using System;
using System.Diagnostics.CodeAnalysis;

namespace NModbus.Utility
{
    /// <summary>
    ///     Possible options for DiscriminatedUnion type.
    /// </summary>
    public enum DiscriminatedUnionOption
    {
        /// <summary>
        ///     Option A.
        /// </summary>
        [SuppressMessage("Microsoft.Naming", "CA1704:IdentifiersShouldBeSpelledCorrectly", MessageId = "A")]
        A,

        /// <summary>
        ///     Option B.
        /// </summary>
        [SuppressMessage("Microsoft.Naming", "CA1704:IdentifiersShouldBeSpelledCorrectly", MessageId = "B")]
        B
    }

    /// <summary>
    ///     A data type that can store one of two possible strongly typed options.
    /// </summary>
    /// <typeparam name="TA">The type of option A.</typeparam>
    /// <typeparam name="TB">The type of option B.</typeparam>
    public class DiscriminatedUnion<TA, TB>
    {
        private TA? optionA;
        private TB? optionB;
        private DiscriminatedUnionOption option;

        /// <summary>
        ///     Gets the value of option A.
        /// </summary>
        [SuppressMessage("Microsoft.Naming", "CA1704:IdentifiersShouldBeSpelledCorrectly", MessageId = "A")]
        public TA? A
        {
            get
            {
                if (this.Option != DiscriminatedUnionOption.A)
                {
                    string msg = $"{DiscriminatedUnionOption.A} is not a valid option for this discriminated union instance.";
                    throw new InvalidOperationException(msg);
                }

                return this.optionA;
            }
        }

        /// <summary>
        ///     Gets the value of option B.
        /// </summary>
        [SuppressMessage("Microsoft.Naming", "CA1704:IdentifiersShouldBeSpelledCorrectly", MessageId = "B")]
        public TB? B
        {
            get
            {
                if (this.Option != DiscriminatedUnionOption.B)
                {
                    string msg = $"{DiscriminatedUnionOption.B} is not a valid option for this discriminated union instance.";
                    throw new InvalidOperationException(msg);
                }

                return this.optionB;
            }
        }

        /// <summary>
        ///     Gets the discriminated value option set for this instance.
        /// </summary>
        public DiscriminatedUnionOption Option => this.option;

        /// <summary>
        ///     Factory method for creating DiscriminatedUnion with option A set.
        /// </summary>
        [SuppressMessage("Microsoft.Design", "CA1000:DoNotDeclareStaticMembersOnGenericTypes", Justification = "Factory method.")]
        [SuppressMessage("Microsoft.Naming", "CA1704:IdentifiersShouldBeSpelledCorrectly", MessageId = "0#a")]
        public static DiscriminatedUnion<TA, TB> CreateA(TA a)
        {
            return new DiscriminatedUnion<TA, TB>() { option = DiscriminatedUnionOption.A, optionA = a };
        }

        /// <summary>
        ///     Factory method for creating DiscriminatedUnion with option B set.
        /// </summary>
        [SuppressMessage("Microsoft.Design", "CA1000:DoNotDeclareStaticMembersOnGenericTypes", Justification = "Factory method.")]
        [SuppressMessage("Microsoft.Naming", "CA1704:IdentifiersShouldBeSpelledCorrectly", MessageId = "0#b")]
        public static DiscriminatedUnion<TA, TB> CreateB(TB b)
        {
            return new DiscriminatedUnion<TA, TB>() { option = DiscriminatedUnionOption.B, optionB = b };
        }

        /// <summary>
        ///     Returns a <see cref="T:System.String" /> that represents the current <see cref="T:System.Object" />.
        /// </summary>
        /// <returns>
        ///     A <see cref="T:System.String" /> that represents the current <see cref="T:System.Object" />.
        /// </returns>
        public override string ToString()
        {
            string value = string.Empty;

            switch (Option)
            {
                case DiscriminatedUnionOption.A:
                    value = A?.ToString() ?? string.Empty;
                    break;
                case DiscriminatedUnionOption.B:
                    value = B?.ToString() ?? string.Empty;
                    break;
            }

            return value ?? string.Empty;
        }
    }
}