using System.Security.Cryptography;
using System.Text;
using Grpc.Core;
using Grpc.Core.Interceptors;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using ZB.MOM.WW.OtOpcUa.Cluster;

namespace ZB.MOM.WW.OtOpcUa.Host.Configuration;

/// <summary>
///     Gates the driver-node telemetry stream endpoint (per-cluster mesh Phase 5). The
///     <c>TelemetryStreamService.Subscribe</c> RPC streams a node's live alerts / script-logs /
///     driver-health / driver-resilience-status to whoever dials it; anything able to reach a driver
///     node's telemetry port could otherwise pull that live feed out-of-band. This interceptor is the
///     ONLY inbound auth on that endpoint.
/// </summary>
/// <remarks>
///     <para>
///         <b>Scoped by method path.</b> Only calls under
///         <c>/telemetry.v1.TelemetryStreamService/</c> are gated; every other method on the shared
///         gRPC pipeline (config-serve, LocalDb sync) passes through untouched, so this can share one
///         <c>AddGrpc</c> registration with <see cref="ConfigServeAuthInterceptor"/> and
///         <see cref="LocalDbSyncAuthInterceptor"/>.
///     </para>
///     <para>
///         <b>Fail-closed.</b> With no <c>Telemetry:ApiKey</c> configured, NO dial is accepted,
///         authenticated or not. Treating "no key" as "no auth required" would expose the live
///         telemetry feed on exactly the default shape most nodes ship with. The caller dialing in
///         must present the same <c>TelemetryDial:ApiKey</c> (the shared node key), so a key typo
///         stops telemetry delivery outright rather than degrading to unauthenticated.
///     </para>
///     <para>
///         Comparison is <see cref="CryptographicOperations.FixedTimeEquals"/> over UTF-8 bytes, so a
///         wrong key cannot be recovered byte-by-byte from response timing. Length differences are
///         unavoidably observable and are not sensitive.
///     </para>
///     <para>
///         <b>Exactly one public constructor.</b> <c>Grpc.AspNetCore</c> silently stops invoking an
///         interceptor that has more than one public ctor — see
///         <c>TelemetryStreamAuthInterceptorTests.HasExactlyOnePublicConstructor</c>.
///     </para>
/// </remarks>
public sealed class TelemetryStreamAuthInterceptor : Interceptor
{
    private const string ServicePrefix = "/telemetry.v1.TelemetryStreamService/";
    private const string AuthorizationHeader = "authorization";
    private const string BearerPrefix = "Bearer ";

    private readonly IOptions<TelemetryOptions> _options;
    private readonly ILogger<TelemetryStreamAuthInterceptor> _logger;

    /// <summary>Creates the interceptor.</summary>
    /// <param name="options">Telemetry options; <c>ApiKey</c> is the expected bearer token.</param>
    /// <param name="logger">Logger for denial diagnostics.</param>
    public TelemetryStreamAuthInterceptor(
        IOptions<TelemetryOptions> options,
        ILogger<TelemetryStreamAuthInterceptor> logger)
    {
        ArgumentNullException.ThrowIfNull(options);
        ArgumentNullException.ThrowIfNull(logger);

        _options = options;
        _logger = logger;
    }

    /// <inheritdoc />
    public override Task<TResponse> UnaryServerHandler<TRequest, TResponse>(
        TRequest request,
        ServerCallContext context,
        UnaryServerMethod<TRequest, TResponse> continuation)
    {
        Authorize(context);
        return continuation(request, context);
    }

    /// <inheritdoc />
    public override Task DuplexStreamingServerHandler<TRequest, TResponse>(
        IAsyncStreamReader<TRequest> requestStream,
        IServerStreamWriter<TResponse> responseStream,
        ServerCallContext context,
        DuplexStreamingServerMethod<TRequest, TResponse> continuation)
    {
        Authorize(context);
        return continuation(requestStream, responseStream, context);
    }

    /// <inheritdoc />
    public override Task<TResponse> ClientStreamingServerHandler<TRequest, TResponse>(
        IAsyncStreamReader<TRequest> requestStream,
        ServerCallContext context,
        ClientStreamingServerMethod<TRequest, TResponse> continuation)
    {
        Authorize(context);
        return continuation(requestStream, context);
    }

    /// <inheritdoc />
    public override Task ServerStreamingServerHandler<TRequest, TResponse>(
        TRequest request,
        IServerStreamWriter<TResponse> responseStream,
        ServerCallContext context,
        ServerStreamingServerMethod<TRequest, TResponse> continuation)
    {
        Authorize(context);
        return continuation(request, responseStream, context);
    }

    /// <summary>
    ///     Throws <see cref="RpcException"/> with <see cref="StatusCode.PermissionDenied"/> if this is
    ///     a telemetry-stream call that does not carry the configured bearer token. Non-telemetry
    ///     calls return immediately.
    /// </summary>
    private void Authorize(ServerCallContext context)
    {
        if (!context.Method.StartsWith(ServicePrefix, StringComparison.Ordinal))
            return;

        var expected = _options.Value.ApiKey;
        // Whitespace-only counts as unconfigured: an all-blank expected key would otherwise be
        // satisfied by an all-blank bearer token, which is no gate at all.
        if (string.IsNullOrWhiteSpace(expected))
        {
            _logger.LogWarning(
                "Rejected a telemetry stream call to {Method}: no Telemetry:ApiKey is configured, so " +
                "the telemetry endpoint is closed. Configure the shared node key on this node and every " +
                "dialing caller.",
                context.Method);
            throw new RpcException(new Status(
                StatusCode.PermissionDenied,
                "Telemetry stream is not accepting connections: no API key is configured on this node."));
        }

        var presented = ExtractBearerToken(context.RequestHeaders);
        if (presented is null || !FixedTimeEquals(presented, expected))
        {
            _logger.LogWarning(
                "Rejected a telemetry stream call to {Method}: {Reason}.",
                context.Method,
                presented is null ? "no bearer token presented" : "bearer token did not match");
            throw new RpcException(new Status(
                StatusCode.PermissionDenied,
                "Telemetry stream authentication failed."));
        }
    }

    private static string? ExtractBearerToken(Metadata headers)
    {
        // gRPC lowercases header keys on the wire; compare case-insensitively anyway so a hand-built
        // Metadata in a test behaves the same as a real request.
        foreach (var entry in headers)
        {
            if (!string.Equals(entry.Key, AuthorizationHeader, StringComparison.OrdinalIgnoreCase))
                continue;

            var value = entry.Value;
            if (value is not null && value.StartsWith(BearerPrefix, StringComparison.OrdinalIgnoreCase))
                return value[BearerPrefix.Length..];
        }

        return null;
    }

    private static bool FixedTimeEquals(string presented, string expected)
        => CryptographicOperations.FixedTimeEquals(
            Encoding.UTF8.GetBytes(presented), Encoding.UTF8.GetBytes(expected));
}