using System.Security.Claims;
using System.Threading.RateLimiting;
using Asp.Versioning;
using FluentValidation.AspNetCore;
using InboxIntel.Api.Auth;
using InboxIntel.Application;
using InboxIntel.Application.Abstractions;
using InboxIntel.Infrastructure;
using InboxIntel.Infrastructure.Configuration;
using InboxIntel.Infrastructure.Persistence;
using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Authentication.Google;
using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.DataProtection;
using Microsoft.AspNetCore.HttpOverrides;
using Microsoft.AspNetCore.RateLimiting;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Options;
using Npgsql;
using OpenTelemetry.Metrics;
using OpenTelemetry.Resources;
using OpenTelemetry.Trace;
using Serilog;

var builder = WebApplication.CreateBuilder(args);

// Structured logging (Serilog). Note: token values are never logged.
builder.Host.UseSerilog((ctx, cfg) => cfg
    .ReadFrom.Configuration(ctx.Configuration)
    .Enrich.FromLogContext()
    .WriteTo.Console());

// Persist Data Protection keys so encrypted refresh tokens survive restarts.
// AUDIT M-3: optionally encrypt the Data Protection key ring with an X.509 certificate so
// the keys are not readable in plaintext from the /keys volume (which would otherwise let
// anyone with volume access decrypt all stored refresh tokens). Configure
// DataProtection:CertificatePath (+ CertificatePassword) to enable; without it, keys are
// persisted unprotected and a startup warning documents the residual risk.
var dp = builder.Services.AddDataProtection()
    .PersistKeysToFileSystem(new DirectoryInfo(builder.Configuration["DataProtection:KeyPath"] ?? "/keys"))
    .SetApplicationName("InboxIntel");
var dpCertPath = builder.Configuration["DataProtection:CertificatePath"];
if (!string.IsNullOrWhiteSpace(dpCertPath))
{
    dp.ProtectKeysWithCertificate(System.Security.Cryptography.X509Certificates.X509CertificateLoader
        .LoadPkcs12FromFile(dpCertPath, builder.Configuration["DataProtection:CertificatePassword"]));
}

builder.Services.AddApplication();
builder.Services.AddInfrastructure(builder.Configuration);

builder.Services.AddHttpContextAccessor();
builder.Services.AddScoped<ICurrentUser, CurrentUser>();

// Authentication: cookie session established via Google OAuth2 (only login method).
var google = builder.Configuration.GetSection(GoogleOAuthOptions.SectionName).Get<GoogleOAuthOptions>() ?? new();
builder.Services.AddAuthentication(options =>
    {
        options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
        // Challenge via the cookie scheme so unauthenticated API (XHR) calls get a
        // 401 instead of a redirect to Google. The SPA's axios interceptor turns
        // that 401 into a top-level navigation to /auth/login, which then starts
        // the Google flow explicitly. (A 302 to Google on an XHR is CORS-blocked.)
        options.DefaultChallengeScheme = CookieAuthenticationDefaults.AuthenticationScheme;
    })
    .AddCookie(options =>
    {
        options.Cookie.HttpOnly = true;
        options.Cookie.SameSite = SameSiteMode.Lax;
        // V-04: never emit the session cookie over plain HTTP in non-dev. Behind nginx
        // the forwarded proto (now only trusted from known proxies, see below) makes
        // Always work; local http://localhost dev still functions via SameAsRequest.
        options.Cookie.SecurePolicy = builder.Environment.IsDevelopment()
            ? CookieSecurePolicy.SameAsRequest
            : CookieSecurePolicy.Always;
        options.Cookie.Name = "inboxintel.session";
        options.ExpireTimeSpan = TimeSpan.FromDays(7);
        options.SlidingExpiration = true;
        // AUDIT M-1: sliding expiration alone lets a stolen cookie renew forever. Stamp an
        // absolute start at sign-in and reject principals older than the configured cap,
        // forcing a full re-login. (Pre-existing sessions without the stamp are rejected
        // once — a single re-login, then they carry the stamp.)
        var absoluteDays = builder.Configuration.GetValue("Auth:AbsoluteSessionDays", 30);
        options.Events.OnSigningIn = ctx =>
        {
            ctx.Properties.SetString("abs-start", DateTimeOffset.UtcNow.ToString("O"));
            return Task.CompletedTask;
        };
        options.Events.OnValidatePrincipal = async ctx =>
        {
            if (SessionLifetime.IsExpired(ctx.Properties.GetString("abs-start"), DateTimeOffset.UtcNow, TimeSpan.FromDays(absoluteDays)))
            {
                ctx.RejectPrincipal();
                await ctx.HttpContext.SignOutAsync(CookieAuthenticationDefaults.AuthenticationScheme);
            }
        };
        // API-style behaviour: return status codes rather than redirecting to a login page.
        options.Events.OnRedirectToLogin = ctx =>
        {
            ctx.Response.StatusCode = StatusCodes.Status401Unauthorized;
            return Task.CompletedTask;
        };
        options.Events.OnRedirectToAccessDenied = ctx =>
        {
            ctx.Response.StatusCode = StatusCodes.Status403Forbidden;
            return Task.CompletedTask;
        };
    })
    .AddGoogle(options =>
    {
        options.ClientId = google.ClientId;
        options.ClientSecret = google.ClientSecret;
        options.AccessType = "offline";       // request a refresh token
        options.SaveTokens = true;
        foreach (var scope in google.Scopes) options.Scope.Add(scope);
        options.Events.OnCreatingTicket = GoogleAuthEvents.OnCreatingTicketAsync;
        // Force the consent screen so Google ALWAYS returns a refresh token.
        // Without this, Google omits the refresh token on re-authorisation,
        // leaving offline Gmail sync with no usable credential.
        options.Events.OnRedirectToAuthorizationEndpoint = context =>
        {
            context.Response.Redirect(context.RedirectUri + "&prompt=consent");
            return Task.CompletedTask;
        };
    });

