using System.Collections.Concurrent;
using System.Runtime.CompilerServices;
using System.Security.Cryptography;
using System.Text;
using Grpc.Core;
using ZB.MOM.WW.MxGateway.Contracts.Proto.Galaxy;

namespace ZB.MOM.WW.MxGateway.Server.Galaxy;

/// <summary>
///     Projects one level of children of a parent object out of an immutable
///     <see cref="GalaxyHierarchyCacheEntry"/>. Pure and side-effect free. Memoizes the
///     filtered child list per cache-entry instance so repeated paging is an O(pageSize)
///     slice rather than an O(siblings) filter scan per page. The memo is keyed on the
///     immutable cache entry, so when the cache publishes a new entry the stale memo
///     becomes unreachable and is reclaimed with it.
/// </summary>
public static class GalaxyBrowseProjector
{
    private static readonly ConditionalWeakTable<
        GalaxyHierarchyCacheEntry,
        ConcurrentDictionary<string, FilteredChildren>> FilteredChildrenCache = new();

    /// <summary>Projects one page of direct children of the resolved parent.</summary>
    /// <param name="entry">The Galaxy hierarchy cache entry to query.</param>
    /// <param name="request">The browse-children request.</param>
    /// <param name="browseSubtreeGlobs">Optional API-key browse-subtree constraints.</param>
    /// <param name="offset">Zero-based offset into the filtered child list.</param>
    /// <param name="pageSize">Maximum number of children to return.</param>
    public static GalaxyBrowseChildrenResult ProjectChildren(
        GalaxyHierarchyCacheEntry entry,
        BrowseChildrenRequest request,
        IReadOnlyList<string>? browseSubtreeGlobs,
        int offset,
        int pageSize)
    {
        ArgumentNullException.ThrowIfNull(entry);
        ArgumentNullException.ThrowIfNull(request);
        if (offset < 0)
        {
            throw new ArgumentOutOfRangeException(nameof(offset), offset, "Offset must be greater than or equal to zero.");
        }
        if (pageSize <= 0)
        {
            throw new ArgumentOutOfRangeException(nameof(pageSize), pageSize, "Page size must be greater than zero.");
        }

        int parentId = ResolveParentId(entry, request);
        string filterSignature = ComputeFilterSignature(request, browseSubtreeGlobs, parentId);
        FilteredChildren filtered = GetFilteredChildren(entry, request, browseSubtreeGlobs, parentId, filterSignature);

        bool includeAttributes = IncludeAttributes(request);
        int end = (int)Math.Min((long)offset + pageSize, filtered.Children.Count);
        List<GalaxyObject> page = new(Math.Max(0, end - offset));
        List<bool> hasChildren = new(Math.Max(0, end - offset));
        for (int index = offset; index < end; index++)
        {
            page.Add(CloneObject(filtered.Children[index].Object, includeAttributes));
            hasChildren.Add(filtered.HasMatchingDescendant[index]);
        }

        return new GalaxyBrowseChildrenResult(page, hasChildren, filtered.Children.Count, filterSignature);
    }

    /// <summary>
    ///     Resolves the request's parent oneof to a gobject id, throwing
    ///     <see cref="RpcException"/> with <see cref="StatusCode.NotFound"/> when the
    ///     parent does not exist. Public so the gRPC handler can compute the same
    ///     parent id (needed for the page-token signature) without reimplementing the
    ///     resolution rules.
    /// </summary>
    /// <param name="entry">The Galaxy hierarchy cache entry to query.</param>
    /// <param name="request">The browse-children request.</param>
    public static int ResolveParentId(GalaxyHierarchyCacheEntry entry, BrowseChildrenRequest request)
    {
        switch (request.ParentCase)
        {
            case BrowseChildrenRequest.ParentOneofCase.None:
                return 0;
            case BrowseChildrenRequest.ParentOneofCase.ParentGobjectId:
                if (request.ParentGobjectId == 0)
                {
                    return 0;
                }
                if (!entry.Index.ObjectViewsById.ContainsKey(request.ParentGobjectId))
                {
                    throw new RpcException(new Status(StatusCode.NotFound, "BrowseChildren parent was not found."));
                }
                return request.ParentGobjectId;
            case BrowseChildrenRequest.ParentOneofCase.ParentTagName:
            {
                if (!entry.Index.ObjectViewsByTagName.TryGetValue(request.ParentTagName, out GalaxyObjectView? match))
                {
                    throw new RpcException(new Status(StatusCode.NotFound, "BrowseChildren parent was not found."));
                }
                return match.Object.GobjectId;
            }
            case BrowseChildrenRequest.ParentOneofCase.ParentContainedPath:
            {
                if (!entry.Index.ObjectViewsByContainedPath.TryGetValue(request.ParentContainedPath, out GalaxyObjectView? match))
                {
                    throw new RpcException(new Status(StatusCode.NotFound, "BrowseChildren parent was not found."));
                }
                return match.Object.GobjectId;
            }
            default:
                return 0;
        }
    }

