using System;
using System.Linq;

namespace Svrnty.CQRS.Events.Abstractions.Schema;

/// <summary>
/// Marks an event type with version information for schema evolution.
/// </summary>
/// <remarks>
/// <para>
/// This attribute enables automatic schema versioning and upcasting.
/// Use it to track event evolution over time and specify upcast relationships.
/// </para>
/// <para>
/// <strong>Example:</strong>
/// <code>
/// // Version 1 (initial)
/// [EventVersion(1)]
/// public record UserCreatedEventV1 : CorrelatedEvent
/// {
///     public string Name { get; init; }
/// }
///
/// // Version 2 (added Email property)
/// [EventVersion(2, UpcastFrom = typeof(UserCreatedEventV1))]
/// public record UserCreatedEventV2 : CorrelatedEvent
/// {
///     public string Name { get; init; }
///     public string Email { get; init; }
///
///     // Static upcaster method (convention-based)
///     public static UserCreatedEventV2 UpcastFrom(UserCreatedEventV1 v1)
///     {
///         return new UserCreatedEventV2
///         {
///             EventId = v1.EventId,
///             CorrelationId = v1.CorrelationId,
///             OccurredAt = v1.OccurredAt,
///             Name = v1.Name,
///             Email = "unknown@example.com" // Default value for new property
///         };
///     }
/// }
/// </code>
/// </para>
/// </remarks>
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = false, Inherited = false)]
public sealed class EventVersionAttribute : Attribute
{
    /// <summary>
    /// Gets the version number of this event schema.
    /// </summary>
    /// <remarks>
    /// Version numbers should start at 1 and increment sequentially.
    /// Version 1 represents the initial event schema.
    /// </remarks>
    public int Version { get; }

    /// <summary>
    /// Gets the type of the previous version this event can upcast from.
    /// </summary>
    /// <remarks>
    /// <para>
    /// Should be null for version 1 (initial version).
    /// For versions > 1, specify the immediate previous version.
    /// </para>
    /// <para>
    /// Multi-hop upcasting is automatic: V1 → V2 → V3
    /// You only need to specify the immediate previous version.
    /// </para>
    /// </remarks>
    public Type? UpcastFrom { get; init; }

    /// <summary>
    /// Gets or sets the event type name used for schema identification.
    /// </summary>
    /// <remarks>
    /// <para>
    /// If not specified, defaults to the class name without version suffix.
    /// Example: "UserCreatedEventV2" → "UserCreatedEvent"
    /// </para>
    /// <para>
    /// All versions of the same event should use the same EventTypeName.
    /// </para>
    /// </remarks>
    public string? EventTypeName { get; init; }

    /// <summary>
    /// Initializes a new instance of the <see cref="EventVersionAttribute"/> class.
    /// </summary>
    /// <param name="version">The version number (must be >= 1).</param>
    /// <exception cref="ArgumentOutOfRangeException">Thrown if version is less than 1.</exception>
    public EventVersionAttribute(int version)
    {
        if (version < 1)
            throw new ArgumentOutOfRangeException(nameof(version), "Version must be >= 1.");

        Version = version;
    }

    /// <summary>
    /// Gets the normalized event type name from a CLR type.
    /// </summary>
    /// <param name="eventType">The CLR type of the event.</param>
    /// <returns>The normalized event type name (without version suffix).</returns>
    /// <remarks>
    /// Removes common version suffixes: V1, V2, V3, etc.
    /// Example: "UserCreatedEventV2" → "UserCreatedEvent"
    /// </remarks>
    public static string GetEventTypeName(Type eventType)
    {
        var attribute = eventType.GetCustomAttributes(typeof(EventVersionAttribute), false)
            .FirstOrDefault() as EventVersionAttribute;

        if (attribute?.EventTypeName != null)
            return attribute.EventTypeName;

        // Remove version suffix (V1, V2, etc.) from type name
        var typeName = eventType.Name;
        var versionSuffixIndex = typeName.LastIndexOf('V');

        if (versionSuffixIndex > 0 && versionSuffixIndex < typeName.Length - 1)
        {
            var suffix = typeName.Substring(versionSuffixIndex + 1);
            if (int.TryParse(suffix, out _))
            {
                return typeName.Substring(0, versionSuffixIndex);
            }
        }

        return typeName;
    }
}