using System.Collections;
using DigitalData.Core.Exceptions;
using MediatR;

namespace EnvelopeGenerator.Application.Common.Extensions;

/// <summary>
/// Extension methods for <see cref="IMediator"/> that enforce non-null and non-empty responses.
/// </summary>
public static class MediatorExtensions
{
    /// <summary>
    /// Sends a request via MediatR and throws a custom exception produced by <paramref name="exceptionFactory"/>
    /// when the response is <c>null</c> or an empty collection.
    /// </summary>
    /// <typeparam name="TResponse">The expected response type.</typeparam>
    /// <typeparam name="TException">The exception type to throw.</typeparam>
    /// <param name="mediator">The mediator instance.</param>
    /// <param name="request">The MediatR request whose response may be <c>null</c>.</param>
    /// <param name="exceptionFactory">A factory that creates the exception to throw when the response is absent.</param>
    /// <param name="cancel">Cancellation token.</param>
    /// <returns>A guaranteed non-null <typeparamref name="TResponse"/>.</returns>
    /// <exception cref="Exception">The exception produced by <paramref name="exceptionFactory"/>.</exception>
    public static async Task<TResponse> SendOrThrowAsync<TResponse, TException>(this IMediator mediator, IRequest<TResponse?> request, Func<TException> exceptionFactory, CancellationToken cancel = default)
        where TException : Exception
    {
        if (await mediator.Send(request, cancel) is TResponse res)
        {
            if (res is not string && res is IEnumerable enumerable && !enumerable.Cast<object>().Any())
                throw exceptionFactory();

            return res;
        }

        throw exceptionFactory();
    }

    /// <summary>
    /// Sends a request via MediatR and throws <see cref="NotFoundException"/> when the response is <c>null</c> or an empty collection.
    /// </summary>
    /// <typeparam name="TResponse">The expected response type.</typeparam>
    /// <param name="mediator">The mediator instance.</param>
    /// <param name="request">The MediatR request whose response may be <c>null</c>.</param>
    /// <param name="exceptionMessage">Optional message for the <see cref="NotFoundException"/>.</param>
    /// <param name="cancel">Cancellation token.</param>
    /// <returns>A guaranteed non-null <typeparamref name="TResponse"/>.</returns>
    /// <exception cref="NotFoundException">Thrown when the response is <c>null</c> or an empty collection.</exception>
    public static async Task<TResponse> SendOrNotFoundAsync<TResponse>(this IMediator mediator, IRequest<TResponse?> request, string? exceptionMessage, CancellationToken cancel = default)
        => await mediator.SendOrThrowAsync(request, () => new NotFoundException(exceptionMessage ?? $"The requested resource of type {typeof(TResponse).Name} was not found."), cancel);

    /// <inheritdoc cref="SendOrNotFoundAsync{TResponse}(IMediator, IRequest{TResponse}, string, CancellationToken)"/>
    public static async Task<TResponse> SendOrNotFoundAsync<TResponse>(this IMediator mediator, IRequest<TResponse?> request, CancellationToken cancel = default)
        => await mediator.SendOrNotFoundAsync(request, null, cancel);
}