    private static FilteredChildren GetFilteredChildren(
        GalaxyHierarchyCacheEntry entry,
        BrowseChildrenRequest request,
        IReadOnlyList<string>? browseSubtreeGlobs,
        int parentId,
        string filterSignature)
    {
        ConcurrentDictionary<string, FilteredChildren> memo =
            FilteredChildrenCache.GetValue(entry, static _ => new ConcurrentDictionary<string, FilteredChildren>(StringComparer.Ordinal));

        return memo.GetOrAdd(
            filterSignature,
            static (_, state) =>
            {
                IReadOnlyDictionary<int, IReadOnlyList<GalaxyObjectView>> map = state.Entry.Index.ChildrenByParent;
                IReadOnlyList<GalaxyObjectView> directChildren = map.TryGetValue(state.ParentId, out IReadOnlyList<GalaxyObjectView>? list)
                    ? list
                    : Array.Empty<GalaxyObjectView>();

                List<GalaxyObjectView> matched = [];
                List<bool> hasMatching = [];
                foreach (GalaxyObjectView view in directChildren)
                {
                    if (!MatchesBrowseSubtrees(view, state.BrowseSubtreeGlobs))
                    {
                        continue;
                    }
                    if (!MatchesFilters(view.Object, state.Request))
                    {
                        // Even if the direct child itself fails the filter, a matching
                        // descendant should still surface its ancestor — but only when
                        // there is one. Mirror the dashboard browse-tree semantics: if a
                        // descendant matches, include the parent with has-children true.
                        if (HasMatchingDescendant(view, state.Entry.Index, state.Request, state.BrowseSubtreeGlobs))
                        {
                            matched.Add(view);
                            hasMatching.Add(true);
                        }
                        continue;
                    }
                    matched.Add(view);
                    hasMatching.Add(HasMatchingDescendant(view, state.Entry.Index, state.Request, state.BrowseSubtreeGlobs));
                }

                return new FilteredChildren(matched, hasMatching);
            },
            (Entry: entry, ParentId: parentId, Request: request, BrowseSubtreeGlobs: browseSubtreeGlobs));
    }

    private static bool HasMatchingDescendant(
        GalaxyObjectView parent,
        GalaxyHierarchyIndex index,
        BrowseChildrenRequest request,
        IReadOnlyList<string>? browseSubtreeGlobs)
    {
        if (!index.ChildrenByParent.TryGetValue(parent.Object.GobjectId, out IReadOnlyList<GalaxyObjectView>? children))
        {
            return false;
        }

