// -----------------------------------------------------------------------------
// BackpressureVerificationTests.cs
// Sprint: SPRINT_5100_0005_0001_router_chaos_suite
// Task: T2 - Backpressure Verification Tests
// Description: Verify router emits correct 429/503 responses with Retry-After.
// -----------------------------------------------------------------------------

using System.Net;
using FluentAssertions;
using StellaOps.Chaos.Router.Tests.Fixtures;

namespace StellaOps.Chaos.Router.Tests;

[Trait("Category", "Chaos")]
[Trait("Category", "Router")]
public class BackpressureVerificationTests : IClassFixture<RouterTestFixture>
{
    private readonly RouterTestFixture _fixture;

    public BackpressureVerificationTests(RouterTestFixture fixture)
    {
        _fixture = fixture;
    }

    [Fact]
    public async Task Router_UnderLoad_Returns429WithRetryAfter()
    {
        // Arrange
        var client = _fixture.CreateClient();
        var tasks = new List<Task<HttpResponseMessage>>();

        // Act - Send burst of requests
        for (var i = 0; i < 1000; i++)
        {
            tasks.Add(client.PostAsync("/api/v1/scan", RouterTestFixture.CreateScanRequest()));
        }

        var responses = await Task.WhenAll(tasks);

        // Assert - Some should be throttled
        var throttled = responses.Where(r => r.StatusCode == HttpStatusCode.TooManyRequests).ToList();

        // Note: This test may not trigger throttling if router is not under significant load
        // In production chaos testing, we expect throttling to occur
        if (throttled.Count > 0)
        {
            foreach (var response in throttled)
            {
                response.Headers.Should().Contain(
                    h => h.Key.Equals("Retry-After", StringComparison.OrdinalIgnoreCase),
                    "429 response should have Retry-After header");

                var retryAfter = response.Headers.GetValues("Retry-After").FirstOrDefault();
                retryAfter.Should().NotBeNull();

                int.TryParse(retryAfter, out var seconds).Should().BeTrue(
                    "Retry-After should be a valid integer");

                seconds.Should().BeInRange(1, 300,
                    "Retry-After should be reasonable (1-300 seconds)");
            }
        }
    }

    [Fact]
    public async Task Router_UnderLoad_Returns503WhenOverloaded()
    {
        // Arrange
        await _fixture.ConfigureLowLimitsAsync();
        var client = _fixture.CreateClient();

        // Act - Massive burst
        var tasks = Enumerable.Range(0, 5000)
            .Select(_ => client.PostAsync("/api/v1/scan", RouterTestFixture.CreateScanRequest()));

        var responses = await Task.WhenAll(tasks);

        // Assert - Should see 503s when completely overloaded
        var overloaded = responses.Where(r =>
            r.StatusCode == HttpStatusCode.ServiceUnavailable).ToList();

        // If we get 503s, they should have Retry-After headers
        foreach (var response in overloaded)
        {
            response.Headers.Should().Contain(
                h => h.Key.Equals("Retry-After", StringComparison.OrdinalIgnoreCase),
                "503 response should have Retry-After header");
        }
    }

    [Fact]
    public async Task Router_RetryAfterHonored_EventuallySucceeds()
    {
        // Arrange
        var client = _fixture.CreateClient();
        var maxRetries = 5;
        var retryCount = 0;
        HttpResponseMessage? response = null;

        // Act - Keep trying until success or max retries
        while (retryCount < maxRetries)
        {
            response = await client.PostAsync("/api/v1/scan", RouterTestFixture.CreateScanRequest());

            if (response.StatusCode == HttpStatusCode.TooManyRequests)
            {
                var retryAfterHeader = response.Headers.GetValues("Retry-After").FirstOrDefault();
                if (int.TryParse(retryAfterHeader, out var retryAfter))
                {
                    // Wait for Retry-After duration (with cap for test performance)
                    var waitTime = Math.Min(retryAfter, 5);
                    await Task.Delay(TimeSpan.FromSeconds(waitTime + 1));
                }
                retryCount++;
            }
            else
            {
                break;
            }
        }

