using System;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;

public static class UIBackStack
{
    private sealed class Entry
    {
        public UnityEngine.Object owner;
        public Func<bool> canHandle;
        public Func<bool> handleBack;
        public long sequence;
    }

    private static readonly List<Entry> entries = new List<Entry>();
    private static long nextSequence;
    private static bool hookedSceneEvents;

    public static void RegisterOrBump(UnityEngine.Object owner, Func<bool> canHandle, Func<bool> handleBack)
    {
        if (owner == null || handleBack == null)
        {
            return;
        }

        EnsureSceneHook();
        PurgeInvalidEntries();

        for (int i = 0; i < entries.Count; i++)
        {
            Entry existing = entries[i];
            if (ReferenceEquals(existing.owner, owner))
            {
                existing.canHandle = canHandle;
                existing.handleBack = handleBack;
                existing.sequence = ++nextSequence;
                return;
            }
        }

        entries.Add(new Entry
        {
            owner = owner,
            canHandle = canHandle,
            handleBack = handleBack,
            sequence = ++nextSequence
        });
    }

    public static void Unregister(UnityEngine.Object owner)
    {
        if (owner == null)
        {
            return;
        }

        for (int i = entries.Count - 1; i >= 0; i--)
        {
            if (ReferenceEquals(entries[i].owner, owner))
            {
                entries.RemoveAt(i);
            }
        }
    }

    public static bool TryHandleTop()
    {
        EnsureSceneHook();
        PurgeInvalidEntries();

        entries.Sort((a, b) => a.sequence.CompareTo(b.sequence));

        for (int i = entries.Count - 1; i >= 0; i--)
        {
            Entry entry = entries[i];
            if (!CanEntryHandle(entry))
            {
                continue;
            }

            try
            {
                if (entry.handleBack())
                {
                    PurgeInvalidEntries();
                    return true;
                }
            }
            catch (Exception ex)
            {
                Debug.LogWarning($"[UIBackStack] HandleBack failed: {ex.Message}");
            }
        }

        PurgeInvalidEntries();
        return false;
    }

    private static bool CanEntryHandle(Entry entry)
    {
        if (entry == null || entry.owner == null)
        {
            return false;
        }

        try
        {
            return entry.canHandle == null || entry.canHandle();
        }
        catch (Exception ex)
        {
            Debug.LogWarning($"[UIBackStack] CanHandle failed: {ex.Message}");
            return false;
        }
    }

    private static void PurgeInvalidEntries()
    {
        for (int i = entries.Count - 1; i >= 0; i--)
        {
            Entry entry = entries[i];
            if (entry == null || entry.owner == null)
            {
                entries.RemoveAt(i);
                continue;
            }

            if (entry.owner is Behaviour behaviour && behaviour.gameObject == null)
            {
                entries.RemoveAt(i);
                continue;
            }

            if (entry.owner is GameObject ownerGo && ownerGo == null)
            {
                entries.RemoveAt(i);
            }
        }
    }

    private static void EnsureSceneHook()
    {
        if (hookedSceneEvents)
        {
            return;
        }

        hookedSceneEvents = true;
        SceneManager.sceneLoaded += HandleSceneLoaded;
    }

    private static void HandleSceneLoaded(Scene scene, LoadSceneMode mode)
    {
        PurgeInvalidEntries();
    }
}