        // Defend against pathological cycles in Galaxy data (e.g. a corrupt A→B→A chain).
        // BuildContainedPath uses the same visited-id pattern; mirror it so this walk
        // terminates even when ChildrenByParent forms a cycle.
        HashSet<int> visited = new() { parent.Object.GobjectId };
        Stack<GalaxyObjectView> stack = new();
        foreach (GalaxyObjectView child in children)
        {
            if (visited.Add(child.Object.GobjectId))
            {
                stack.Push(child);
            }
        }
        while (stack.Count > 0)
        {
            GalaxyObjectView candidate = stack.Pop();
            if (MatchesBrowseSubtrees(candidate, browseSubtreeGlobs)
                && MatchesFilters(candidate.Object, request))
            {
                return true;
            }
            if (index.ChildrenByParent.TryGetValue(candidate.Object.GobjectId, out IReadOnlyList<GalaxyObjectView>? grandchildren))
            {
                foreach (GalaxyObjectView grandchild in grandchildren)
                {
                    if (visited.Add(grandchild.Object.GobjectId))
                    {
                        stack.Push(grandchild);
                    }
                }
            }
        }
        return false;
    }

    private static bool MatchesBrowseSubtrees(GalaxyObjectView view, IReadOnlyList<string>? browseSubtreeGlobs)
    {
        return browseSubtreeGlobs is null
            || browseSubtreeGlobs.Count == 0
            || browseSubtreeGlobs.Any(glob => GalaxyGlobMatcher.IsMatch(view.ContainedPath, glob));
    }

    private static bool MatchesFilters(GalaxyObject obj, BrowseChildrenRequest request)
    {
        if (request.CategoryIds.Count > 0 && !request.CategoryIds.Contains(obj.CategoryId))
        {
            return false;
        }
        foreach (string templateFilter in request.TemplateChainContains)
        {
            if (!obj.TemplateChain.Any(template => template.Contains(templateFilter, StringComparison.OrdinalIgnoreCase)))
            {
                return false;
            }
        }
        if (!string.IsNullOrWhiteSpace(request.TagNameGlob)
            && !GalaxyGlobMatcher.IsMatch(obj.TagName, request.TagNameGlob))
        {
            return false;
        }
        if (request.AlarmBearingOnly && !obj.Attributes.Any(attribute => attribute.IsAlarm))
        {
            return false;
        }
        if (request.HistorizedOnly && !obj.Attributes.Any(attribute => attribute.IsHistorized))
        {
            return false;
        }
        return true;
    }

    private static bool IncludeAttributes(BrowseChildrenRequest request)
    {
        return !request.HasIncludeAttributes || request.IncludeAttributes;
    }

    private static GalaxyObject CloneObject(GalaxyObject source, bool includeAttributes)
    {
        GalaxyObject clone = source.Clone();
        if (!includeAttributes)
        {
            clone.Attributes.Clear();
        }
        return clone;
    }

    /// <summary>Computes a stable filter signature for memoization purposes.</summary>
    /// <param name="request">The browse-children request.</param>
    /// <param name="browseSubtreeGlobs">Optional API-key browse-subtree constraints.</param>
    /// <param name="parentId">Resolved parent gobject id (0 for roots).</param>
    public static string ComputeFilterSignature(
        BrowseChildrenRequest request,
        IReadOnlyList<string>? browseSubtreeGlobs,
        int parentId)
    {
        StringBuilder builder = new();
        builder.Append("parent=").Append(parentId.ToString(System.Globalization.CultureInfo.InvariantCulture));
        builder.Append("|cat=").AppendJoin(',', request.CategoryIds.Order());
        builder.Append("|tpl=").AppendJoin(',', request.TemplateChainContains.Order(StringComparer.OrdinalIgnoreCase));
        builder.Append("|glob=").Append(request.TagNameGlob);
        builder.Append("|attrs=").Append(request.HasIncludeAttributes ? request.IncludeAttributes.ToString() : "unset");
        builder.Append("|alarm=").Append(request.AlarmBearingOnly);
        builder.Append("|hist=").Append(request.HistorizedOnly);
        builder.Append("|browse=").AppendJoin(',', (browseSubtreeGlobs ?? Array.Empty<string>()).Order(StringComparer.OrdinalIgnoreCase));
        byte[] hash = SHA256.HashData(Encoding.UTF8.GetBytes(builder.ToString()));
        return Convert.ToHexString(hash, 0, 12);
    }

    private sealed record FilteredChildren(
        IReadOnlyList<GalaxyObjectView> Children,
        IReadOnlyList<bool> HasMatchingDescendant);
}