builder.Services.AddAuthorization();

// RECOMMENDATIONS #5: OpenTelemetry traces + metrics (ASP.NET, outbound HTTP, Npgsql).
// The OTLP exporter only activates when Otel:Endpoint (or the standard
// OTEL_EXPORTER_OTLP_ENDPOINT env var) is configured — zero overhead otherwise.
// Logs stay on Serilog. Pair with the compose "observability" profile (grafana/otel-lgtm).
var otlpEndpoint = builder.Configuration["Otel:Endpoint"]
    ?? Environment.GetEnvironmentVariable("OTEL_EXPORTER_OTLP_ENDPOINT");
if (!string.IsNullOrWhiteSpace(otlpEndpoint))
{
    builder.Services.AddOpenTelemetry()
        .ConfigureResource(r => r.AddService("inboxintel-api"))
        .WithTracing(t => t
            .AddAspNetCoreInstrumentation()
            .AddHttpClientInstrumentation()
            .AddNpgsql()
            .AddOtlpExporter(o => o.Endpoint = new Uri(otlpEndpoint)))
        .WithMetrics(m => m
            .AddAspNetCoreInstrumentation()
            .AddHttpClientInstrumentation()
            .AddNpgsqlInstrumentation()
            .AddOtlpExporter(o => o.Endpoint = new Uri(otlpEndpoint)));
}

builder.Services.AddApiVersioning(o =>
{
    o.DefaultApiVersion = new ApiVersion(1, 0);
    o.AssumeDefaultVersionWhenUnspecified = true;
    o.ReportApiVersions = true;
    o.ApiVersionReader = new UrlSegmentApiVersionReader();
}).AddApiExplorer(o => { o.GroupNameFormat = "'v'VVV"; o.SubstituteApiVersionInUrl = true; });

builder.Services.AddControllers();
// AUDIT H-1: the validators in InboxIntel.Application/Validation were registered but never
// executed (FluentValidation 11.x needs explicit auto-validation). This wires them into
// model binding so invalid DTOs 400 at the boundary instead of reaching services.
builder.Services.AddFluentValidationAutoValidation();

// AUDIT H-2: rate limiting. Global per-user (or per-IP when anonymous) window, plus stricter
// named policies for auth and expensive endpoints (export/unsubscribe/AI). Limits are
// config-driven so tests and deployments can tune them.
var rl = builder.Configuration.GetSection("RateLimiting");
int Limit(string key, int def) => rl.GetValue(key, def);
var rlWindow = TimeSpan.FromSeconds(Limit("WindowSeconds", 60));
static string Partition(HttpContext ctx) =>
    ctx.User.Identity?.IsAuthenticated == true
        ? ctx.User.FindFirstValue("inboxintel:uid") ?? "auth-unknown"
        : ctx.Connection.RemoteIpAddress?.ToString() ?? "anon";
builder.Services.AddRateLimiter(o =>
{
    o.RejectionStatusCode = StatusCodes.Status429TooManyRequests;
    o.GlobalLimiter = PartitionedRateLimiter.Create<HttpContext, string>(ctx =>
        RateLimitPartition.GetFixedWindowLimiter(Partition(ctx), _ =>
            new FixedWindowRateLimiterOptions { PermitLimit = Limit("GlobalPermitLimit", 300), Window = rlWindow, QueueLimit = 0 }));
    o.AddPolicy("auth", ctx =>
        RateLimitPartition.GetFixedWindowLimiter(Partition(ctx), _ =>
            new FixedWindowRateLimiterOptions { PermitLimit = Limit("AuthPermitLimit", 10), Window = rlWindow, QueueLimit = 0 }));
    o.AddPolicy("expensive", ctx =>
        RateLimitPartition.GetFixedWindowLimiter(Partition(ctx), _ =>
            new FixedWindowRateLimiterOptions { PermitLimit = Limit("ExpensivePermitLimit", 20), Window = rlWindow, QueueLimit = 0 }));
});

builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
// V-06: RFC7807 ProblemDetails so the global exception handler returns a safe,
// generic error body instead of leaking framework stack traces / internal messages.
builder.Services.AddProblemDetails();

builder.Services.AddCors(o => o.AddPolicy("frontend", p => p
    .WithOrigins(builder.Configuration.GetSection("Cors:Origins").Get<string[]>() ?? new[] { "http://localhost:5173" })
    .AllowAnyHeader().AllowAnyMethod().AllowCredentials()));

var app = builder.Build();

// Apply migrations on startup so `docker-compose up` yields a ready schema.
using (var scope = app.Services.CreateScope())
{
    var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
    if (db.Database.IsRelational() && app.Configuration.GetValue("Database:AutoMigrate", true))
    {
        // AUDIT M-4: the shipped appsettings no longer carries a guessable default DB
        // password. Fail fast with a clear message rather than connecting with weak or
        // missing credentials (compose/staging/prod inject the full connection string).
        var connStr = app.Configuration.GetConnectionString("Postgres") ?? string.Empty;
        var csb = new Npgsql.NpgsqlConnectionStringBuilder(connStr);
        if (string.IsNullOrWhiteSpace(csb.Password))
            throw new InvalidOperationException(
                "ConnectionStrings:Postgres has no password. Set the full connection string via " +
                "environment/user-secrets (see README) — a default password is deliberately not shipped.");
        await db.Database.MigrateAsync();
    }
}

// Honor X-Forwarded-* from the nginx reverse proxy so OAuth redirect URIs and
// cookie Secure flags reflect the external scheme/host, not the container's.
// V-08: only trust these headers from KNOWN proxy networks (configurable). The
// default covers private/Docker ranges so the compose nginx works, while a client
// reaching the API directly can no longer spoof scheme/host/forwarded-for.
var forwardedOptions = new ForwardedHeadersOptions
{
    ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto | ForwardedHeaders.XForwardedHost,
    ForwardLimit = app.Configuration.GetValue<int?>("ForwardedHeaders:ForwardLimit") ?? 1
};
forwardedOptions.KnownIPNetworks.Clear();
forwardedOptions.KnownProxies.Clear();
var trustedNetworks = app.Configuration.GetSection("ForwardedHeaders:KnownNetworks").Get<string[]>()
    ?? new[] { "10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16", "127.0.0.0/8", "::1/128" };
foreach (var cidr in trustedNetworks)
{
    if (System.Net.IPNetwork.TryParse(cidr, out var network))
        forwardedOptions.KnownIPNetworks.Add(network);
}
app.UseForwardedHeaders(forwardedOptions);

if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}
else
{
    // V-06: generic ProblemDetails for unhandled exceptions (no stack traces to clients).
    app.UseExceptionHandler();
    // V-11: HSTS once TLS is enforced at the proxy (forwarded proto now trustworthy).
    app.UseHsts();
}

// V-11: baseline security response headers. CSP is report-only for now so it can be
// tuned against the SPA before enforcing (the SPA itself is also served with headers
// by its nginx). Applied to API responses here as defense-in-depth.
app.Use(async (ctx, next) =>
{
    var h = ctx.Response.Headers;
    h["X-Content-Type-Options"] = "nosniff";
    h["X-Frame-Options"] = "DENY";
    h["Referrer-Policy"] = "no-referrer";
    h["Cross-Origin-Opener-Policy"] = "same-origin";
    await next();
});

app.UseSerilogRequestLogging();
app.UseCors("frontend");
app.UseAuthentication();
// AUDIT H-2: after authentication so authenticated traffic partitions per-user; anonymous
// traffic partitions per-IP. Endpoint policies ("auth", "expensive") apply via attributes.
app.UseRateLimiter();
app.UseAuthorization();
app.MapControllers();

app.Run();

public partial class Program { }

/// <summary>
/// AUDIT M-1: absolute session lifetime check, extracted for unit testing. A session with no
/// issued stamp (pre-dating this feature) is treated as expired — one forced re-login.
/// </summary>
public static class SessionLifetime
{
    public static bool IsExpired(string? issuedAtIso, DateTimeOffset now, TimeSpan maxAge)
        => !DateTimeOffset.TryParse(issuedAtIso, out var issued) || now - issued > maxAge;
}