using System.Reflection;
using Akka.Actor;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;

namespace ZB.MOM.WW.ScadaBridge.Host.Tests;

/// <summary>
/// Deferred register row 50, mitigation 2: a reflection guard that no Akka actor holds
/// captive scope state — a scoped service, or an <see cref="IServiceScope"/>, cached for
/// the actor's lifetime.
/// </summary>
/// <remarks>
/// <para>
/// <b>Why this exists separately from scope validation.</b> Register row 45 was exactly this
/// bug: <c>NotificationOutboxActor</c> cached delivery adapters resolved from an
/// actor-lifetime <c>IServiceScope</c>, so <c>MaxParallelDeliveries</c> concurrent deliveries
/// shared one EF <c>DbContext</c> and EF threw "a second operation was started on this
/// context instance" continuously on the rig (fixed 9e769735). .NET's own
/// <c>ValidateScopes</c> could never have caught it: the actors take the ROOT
/// <see cref="IServiceProvider"/> and call <c>GetRequiredService</c> by hand, which the
/// container's static graph analysis cannot see through. This guard closes that half.
/// </para>
/// <para>
/// <b>What is allowed.</b> Holding <see cref="IServiceProvider"/> or
/// <see cref="IServiceScopeFactory"/> for the actor's lifetime is the CORRECT pattern and is
/// deliberately not flagged — every actor here does that and opens a fresh scope per
/// operation. What is forbidden is holding the SCOPE, or anything resolved from one, past
/// the operation that opened it.
/// </para>
/// <para>
/// <b>How "scoped" is decided.</b> The caller supplies the scoped set derived from the actual
/// DI registrations of a real composition root, so the guard tracks the container rather than
/// a hand-maintained list — re-registering a service from singleton to scoped automatically
/// brings any actor field of that type into scope for the check. Two types are flagged
/// unconditionally on top of that set, because they are wrong in an actor field whatever the
/// container says: <see cref="IServiceScope"/> itself, and any <see cref="DbContext"/>.
/// </para>
/// <para>
/// Auto-property backing fields need no special handling: the compiler emits them as private
/// instance fields, so <c>GetFields(NonPublic | Instance)</c> already returns them.
/// </para>
/// </remarks>
internal static class CaptiveDependencyGuard
{
    /// <summary>One offending field on one actor type.</summary>
    /// <param name="ActorType">The actor class declaring the field.</param>
    /// <param name="FieldName">The field's name (a compiler-generated backing field keeps its mangled name).</param>
    /// <param name="FieldType">The field's declared type.</param>
    /// <param name="Reason">Why the field was flagged.</param>
    internal sealed record Violation(Type ActorType, string FieldName, Type FieldType, string Reason)
    {
        /// <inheritdoc/>
        public override string ToString() =>
            $"{ActorType.FullName}.{FieldName} : {FieldType.FullName} — {Reason}";
    }

    /// <summary>
    /// Field types that are always correct to hold for an actor's lifetime: they ARE the
    /// per-operation-scope pattern rather than a violation of it.
    /// </summary>
    internal static readonly IReadOnlyList<Type> AllowedProviderTypes =
    [
        typeof(IServiceProvider),
        typeof(IServiceScopeFactory),
    ];

    /// <summary>
    /// Fields that hold a scoped-typed value but are provably safe, keyed
    /// <c>"Namespace.ActorType.fieldName"</c>. Every entry carries its safety argument —
    /// nothing is exempted silently.
    /// </summary>
    /// <remarks>
    /// <para>
    /// EMPTY, and pinned empty by <c>CaptiveDependencyGuardTests.Allowlist_IsEmpty</c>.
    /// </para>
    /// <para>
    /// It previously held two entries — <c>AuditLogIngestActor._injectedRepository</c> and
    /// <c>SiteCallAuditActor._injectedRepository</c> — nullable repository fields assigned only
    /// by explicitly test-mode constructors. They were safe solely because no production call
    /// site picked those overloads, which left the row-45 captive-dependency bug one
    /// constructor call away. Register row 61 deleted the constructors instead: both actors now
    /// have a single <see cref="IServiceProvider"/> constructor, the fields are gone, and the
    /// test fixtures build stub providers that hand a repository out of a scope. With the shape
    /// removed there is nothing left to exempt.
    /// </para>
    /// <para>
    /// Keep it empty. A new entry means someone reintroduced a scoped service cached on an
    /// actor field; prefer fixing the actor.
    /// </para>
    /// </remarks>
    internal static readonly IReadOnlySet<string> Allowlist =
        new HashSet<string>(StringComparer.Ordinal);

