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using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;

namespace Opc.Ua.Gds.Server
{
    /// <summary>
    /// Represents the state of a key-credential request.
    /// </summary>
    public enum KeyCredentialRequestState
    {
        /// <summary>Request is new / pending approval.</summary>
        New,

        /// <summary>Request has been approved.</summary>
        Approved,

        /// <summary>Request has been rejected.</summary>
        Rejected,

        /// <summary>Request has been completed.</summary>
        Completed
    }

    /// <summary>
    /// Represents a single key-credential request record.
    /// </summary>
    public sealed class KeyCredentialRequestRecord
    {
        /// <summary>The unique request identifier.</summary>
        public NodeId RequestId { get; set; }

        /// <summary>The ApplicationUri that requested the credential.</summary>
        public string? ApplicationUri { get; set; }

        /// <summary>The public key supplied by the requester.</summary>
        public ByteString PublicKey { get; set; }

        /// <summary>The security policy URI for the credential.</summary>
        public string? SecurityPolicyUri { get; set; }

        /// <summary>The roles requested by the caller.</summary>
        public ArrayOf<NodeId> RequestedRoles { get; set; }

        /// <summary>Current state of the request.</summary>
        public KeyCredentialRequestState State { get; set; }

        /// <summary>The credential identifier (populated after approval).</summary>
        public string? CredentialId { get; set; }

        /// <summary>The credential secret (populated after approval).</summary>
        public ByteString CredentialSecret { get; set; }

        /// <summary>Thumbprint of the certificate that protects the credential.</summary>
        public string? CertificateThumbprint { get; set; }

        /// <summary>The security policy URI actually assigned.</summary>
        public string? GrantedSecurityPolicyUri { get; set; }

        /// <summary>The roles actually granted.</summary>
        public ArrayOf<NodeId> GrantedRoles { get; set; }

        /// <summary>When the request was created.</summary>
        public DateTime CreatedAt { get; set; }
    }

    /// <summary>
    /// Result of <see cref="IKeyCredentialRequestStore.FinishRequestAsync"/>.
    /// </summary>
    public readonly record struct FinishKeyCredentialRequestResult
    {
        /// <summary>The state of the request.</summary>
        public KeyCredentialRequestState State { get; init; }

        /// <summary>The issued credential id, or <c>null</c> when cancelled.</summary>
        public string? CredentialId { get; init; }

        /// <summary>The issued credential secret, or <c>default</c> when cancelled.</summary>
        public ByteString CredentialSecret { get; init; }

        /// <summary>The thumbprint of the protecting certificate.</summary>
        public string? CertificateThumbprint { get; init; }

        /// <summary>The assigned security policy URI.</summary>
        public string? SecurityPolicyUri { get; init; }

        /// <summary>The granted roles.</summary>
        public ArrayOf<NodeId> GrantedRoles { get; init; }
    }

    /// <summary>
    /// An abstract interface to the key-credential request store.
    /// Mirrors <see cref="ICertificateRequest"/> but for OPC 10000-12
    /// §8 KeyCredentialService requests.
    /// </summary>
    public interface IKeyCredentialRequestStore
    {
        /// <summary>
        /// Start a new key-credential request.
        /// </summary>
        ValueTask<NodeId> StartRequestAsync(
            string applicationUri,
            ByteString publicKey,
            string? securityPolicyUri,
            ArrayOf<NodeId> requestedRoles,
            CancellationToken cancellationToken = default);

        /// <summary>
        /// Finish (approve or cancel) a key-credential request.
        /// </summary>
        /// <param name="requestId">The request to finish.</param>
        /// <param name="cancelRequest">
        /// When <c>true</c> the request is cancelled; when <c>false</c>
        /// the credential is issued.
        /// </param>
        /// <param name="cancellationToken">The cancellation token.</param>
        /// <returns>The state of the request and any issued credential data.</returns>
        ValueTask<FinishKeyCredentialRequestResult> FinishRequestAsync(
            NodeId requestId,
            bool cancelRequest,
            CancellationToken cancellationToken = default);

        /// <summary>
        /// Revoke a previously issued credential.
        /// </summary>
        /// <param name="credentialId">The credential to revoke.</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        ValueTask RevokeAsync(string credentialId, CancellationToken cancellationToken = default);
    }

    /// <summary>
    /// In-memory implementation of <see cref="IKeyCredentialRequestStore"/>.
    /// Suitable for testing and single-process GDS deployments.
    /// </summary>
    /// <remarks>
    /// When an <see cref="ISecretStore"/> is supplied, credential secrets
    /// are persisted through the store rather than held in-process. This
    /// allows a production deployment to plug in Key Vault, DPAPI,
    /// Kubernetes secrets, or any other backend that implements
    /// <see cref="ISecretStore"/> without changing the GDS code.
    /// </remarks>
    public sealed class InMemoryKeyCredentialRequestStore : IKeyCredentialRequestStore
    {
        private int m_nextId;
        private readonly ConcurrentDictionary<NodeId, KeyCredentialRequestRecord> m_requests = new();
        private readonly ConcurrentDictionary<string, KeyCredentialRequestRecord> m_credentials = new(StringComparer.Ordinal);
        private readonly ISecretStore m_secretStore;

