#nullable enable
using System;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UIElements;

namespace UVC.UIToolkit
{
    /// <summary>
    /// UIToolkit 테마 관리자
    /// 런타임에서 Light/Dark 테마 전환을 지원합니다.
    /// </summary>
    public class UTKThemeManager
    {
        #region Singleton
        private static UTKThemeManager? _instance;
        public static UTKThemeManager Instance => _instance ??= new UTKThemeManager();

        private UTKThemeManager()
        {
            LoadStyleSheets();
        }
        #endregion

        #region Constants
        private const string VARIABLES_PATH = "UIToolkit/Style/UTKVariables";
        private const string THEME_DARK_PATH = "UIToolkit/Style/UTKThemeDark";
        private const string THEME_LIGHT_PATH = "UIToolkit/Style/UTKThemeLight";
        private const string COMPONENTS_PATH = "UIToolkit/Style/UTKComponents";
        private const string DEFAULT_STYLE_PATH = "UIToolkit/Style/UTKDefaultStyle";
        #endregion

        #region Fields
        private StyleSheet? _variablesSheet;
        private StyleSheet? _themeDarkSheet;
        private StyleSheet? _themeLightSheet;
        private StyleSheet? _componentsSheet;
        private StyleSheet? _defaultStyleSheet;

        private readonly HashSet<VisualElement> _registeredRoots = new();
        private UTKTheme _currentTheme = UTKTheme.Dark;
        #endregion

        #region Properties
        /// <summary>현재 적용된 테마</summary>
        public UTKTheme CurrentTheme => _currentTheme;

        /// <summary>다크 테마 여부</summary>
        public bool IsDarkTheme => _currentTheme == UTKTheme.Dark;
        #endregion

        #region Events
        /// <summary>테마 변경 이벤트</summary>
        public event Action<UTKTheme>? OnThemeChanged;
        #endregion

        #region Initialization
        private void LoadStyleSheets()
        {
            _variablesSheet = Resources.Load<StyleSheet>(VARIABLES_PATH);
            _themeDarkSheet = Resources.Load<StyleSheet>(THEME_DARK_PATH);
            _themeLightSheet = Resources.Load<StyleSheet>(THEME_LIGHT_PATH);
            _componentsSheet = Resources.Load<StyleSheet>(COMPONENTS_PATH);
            _defaultStyleSheet = Resources.Load<StyleSheet>(DEFAULT_STYLE_PATH);

            if (_variablesSheet == null)
                Debug.LogWarning($"[UTKThemeManager] UTKVariables.uss not found at: {VARIABLES_PATH}");
            if (_themeDarkSheet == null)
                Debug.LogWarning($"[UTKThemeManager] UTKThemeDark.uss not found at: {THEME_DARK_PATH}");
            if (_themeLightSheet == null)
                Debug.LogWarning($"[UTKThemeManager] UTKThemeLight.uss not found at: {THEME_LIGHT_PATH}");
            if (_componentsSheet == null)
                Debug.LogWarning($"[UTKThemeManager] UTKComponents.uss not found at: {COMPONENTS_PATH}");
        }
        #endregion

        #region Public Methods
        /// <summary>
        /// VisualElement 루트에 테마 스타일시트를 등록합니다.
        /// UIDocument의 rootVisualElement나 커스텀 VisualElement를 전달하세요.
        /// </summary>
        /// <param name="root">스타일시트를 적용할 VisualElement</param>
        /// <param name="includeDefaultStyle">UTKDefaultStyle.uss 포함 여부 (Unity 기본 요소 스타일)</param>
        public void RegisterRoot(VisualElement root, bool includeDefaultStyle = true)
        {
            if (root == null || _registeredRoots.Contains(root))
                return;

            _registeredRoots.Add(root);
            ApplyThemeToRoot(root, includeDefaultStyle);
        }

        /// <summary>
        /// 등록된 루트에서 스타일시트를 제거합니다.
        /// </summary>
        /// <param name="root">제거할 VisualElement</param>
        public void UnregisterRoot(VisualElement root)
        {
            if (root == null)
                return;

            _registeredRoots.Remove(root);
            RemoveThemeFromRoot(root);
        }

        /// <summary>
        /// 테마를 전환합니다.
        /// </summary>
        /// <param name="theme">적용할 테마</param>
        public void SetTheme(UTKTheme theme)
        {
            if (_currentTheme == theme)
                return;

            _currentTheme = theme;

            // 모든 등록된 루트에 테마 적용
            foreach (var root in _registeredRoots)
            {
                ApplyThemeToRoot(root, true);
            }

            OnThemeChanged?.Invoke(_currentTheme);
            Debug.Log($"[UTKThemeManager] Theme changed to: {_currentTheme}");
        }

        /// <summary>
        /// 테마를 토글합니다 (Dark ↔ Light).
        /// </summary>
        public void ToggleTheme()
        {
            SetTheme(_currentTheme == UTKTheme.Dark ? UTKTheme.Light : UTKTheme.Dark);
        }

        /// <summary>
        /// 스타일시트만 반환합니다 (수동 적용용).
        /// </summary>
        /// <returns>현재 테마에 해당하는 스타일시트 배열</returns>
        public StyleSheet?[] GetStyleSheets(bool includeDefaultStyle = true)
        {
            var currentThemeSheet = _currentTheme == UTKTheme.Dark ? _themeDarkSheet : _themeLightSheet;

            if (includeDefaultStyle)
            {
                return new[] { _variablesSheet, currentThemeSheet, _componentsSheet, _defaultStyleSheet };
            }
            else
            {
                return new[] { _variablesSheet, currentThemeSheet, _componentsSheet };
            }
        }