    /// <summary>
    /// Scans the given actor types for instance fields holding captive scope state.
    /// </summary>
    /// <param name="actorTypes">Actor classes to inspect.</param>
    /// <param name="scopedServiceTypes">
    /// Service and implementation types registered with <see cref="ServiceLifetime.Scoped"/>
    /// in the composition root being checked.
    /// </param>
    /// <param name="applyAllowlist">
    /// False makes the scan report allowlisted fields too — used by the guard's own self-tests
    /// so the allowlist cannot quietly grow to cover a real regression.
    /// </param>
    /// <returns>Every violation found, in declaration order.</returns>
    internal static IReadOnlyList<Violation> Scan(
        IEnumerable<Type> actorTypes,
        IReadOnlySet<Type> scopedServiceTypes,
        bool applyAllowlist = true)
    {
        ArgumentNullException.ThrowIfNull(actorTypes);
        ArgumentNullException.ThrowIfNull(scopedServiceTypes);

        var violations = new List<Violation>();

        foreach (var actorType in actorTypes)
        {
            foreach (var field in InstanceFields(actorType))
            {
                var fieldType = field.FieldType;

                if (AllowedProviderTypes.Contains(fieldType))
                {
                    // The correct pattern — create a scope per operation.
                    continue;
                }

                string? reason = null;
                if (fieldType == typeof(IServiceScope) || fieldType == typeof(AsyncServiceScope))
                {
                    reason = "holds a DI scope for the actor's lifetime; open a scope per operation instead";
                }
                else if (typeof(DbContext).IsAssignableFrom(fieldType))
                {
                    reason = "holds an EF DbContext for the actor's lifetime; resolve it inside a per-operation scope";
                }
                else if (scopedServiceTypes.Contains(fieldType))
                {
                    reason = "holds a service registered Scoped; resolve it inside a per-operation scope";
                }

                if (reason is null)
                {
                    continue;
                }

                if (applyAllowlist && Allowlist.Contains($"{actorType.FullName}.{field.Name}"))
                {
                    continue;
                }

                violations.Add(new Violation(actorType, field.Name, fieldType, reason));
            }
        }

        return violations;
    }

    /// <summary>
    /// Every instance field declared by the actor class and by its ScadaBridge-owned base
    /// classes. The walk stops at the first non-ScadaBridge base, which is how Akka's own
    /// <see cref="ActorBase"/> internals are excluded without naming them.
    /// </summary>
    /// <param name="actorType">The actor class to walk.</param>
    /// <returns>The instance fields, most-derived first.</returns>
    private static IEnumerable<FieldInfo> InstanceFields(Type actorType)
    {
        for (var t = actorType; t is not null && IsScadaBridgeType(t); t = t.BaseType)
        {
            foreach (var field in t.GetFields(
                BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.DeclaredOnly))
            {
                yield return field;
            }
        }
    }

    /// <summary>Whether a type belongs to a first-party ScadaBridge assembly.</summary>
    /// <param name="type">The type to test.</param>
    /// <returns>True when the declaring assembly is a ScadaBridge assembly.</returns>
    private static bool IsScadaBridgeType(Type type) =>
        type.Assembly.GetName().Name?.StartsWith(AssemblyPrefix, StringComparison.Ordinal) == true;

    private const string AssemblyPrefix = "ZB.MOM.WW.ScadaBridge";