        /// <summary>
        /// Creates a store that holds credential secrets in-process.
        /// </summary>
        public InMemoryKeyCredentialRequestStore()
            : this(new InMemorySecretStore("KeyCredential"))
        {
        }

        /// <summary>
        /// Creates a store that delegates credential-secret persistence
        /// to the supplied <paramref name="secretStore"/>.
        /// </summary>
        /// <param name="secretStore">
        /// The secret store used to persist, materialise and purge
        /// credential secrets. Pass an <see cref="InMemorySecretStore"/>
        /// for testing or a Key Vault / DPAPI store for production.
        /// </param>
        public InMemoryKeyCredentialRequestStore(ISecretStore secretStore)
        {
            m_secretStore = secretStore ?? throw new ArgumentNullException(nameof(secretStore));
        }

        /// <inheritdoc/>
        public async ValueTask<NodeId> StartRequestAsync(
            string applicationUri,
            ByteString publicKey,
            string? securityPolicyUri,
            ArrayOf<NodeId> requestedRoles,
            CancellationToken cancellationToken = default)
        {
            int id = Interlocked.Increment(ref m_nextId);
            var requestId = new NodeId((uint)id);
            string credentialId = $"KC-{id}";

            // generate and persist the credential secret via ISecretStore
            byte[] secretBytes = GenerateRandomBytes(32);
            var secretId = new SecretIdentifier(credentialId, m_secretStore.StoreType);
            await m_secretStore.SetAsync(secretId, secretBytes, cancellationToken).ConfigureAwait(false);

            var record = new KeyCredentialRequestRecord
            {
                RequestId = requestId,
                ApplicationUri = applicationUri,
                PublicKey = publicKey,
                SecurityPolicyUri = securityPolicyUri,
                RequestedRoles = requestedRoles,
                State = KeyCredentialRequestState.Approved,
                CreatedAt = DateTime.UtcNow,
                CredentialId = credentialId,
                CredentialSecret = ByteString.From(secretBytes),
                CertificateThumbprint = null,
                GrantedSecurityPolicyUri = securityPolicyUri,
                GrantedRoles = requestedRoles
            };

            m_requests[requestId] = record;
            m_credentials[record.CredentialId] = record;
            return requestId;
        }

        /// <inheritdoc/>
        public async ValueTask<FinishKeyCredentialRequestResult> FinishRequestAsync(
            NodeId requestId,
            bool cancelRequest,
            CancellationToken cancellationToken = default)
        {
            if (!m_requests.TryGetValue(requestId, out KeyCredentialRequestRecord? record))
            {
                throw new ServiceResultException(
                    StatusCodes.BadNotFound,
                    "The RequestId is not known.");
            }

            if (cancelRequest)
            {
                record.State = KeyCredentialRequestState.Rejected;
                // purge the secret on cancel
                if (record.CredentialId != null)
                {
                    var secretId = new SecretIdentifier(record.CredentialId, m_secretStore.StoreType);
                    await m_secretStore.RemoveAsync(secretId, cancellationToken).ConfigureAwait(false);
                }
                return new FinishKeyCredentialRequestResult { State = record.State };
            }

            if (record.State == KeyCredentialRequestState.Approved)
            {
                record.State = KeyCredentialRequestState.Completed;
            }

            // materialise the secret from the store
            string? credentialId = record.CredentialId;
            var sid = new SecretIdentifier(credentialId!, m_secretStore.StoreType);
            using ISecret? secret = m_secretStore.TryGet(sid);
            ByteString credentialSecret = secret != null
                ? ByteString.From(secret.Bytes.ToArray())
                : record.CredentialSecret;
            return new FinishKeyCredentialRequestResult
            {
                State = record.State,
                CredentialId = credentialId,
                CredentialSecret = credentialSecret,
                CertificateThumbprint = record.CertificateThumbprint,
                SecurityPolicyUri = record.GrantedSecurityPolicyUri,
                GrantedRoles = record.GrantedRoles
            };
        }

        /// <inheritdoc/>
        public async ValueTask RevokeAsync(string credentialId, CancellationToken cancellationToken = default)
        {
            if (!m_credentials.TryGetValue(credentialId, out KeyCredentialRequestRecord? record))
            {
                throw new ServiceResultException(
                    StatusCodes.BadNotFound,
                    "The CredentialId is not known.");
            }

            record.State = KeyCredentialRequestState.Rejected;

            // purge the secret from the backing store
            var secretId = new SecretIdentifier(credentialId, m_secretStore.StoreType);
            await m_secretStore.RemoveAsync(secretId, cancellationToken).ConfigureAwait(false);
        }

        private static byte[] GenerateRandomBytes(int length)
        {
            byte[] buffer = new byte[length];
            using var rng = System.Security.Cryptography.RandomNumberGenerator.Create();
            rng.GetBytes(buffer);
            return buffer;
        }
    }
}