        /// <summary>
        /// 특정 컴포넌트에 필요한 스타일시트만 추가합니다.
        /// 이미 루트에 등록된 경우 호출할 필요 없음.
        /// </summary>
        /// <param name="element">스타일을 적용할 요소</param>
        public void ApplyThemeToElement(VisualElement element)
        {
            if (element == null)
                return;

            var currentThemeSheet = _currentTheme == UTKTheme.Dark ? _themeDarkSheet : _themeLightSheet;
            var otherThemeSheet = _currentTheme == UTKTheme.Dark ? _themeLightSheet : _themeDarkSheet;

            // 이전 테마 제거
            if (otherThemeSheet != null && element.styleSheets.Contains(otherThemeSheet))
                element.styleSheets.Remove(otherThemeSheet);

            // 현재 테마가 이미 있다면 제거 후 다시 추가 (순서 보장)
            if (currentThemeSheet != null && element.styleSheets.Contains(currentThemeSheet))
                element.styleSheets.Remove(currentThemeSheet);

            if (_variablesSheet != null && !element.styleSheets.Contains(_variablesSheet))
                element.styleSheets.Add(_variablesSheet);

            // 현재 테마 적용
            if (currentThemeSheet != null)
                element.styleSheets.Add(currentThemeSheet);

            if (_componentsSheet != null && !element.styleSheets.Contains(_componentsSheet))
                element.styleSheets.Add(_componentsSheet);
        }
        #endregion

        #region Private Methods
        private void ApplyThemeToRoot(VisualElement root, bool includeDefaultStyle)
        {
            var currentThemeSheet = _currentTheme == UTKTheme.Dark ? _themeDarkSheet : _themeLightSheet;
            var otherThemeSheet = _currentTheme == UTKTheme.Dark ? _themeLightSheet : _themeDarkSheet;

            // 이전 테마 제거
            if (otherThemeSheet != null && root.styleSheets.Contains(otherThemeSheet))
            {
                root.styleSheets.Remove(otherThemeSheet);
            }

            // 현재 테마가 이미 적용되어 있다면 다시 추가해서 순서 보장
            if (currentThemeSheet != null && root.styleSheets.Contains(currentThemeSheet))
            {
                root.styleSheets.Remove(currentThemeSheet);
            }

            // 스타일시트 적용 순서가 중요함 (나중에 추가된 것이 우선)
            // 1. Variables (기본 레이아웃 변수)
            if (_variablesSheet != null && !root.styleSheets.Contains(_variablesSheet))
            {
                root.styleSheets.Add(_variablesSheet);
            }

            // 2. Theme (색상 변수) - 항상 새로 추가하여 최신 테마 적용
            if (currentThemeSheet != null)
            {
                root.styleSheets.Add(currentThemeSheet);
            }

            // 3. Components (컴포넌트 클래스)
            if (_componentsSheet != null && !root.styleSheets.Contains(_componentsSheet))
            {
                root.styleSheets.Add(_componentsSheet);
            }

            // 4. DefaultStyle (Unity 기본 요소 오버라이드)
            if (includeDefaultStyle && _defaultStyleSheet != null && !root.styleSheets.Contains(_defaultStyleSheet))
            {
                root.styleSheets.Add(_defaultStyleSheet);
            }
        }

        private void RemoveThemeFromRoot(VisualElement root)
        {
            if (_variablesSheet != null && root.styleSheets.Contains(_variablesSheet))
                root.styleSheets.Remove(_variablesSheet);
            if (_themeDarkSheet != null && root.styleSheets.Contains(_themeDarkSheet))
                root.styleSheets.Remove(_themeDarkSheet);
            if (_themeLightSheet != null && root.styleSheets.Contains(_themeLightSheet))
                root.styleSheets.Remove(_themeLightSheet);
            if (_componentsSheet != null && root.styleSheets.Contains(_componentsSheet))
                root.styleSheets.Remove(_componentsSheet);
            if (_defaultStyleSheet != null && root.styleSheets.Contains(_defaultStyleSheet))
                root.styleSheets.Remove(_defaultStyleSheet);
        }
        #endregion

        #region Static Helpers
        /// <summary>
        /// 현재 테마의 변수 값을 가져옵니다.
        /// VisualElement가 패널에 연결된 후에만 동작합니다.
        /// </summary>
        public static Color GetColor(VisualElement element, CustomStyleProperty<Color> property, Color fallback)
        {
            if (element?.customStyle == null)
                return fallback;

            return element.customStyle.TryGetValue(property, out var color) ? color : fallback;
        }

        /// <summary>
        /// 현재 테마의 float 값을 가져옵니다.
        /// </summary>
        public static float GetFloat(VisualElement element, CustomStyleProperty<float> property, float fallback)
        {
            if (element?.customStyle == null)
                return fallback;

            return element.customStyle.TryGetValue(property, out var value) ? value : fallback;
        }

        /// <summary>
        /// 현재 테마의 int 값을 가져옵니다.
        /// </summary>
        public static int GetInt(VisualElement element, CustomStyleProperty<int> property, int fallback)
        {
            if (element?.customStyle == null)
                return fallback;

            return element.customStyle.TryGetValue(property, out var value) ? value : fallback;
        }
        #endregion
    }

    /// <summary>
    /// 지원하는 테마 종류
    /// </summary>
    public enum UTKTheme
    {
        Dark,
        Light
    }
}