public static bool Validate(string digits)
        {

            // Zero can be passed to minimumLength and maximumLength.
            // It instructs the function to ignore length checking.

            return Validate(digits, 0, 0);
        }


        /// <summary>
        ///     Validates a string of digits with the MOD10 algorithm.
        /// </summary>
        /// <param name="digits">
        ///     The string of digits to check.
        /// </param>
        /// <param name="length">
        ///     The expected length of the string.
        /// </param>
        /// <returns>
        ///     True if the string passes the checksum length and is also
        ///     exactly the specified length.
        /// </returns>
        public static bool Validate(string digits, int length)
        {
            return Validate(digits, length, length);
        }


        /// <summary>
        ///     Validates a string of digits with the MOD10 algorithm.
        /// </summary>
        /// <param name="digits">
        ///     The digits to check.
        /// </param>
        /// <param name="minimumLength">
        ///     The minimum allow length of the string.
        /// </param>
        /// <param name="maximumLength">
        ///     The maximum allowed length of the string.
        /// </param>
        /// <returns>
        ///     True if the string passes the checksum test and is also
        ///     between the minimum and maximum lengths, inclusively.
        /// </returns>
        public static bool Validate(
            string digits,
            int minimumLength,
            int maximumLength)
        {

            // Raise an exception if the minimum length is wrong.
            // No exceptions are raised if the digits parameter
            // is null or otherwise wrong, however, because the parameter
            // is based on user input.

            if (minimumLength < 0)
                throw new ArgumentOutOfRangeException("minimumLength");

            if ((maximumLength < 0) || (maximumLength < minimumLength))
                throw new ArgumentOutOfRangeException("maximumLength");

            // For simplicity in the rest of the code, just
            // convert a null string into an empty string.

            digits = digits == null ? string.Empty : digits;

            if (digits.Length == 0)
                return false;

            if (digits.Length < minimumLength)
                return false;

            if ((maximumLength > 0) && (digits.Length > maximumLength))
                return false;

            int parity = digits.Length % 2;
            int sum = 0;

            for (int index = 0; index < digits.Length; index++)
            {

                // First, make sure this character is a digit.
                // This algorithm requires a string to contain
                // all digits (such as credit card numbers).

                if (!char.IsDigit(digits[index]))
                    return false;

                int digit = Convert.ToInt32(char.GetNumericValue(digits[index]));

                if ((index % 2) == parity)
                {

                    digit *= 2;

                    if (digit > 9)
                    {
                        digit -= 9;
                    }

                }


                sum += digit;
            }

            return (sum % 10) == 0;

        }

    }

}