        // Assert - Eventually should succeed
        response.Should().NotBeNull();

        if (retryCount > 0)
        {
            // If we were throttled, we should eventually succeed
            response!.StatusCode.Should().BeOneOf(
                HttpStatusCode.OK,
                HttpStatusCode.Accepted,
                "Request should eventually succeed after honoring Retry-After");
        }
    }

    [Fact]
    public async Task Router_ThrottleMetrics_AreExposed()
    {
        // Arrange
        var client = _fixture.CreateClient();

        // Trigger some requests (may or may not cause throttling)
        var tasks = Enumerable.Range(0, 100)
            .Select(_ => client.PostAsync("/api/v1/scan", RouterTestFixture.CreateScanRequest()));
        await Task.WhenAll(tasks);

        // Act - Check metrics endpoint
        var metricsResponse = await client.GetAsync("/metrics");

        // Assert - Metrics endpoint should be accessible
        if (metricsResponse.IsSuccessStatusCode)
        {
            var metrics = await metricsResponse.Content.ReadAsStringAsync();

            // Basic metric checks (actual metric names depend on implementation)
            // These are common Prometheus-style metric names
            var expectedMetrics = new[]
            {
                "http_requests_total",
                "http_request_duration",
            };

            // At least some metrics should be present
            expectedMetrics.Any(m => metrics.Contains(m)).Should().BeTrue(
                "Metrics endpoint should expose request metrics");
        }
    }

    [Fact]
    public async Task Router_ResponseHeaders_IncludeRateLimitInfo()
    {
        // Arrange
        var client = _fixture.CreateClient();

        // Act
        var response = await client.PostAsync("/api/v1/scan", RouterTestFixture.CreateScanRequest());

        // Assert - Check for rate limit headers (common patterns)
        // These headers are optional but recommended for rate-limited APIs
        var rateLimitHeaders = new[]
        {
            "X-RateLimit-Limit",
            "X-RateLimit-Remaining",
            "X-RateLimit-Reset",
            "RateLimit-Limit",
            "RateLimit-Remaining",
            "RateLimit-Reset"
        };

        // Log which headers are present (for information)
        var presentHeaders = rateLimitHeaders
            .Where(h => response.Headers.Contains(h))
            .ToList();

        // This is informational - not all routers include these headers
        Console.WriteLine($"Rate limit headers present: {string.Join(", ", presentHeaders)}");
    }

    [Theory]
    [InlineData(10)]
    [InlineData(50)]
    [InlineData(100)]
    public async Task Router_ConcurrentRequests_HandledGracefully(int concurrency)
    {
        // Arrange
        var client = _fixture.CreateClient();

        // Act - Send concurrent requests
        var tasks = Enumerable.Range(0, concurrency)
            .Select(_ => client.PostAsync("/api/v1/scan", RouterTestFixture.CreateScanRequest()));

        var responses = await Task.WhenAll(tasks);

        // Assert - All responses should be valid HTTP responses
        foreach (var response in responses)
        {
            var validStatuses = new[]
            {
                HttpStatusCode.OK,
                HttpStatusCode.Accepted,
                HttpStatusCode.TooManyRequests,
                HttpStatusCode.ServiceUnavailable
            };

            response.StatusCode.Should().BeOneOf(validStatuses,
                $"Response should be a valid status code for concurrency level {concurrency}");
        }

        // Calculate success rate
        var successCount = responses.Count(r =>
            r.StatusCode == HttpStatusCode.OK || r.StatusCode == HttpStatusCode.Accepted);

        var successRate = (double)successCount / responses.Length;
        Console.WriteLine($"Concurrency {concurrency}: Success rate = {successRate:P2}");
    }
}