using Shouldly;
using Xunit;
using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
using ZB.MOM.WW.OtOpcUa.OpcUaServer.Security;

namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests;

/// <summary>
///     #520 stage 6 — the decision cache behind the request-rate seams.
/// </summary>
/// <remarks>
///     The risk in a cache like this is not that it is slow; it is that it answers a question slightly
///     different from the one asked. Each test below pins one dimension the key must discriminate on,
///     because a key that collides across any of them hands one session another's authority.
/// </remarks>
public sealed class SessionAclDecisionCacheTests
{
    private const string Session = "key-1";
    private const string Node = "Plant/Modbus/dev1/Speed";

    /// <summary>The second identical question is answered from the cache.</summary>
    [Fact]
    public void A_repeated_question_is_not_recomputed()
    {
        var cache = new SessionAclDecisionCache();
        var calls = 0;

        for (var i = 0; i < 5; i++)
            Ask(cache, generation: 1, compute: () => { calls++; return true; }).ShouldBeTrue();

        calls.ShouldBe(1);
    }

    /// <summary>
    ///     A new generation drops everything. This is the invalidation the whole design rests on: grants
    ///     only change by deployment, and a deployment always advances the generation.
    /// </summary>
    [Fact]
    public void A_new_generation_invalidates_every_decision()
    {
        var cache = new SessionAclDecisionCache();

        Ask(cache, generation: 1, compute: () => true).ShouldBeTrue();
        cache.Count.ShouldBe(1);

        // The same question, one deployment later, must be re-decided — and the new answer must win.
        Ask(cache, generation: 2, compute: () => false).ShouldBeFalse();
        cache.Generation.ShouldBe(2);
    }

    /// <summary>
    ///     Two sessions asking about the same node get their own answers. A key that omitted the session
    ///     would let the first caller's verdict decide for everyone.
    /// </summary>
    [Fact]
    public void Two_sessions_do_not_share_a_decision()
    {
        var cache = new SessionAclDecisionCache();

        Ask(cache, generation: 1, session: "granted-key", compute: () => true).ShouldBeTrue();
        Ask(cache, generation: 1, session: "ungranted-key", compute: () => false).ShouldBeFalse();
    }

    /// <summary>Read and Write on one node are separate questions.</summary>
    [Fact]
    public void Two_operations_do_not_share_a_decision()
    {
        var cache = new SessionAclDecisionCache();

        Ask(cache, generation: 1, operation: OpcUaOperation.Read, compute: () => true).ShouldBeTrue();
        Ask(cache, generation: 1, operation: OpcUaOperation.WriteOperate, compute: () => false).ShouldBeFalse();
    }

    /// <summary>
    ///     The two realms are separate namespaces that can carry the SAME <c>s=</c> identifier, so the realm
    ///     is part of the key. Without it, a raw grant would answer for the UNS node of the same path —
    ///     which is exactly the cross-realm confusion the trie itself is careful to avoid.
    /// </summary>
    [Fact]
    public void The_two_realms_do_not_share_a_decision()
    {
        var cache = new SessionAclDecisionCache();

        Ask(cache, generation: 1, realm: AddressSpaceRealm.Raw, compute: () => true).ShouldBeTrue();
        Ask(cache, generation: 1, realm: AddressSpaceRealm.Uns, compute: () => false).ShouldBeFalse();
    }

    /// <summary>Different nodes are different questions.</summary>
    [Fact]
    public void Two_nodes_do_not_share_a_decision()
    {
        var cache = new SessionAclDecisionCache();

        Ask(cache, generation: 1, node: "Plant/Modbus/dev1/Speed", compute: () => true).ShouldBeTrue();
        Ask(cache, generation: 1, node: "Plant/Modbus/dev1/Torque", compute: () => false).ShouldBeFalse();
    }

    /// <summary>
    ///     Node identifiers are path segments, so they compare ordinally — <c>Speed</c> and <c>speed</c> can
    ///     both exist under one device and must not collide in the cache when they do not collide in the
    ///     trie.
    /// </summary>
    [Fact]
    public void Node_identifiers_are_case_sensitive()
    {
        var cache = new SessionAclDecisionCache();

        Ask(cache, generation: 1, node: "Plant/dev1/Speed", compute: () => true).ShouldBeTrue();
        Ask(cache, generation: 1, node: "Plant/dev1/speed", compute: () => false).ShouldBeFalse();
    }

    /// <summary>
    ///     The cache is bounded. A client walking a huge address space must not be able to grow it without
    ///     limit; clearing at the ceiling degrades to recomputation, which is the no-cache steady state.
    /// </summary>
    [Fact]
    public void The_cache_is_bounded()
    {
        var cache = new SessionAclDecisionCache();

        for (var i = 0; i <= SessionAclDecisionCache.MaxEntries + 10; i++)
            Ask(cache, generation: 1, node: $"Plant/dev1/Tag{i}", compute: () => true);

        cache.Count.ShouldBeLessThanOrEqualTo(SessionAclDecisionCache.MaxEntries);
    }

    /// <summary>
    ///     A decision taken while a deployment lands mid-walk is returned but NOT stored, so the new
    ///     generation never inherits a verdict its own trie did not make.
    /// </summary>
    [Fact]
    public void A_decision_overtaken_by_a_deploy_is_not_stored()
    {
        var cache = new SessionAclDecisionCache();

        // The compute callback simulates the deploy landing while the walk is in flight.
        var decided = cache.GetOrAdd(
            generation: 1, Session, OpcUaOperation.Read, AddressSpaceRealm.Raw, Node,
            () =>
            {
                Ask(cache, generation: 2, node: "unrelated", compute: () => true);
                return true;
            });

        decided.ShouldBeTrue("the caller still gets the answer its own walk produced");

        // Nothing was filed under the superseded generation — the only entry is the unrelated one the
        // callback itself made.
        cache.Generation.ShouldBe(2);
        cache.Count.ShouldBe(1);
    }

    private static bool Ask(
        SessionAclDecisionCache cache,
        long generation,
        Func<bool> compute,
        string session = Session,
        OpcUaOperation operation = OpcUaOperation.Read,
        AddressSpaceRealm realm = AddressSpaceRealm.Raw,
        string node = Node)
        => cache.GetOrAdd(generation, session, operation, realm, node, compute);
}