using FluentValidation;
using MediatR;

namespace Shared.Application.Behaviors;

/// <summary>
/// MediatR pipeline behavior that runs all registered FluentValidation validators for a
/// request before invoking its handler, short-circuiting with a <see cref="ValidationException"/>
/// when any validator reports failures.
/// </summary>
public class ValidationBehavior<TRequest, TResponse>(IEnumerable<IValidator<TRequest>> validators)
    : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    public async Task<TResponse> Handle(
        TRequest request,
        RequestHandlerDelegate<TResponse> next,
        CancellationToken cancellationToken
    )
    {
        if (!validators.Any())
        {
            return await next();
        }

        var context = new ValidationContext<TRequest>(request);

        var failures = (
            await Task.WhenAll(
                validators.Select(v => v.ValidateAsync(context, cancellationToken))
            )
        )
            .SelectMany(result => result.Errors)
            .Where(failure => failure != null)
            .ToList();

        if (failures.Count > 0)
        {
            throw new ValidationException(failures);
        }

        return await next();
    }
}