    /// <summary>
    /// Every concrete Akka actor class in the shipped ScadaBridge assemblies. Test assemblies
    /// are excluded deliberately — the guard's own deliberately-violating fixtures live there,
    /// and a test-only actor is not part of any production composition root.
    /// </summary>
    /// <returns>The production actor types, ordered by full name.</returns>
    internal static IReadOnlyList<Type> ProductionActorTypes() =>
        ProductionAssemblies()
            .SelectMany(a => a.GetTypes())
            .Where(t => typeof(ActorBase).IsAssignableFrom(t) && t is { IsAbstract: false, IsInterface: false })
            .OrderBy(t => t.FullName, StringComparer.Ordinal)
            .ToList();

    /// <summary>
    /// Transitively loads and returns the shipped ScadaBridge assemblies.
    /// </summary>
    /// <remarks>
    /// Referenced assemblies load lazily, so enumerating <c>AppDomain.CurrentDomain</c> alone
    /// would silently scan whichever subset the test run happened to touch — a guard that
    /// quietly checks nothing. The reference graph is walked from the Host assembly instead,
    /// and the caller asserts a floor on the actor count so a regression here fails loudly.
    /// </remarks>
    /// <returns>The distinct production ScadaBridge assemblies.</returns>
    private static IReadOnlyList<Assembly> ProductionAssemblies()
    {
        var seen = new Dictionary<string, Assembly>(StringComparer.Ordinal);
        var pending = new Stack<Assembly>();
        pending.Push(typeof(Program).Assembly);

        while (pending.Count > 0)
        {
            var assembly = pending.Pop();
            var name = assembly.GetName().Name;
            if (name is null
                || !name.StartsWith(AssemblyPrefix, StringComparison.Ordinal)
                || name.EndsWith(".Tests", StringComparison.Ordinal)
                || !seen.TryAdd(name, assembly))
            {
                continue;
            }

            foreach (var reference in assembly.GetReferencedAssemblies())
            {
                if (reference.Name?.StartsWith(AssemblyPrefix, StringComparison.Ordinal) != true
                    || seen.ContainsKey(reference.Name))
                {
                    continue;
                }

                try
                {
                    pending.Push(Assembly.Load(reference));
                }
                catch (Exception ex) when (ex is FileNotFoundException or BadImageFormatException)
                {
                    // A ScadaBridge assembly that is referenced but not deployed alongside the
                    // test binaries cannot contribute actors to this run either.
                }
            }
        }

        return seen.Values.ToList();
    }

    /// <summary>
    /// Collects the service and implementation types registered with
    /// <see cref="ServiceLifetime.Scoped"/> in a composition root.
    /// </summary>
    /// <remarks>
    /// Both sides of a descriptor are collected: an actor can just as easily cache the
    /// concrete <c>EmailNotificationDeliveryAdapter</c> as the <c>INotificationDeliveryAdapter</c>
    /// it is registered as, and row 45 in fact cached the interface.
    /// </remarks>
    /// <param name="services">The composition root's service collection.</param>
    /// <returns>The scoped service and implementation types.</returns>
    internal static IReadOnlySet<Type> ScopedTypes(IEnumerable<ServiceDescriptor> services)
    {
        ArgumentNullException.ThrowIfNull(services);

        var scoped = new HashSet<Type>();
        foreach (var descriptor in services.Where(d => d.Lifetime == ServiceLifetime.Scoped))
        {
            scoped.Add(descriptor.ServiceType);

            // ImplementationType throws on a keyed descriptor and vice versa, so branch.
            var implementationType = descriptor.IsKeyedService
                ? descriptor.KeyedImplementationType
                : descriptor.ImplementationType;
            if (implementationType is not null)
            {
                scoped.Add(implementationType);
            }
        }

        return scoped;
    }
}