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 central config-serve artifact endpoint (per-cluster mesh Phase 3). The
///     <c>DeploymentArtifactService</c> streams a driver node the exact configuration it will boot
///     on; anything able to reach central's serve port could otherwise pull the whole deployed config
///     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>/deployment_artifact.v1.DeploymentArtifactService/</c> are gated; every other method on
///         the shared gRPC pipeline (notably the LocalDb sync service) passes through untouched, so
///         this can share one <c>AddGrpc</c> registration with <see cref="LocalDbSyncAuthInterceptor"/>.
///     </para>
///     <para>
///         <b>Fail-closed.</b> With no <c>ConfigServe:ApiKey</c> configured, NO fetch is accepted,
///         authenticated or not. Treating "no key" as "no auth required" would expose the deployed
///         configuration on exactly the default shape most nodes ship with. The node dialing in must
///         present the same <c>ConfigSource:ApiKey</c> (the shared node key), so a key typo stops
///         config 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>
/// </remarks>
public sealed class ConfigServeAuthInterceptor : Interceptor
{
    private const string ServicePrefix = "/deployment_artifact.v1.DeploymentArtifactService/";
    private const string AuthorizationHeader = "authorization";
    private const string BearerPrefix = "Bearer ";

    private readonly IOptions<ConfigServeOptions> _options;
    private readonly ILogger<ConfigServeAuthInterceptor> _logger;

    /// <summary>Creates the interceptor.</summary>
    /// <param name="options">Config-serve options; <c>ApiKey</c> is the expected bearer token.</param>
    /// <param name="logger">Logger for denial diagnostics.</param>
    public ConfigServeAuthInterceptor(
        IOptions<ConfigServeOptions> options,
        ILogger<ConfigServeAuthInterceptor> 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.Unauthenticated"/> if this is
    ///     a config-serve call that does not carry the configured bearer token. Non-serve calls return
    ///     immediately.
    /// </summary>
    private void Authorize(ServerCallContext context)
    {
        if (!context.Method.StartsWith(ServicePrefix, StringComparison.Ordinal))
            return;

        var expected = _options.Value.ApiKey;
        if (string.IsNullOrEmpty(expected))
        {
            _logger.LogWarning(
                "Rejected a config-serve call to {Method}: no ConfigServe:ApiKey is configured, so the " +
                "artifact endpoint is closed. Configure the shared node key on central and every node.",
                context.Method);
            throw new RpcException(new Status(
                StatusCode.Unauthenticated,
                "Config-serve 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 config-serve call to {Method}: {Reason}.",
                context.Method,
                presented is null ? "no bearer token presented" : "bearer token did not match");
            throw new RpcException(new Status(
                StatusCode.Unauthenticated,
                "Config-serve 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));
}