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 node-side command endpoint (ClusterClient→gRPC migration). The
///     <c>NodeCommandService.Execute</c> RPC is how central drives this node — restart a driver,
///     acknowledge an alarm, dispatch a deployment — so anything able to reach a node's command port
///     could otherwise operate the node. This interceptor is the ONLY inbound auth on that endpoint.
/// </summary>
/// <remarks>
///     <para>
///         <b>Scoped by method path.</b> Only calls under
///         <c>/mesh_transport.v1.NodeCommandService/</c> are gated; every other method on the shared
///         gRPC pipeline (LocalDb sync, config-serve, telemetry) passes through untouched, so this can
///         share one <c>AddGrpc</c> registration with <see cref="LocalDbSyncAuthInterceptor"/>,
///         <see cref="ConfigServeAuthInterceptor"/> and <see cref="TelemetryStreamAuthInterceptor"/>.
///     </para>
///     <para>
///         <b>Fail-closed.</b> With no <c>MeshTransport:GrpcPsk</c> configured, NO command is
///         accepted, authenticated or not. Treating "no key" as "no auth required" would expose the
///         command plane on exactly the default shape every node ships with — and unlike the three
///         sibling endpoints, this one is a write surface, not a read one. Central must present the
///         same per-cluster key, so a key typo stops commands outright rather than degrading to
///         unauthenticated.
///     </para>
///     <para>
///         <b>No <c>x-otopcua-cluster</c> requirement here, deliberately.</b> That header is a
///         central-side routing hint — the caller's own claim about which cluster it is addressing.
///         Requiring a node to check it would gate on a value the caller chooses, which proves
///         nothing the key has not already proven; the key IS the cluster boundary, so a caller
///         holding another cluster's PSK is rejected by the comparison below.
///     </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>MeshCommandAuthInterceptorTests.HasExactlyOnePublicConstructor</c>.
///     </para>
/// </remarks>
public sealed class MeshCommandAuthInterceptor : Interceptor
{
    private const string ServicePrefix = "/mesh_transport.v1.NodeCommandService/";
    private const string AuthorizationHeader = "authorization";
    private const string BearerPrefix = "Bearer ";

    private readonly IOptions<MeshTransportOptions> _options;
    private readonly ILogger<MeshCommandAuthInterceptor> _logger;

    /// <summary>Creates the interceptor.</summary>
    /// <param name="options">Mesh transport options; <c>GrpcPsk</c> is the expected bearer token.</param>
    /// <param name="logger">Logger for denial diagnostics.</param>
    public MeshCommandAuthInterceptor(
        IOptions<MeshTransportOptions> options,
        ILogger<MeshCommandAuthInterceptor> 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 node-command call that does not carry the configured bearer token. Non-command calls
    ///     return immediately.
    /// </summary>
    private void Authorize(ServerCallContext context)
    {
        if (!context.Method.StartsWith(ServicePrefix, StringComparison.Ordinal))
            return;

        var expected = _options.Value.GrpcPsk;
        if (string.IsNullOrWhiteSpace(expected))
        {
            _logger.LogWarning(
                "Rejected a mesh command call to {Method}: no MeshTransport:GrpcPsk is configured, so " +
                "the node command endpoint is closed. Set this node's per-cluster pre-shared key " +
                "(MeshTransport__GrpcPsk), ideally as a ${{secret:OT-MESH-PSK-<clusterId>}} reference, " +
                "and give central the same key for this cluster.",
                context.Method);
            throw new RpcException(new Status(
                StatusCode.PermissionDenied,
                "Node command service is not accepting calls: no pre-shared key is configured on this node."));
        }

        var presented = ExtractBearerToken(context.RequestHeaders);
        if (presented is null || !FixedTimeEquals(presented, expected))
        {
            _logger.LogWarning(
                "Rejected a mesh command 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,
                "Node command 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));
}