EnvelopeGenerator/EnvelopeGenerator.Application/Services/EnvelopeGeneratorExtensions.cs

namespace EnvelopeGenerator.Application.Services
{
    /// <summary>
    /// Provides extension methods for decoding and extracting information from an envelope receiver ID.
    /// </summary>
    public static class EnvelopeGeneratorExtensions
    {
        /// <summary>
        /// Validates whether a given string is a correctly formatted Base-64 encoded string.
        /// </summary>
        /// <remarks>
        /// This method checks the string for proper Base-64 formatting, which includes validating
        /// the length of the string (must be divisible by 4). It also checks each character to ensure
        /// it belongs to the Base-64 character set (A-Z, a-z, 0-9, '+', '/', and '=' for padding).
        /// The method ensures that padding characters ('=') only appear at the end of the string and
        /// are in a valid configuration (either one '=' at the end if the string's length % 4 is 3,
        /// or two '==' if the length % 4 is 2).
        /// </remarks>
        /// <param name="input">The Base-64 encoded string to validate.</param>
        /// <returns>
        /// <c>true</c> if the string is a valid Base-64 encoded string; otherwise, <c>false</c>.
        /// </returns>
        /// <example>
        /// <code>
        /// string testString = "TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlzIHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2YgdGhlIG1pbmQsIHRoYXQgYnk=";
        /// bool isValid = IsValidBase64String(testString);
        /// Console.WriteLine(isValid); // Output: true
        /// </code>
        /// </example>
        public static bool IsBase64String(this string input)
        {
            // Check if the string is null or empty
            if (string.IsNullOrEmpty(input))
            {
                return false;
            }

            // Replace valid base-64 padding
            input = input.Trim();
            int mod4 = input.Length % 4;
            if (mod4 > 0)
            {
                // Base-64 string lengths should be divisible by 4
                return false;
            }

            // Check each character to ensure it is valid base-64
            foreach (char c in input)
            {
                if (!char.IsLetterOrDigit(c) && c != '+' && c != '/' && c != '=')
                {
                    // Invalid character detected
                    return false;
                }
            }

            // Ensure no invalid padding scenarios exist
            if (input.EndsWith("==") && (input.Length % 4 == 0) ||
                input.EndsWith("=") && (input.Length % 4 == 3))
            {
                return true;
            }

            return input.IndexOf('=') == -1; // No padding allowed except at the end
        }

        /// <summary>
        /// Decodes the envelope receiver ID and extracts the envelope UUID and receiver signature.
        /// </summary>
        /// <param name="envelopeReceiverId">The base64 encoded string containing the envelope UUID and receiver signature.</param>
        /// <returns>A tuple containing the envelope UUID and receiver signature.</returns>
        public static (string? EnvelopeUuid, string? ReceiverSignature) DecodeEnvelopeReceiverId(this string envelopeReceiverId)
        {
            if (!envelopeReceiverId.IsBase64String())
            {
                return (null, null);
            }
            byte[] bytes = Convert.FromBase64String(envelopeReceiverId);
            string decodedString = System.Text.Encoding.UTF8.GetString(bytes);
            string[] parts = decodedString.Split(new string[] { "::" }, StringSplitOptions.None);

            if (parts.Length > 1)
                return (EnvelopeUuid: parts[0], ReceiverSignature: parts[1]);
            else
                return (string.Empty, string.Empty);
        }

        /// <summary>
        /// Gets the envelope UUID from the decoded envelope receiver ID.
        /// </summary>
        /// <param name="envelopeReceiverId">The base64 encoded string to decode.</param>
        /// <returns>The envelope UUID.</returns>
        public static string GetEnvelopeUuid(this string envelopeReceiverId) => envelopeReceiverId.DecodeEnvelopeReceiverId().EnvelopeUuid;

        /// <summary>
        /// Gets the receiver signature from the decoded envelope receiver ID.
        /// </summary>
        /// <param name="envelopeReceiverId">The base64 encoded string to decode.</param>
        /// <returns>The receiver signature.</returns>
        public static string GetReceiverSignature(this string envelopeReceiverId) => envelopeReceiverId.DecodeEnvelopeReceiverId().ReceiverSignature;
    }
}