source: TFP-WebServer/WebServer/src/UrlHandlers/ItemIconHandler.cs@ 472

using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using UnityEngine;
using Object = UnityEngine.Object;

namespace Webserver.UrlHandlers {
        public class ItemIconHandler : AbsHandler {
                private readonly Dictionary<string, byte[]> icons = new Dictionary<string, byte[]> ();
                private readonly bool logMissingFiles;

                private bool loaded;

                static ItemIconHandler () {
                        Instance = null;
                }

                public ItemIconHandler (bool _logMissingFiles, string _moduleName = null) : base (_moduleName) {
                        logMissingFiles = _logMissingFiles;
                        Instance = this;
                }

                public static ItemIconHandler Instance { get; private set; }

                public override void HandleRequest (RequestContext _context) {
                        if (!loaded) {
                                _context.Response.StatusCode = (int) HttpStatusCode.InternalServerError;
                                Log.Out ("[Web] IconHandler: Icons not loaded");
                                return;
                        }

                        if (!_context.RequestPath.EndsWith (".png", StringComparison.OrdinalIgnoreCase)) {
                                _context.Response.StatusCode = (int) HttpStatusCode.BadRequest;
                                return;
                        }

                        string requestFileName = _context.RequestPath.Remove (0, urlBasePath.Length);
                        int indexOfExtSep = requestFileName.LastIndexOf ('.');
                        if (indexOfExtSep < 0) {
                                _context.Response.StatusCode = (int) HttpStatusCode.BadRequest;
                                return;
                        }

                        requestFileName = requestFileName.Remove (indexOfExtSep);

                        if (!icons.TryGetValue (requestFileName, out byte[] icon)) {
                                _context.Response.StatusCode = (int)HttpStatusCode.NotFound;
                                if (logMissingFiles) {
                                        Log.Out ($"[Web] IconHandler: FileNotFound: \"{_context.RequestPath}\" ");
                                }
                                return;
                        }

                        _context.Response.ContentType = MimeType.GetMimeType (".png");

                        _context.Response.ContentLength64 = icon.Length;
                        _context.Response.OutputStream.Write (icon, 0, icon.Length);
                }

                private class LoadingStats {
                        public int Files;
                        public int Tints;
                        public readonly MicroStopwatch MswTotal = new MicroStopwatch (false);
                        public readonly MicroStopwatch MswLoading = new MicroStopwatch (false);
                        public readonly MicroStopwatch MswEncoding = new MicroStopwatch (false);
                        public readonly MicroStopwatch MswTinting = new MicroStopwatch (false);
                }

                public bool LoadIcons () {
                        
                        lock (icons) {
                                if (loaded) {
                                        return true;
                                }

                                LoadingStats stats = new LoadingStats ();
                                stats?.MswTotal.Start ();

                                // Get list of used tints for all items
                                Dictionary<string, List<Color>> tintedIcons = new Dictionary<string, List<Color>> ();
                                foreach (ItemClass ic in ItemClass.list) {
                                        if (ic == null) {
                                                continue;
                                        }

                                        Color tintColor = ic.GetIconTint ();
                                        if (tintColor == Color.white) {
                                                continue;
                                        }

                                        string name = ic.GetIconName ();
                                        if (!tintedIcons.TryGetValue (name, out List<Color> tintsList)) {
                                                tintsList = new List<Color> ();
                                                tintedIcons.Add (name, tintsList);
                                        }

                                        tintsList.Add (tintColor);
                                }

                                try {
                                        loadIconsFromFolder (GameIO.GetGameDir ("Data/ItemIcons"), tintedIcons, stats);
                                } catch (Exception e) {
                                        Log.Error ("[Web] Failed loading icons from base game");
                                        Log.Exception (e);
                                }

                                // Load icons from mods
                                foreach (Mod mod in ModManager.GetLoadedMods ()) {
                                        try {
                                                string modIconsPath = $"{mod.Path}/ItemIcons";
                                                loadIconsFromFolder (modIconsPath, tintedIcons, stats);
                                        } catch (Exception e) {
                                                Log.Error ($"[Web] Failed loading icons from mod {mod.Name}");
                                                Log.Exception (e);
                                        }
                                }
                                
                                loaded = true;

                                if (stats == null) {
                                        Log.Out ($"[Web] IconHandler: Loaded {icons.Count} icons");
                                } else {
                                        stats.MswTotal.Stop ();
                                        Log.Out ($"[Web] IconHandler: Loaded {icons.Count} icons ({stats.Files} source images with {stats.Tints} tints applied)");
                                        Log.Out ($"[Web] IconHandler: Total time {stats.MswTotal.ElapsedMilliseconds} ms, loading files {stats.MswLoading.ElapsedMilliseconds} ms, tinting files {stats.MswTinting.ElapsedMilliseconds} ms, encoding files {stats.MswEncoding.ElapsedMilliseconds} ms");

                                        int totalSize = 0;
                                        foreach ((string _, byte[] iconData) in icons) {
                                                totalSize += iconData.Length;
                                        }
                                        
                                        Log.Out ($"[Web] IconHandler: Cached {totalSize / 1024} KiB");
                                }

                                return true;
                        }
                }

                private void loadIconsFromFolder (string _path, Dictionary<string, List<Color>> _tintedIcons, LoadingStats _stats) {
                        if (!Directory.Exists (_path)) {
                                return;
                        }

                        foreach (string file in Directory.GetFiles (_path)) {
                                try {
                                        if (!file.EndsWith (".png", StringComparison.OrdinalIgnoreCase)) {
                                                continue;
                                        }

                                        string name = Path.GetFileNameWithoutExtension (file);
                                        Texture2D tex = new Texture2D (1, 1, TextureFormat.ARGB32, false);
                                        
                                        _stats?.MswLoading.Start ();
                                        byte[] sourceBytes = File.ReadAllBytes (file);
                                        if (!tex.LoadImage (sourceBytes)) {
                                                _stats?.MswLoading.Stop ();
                                                continue;
                                        }
                                        _stats?.MswLoading.Stop ();

                                        AddIcon (name, sourceBytes, tex, _tintedIcons, _stats);

                                        Object.Destroy (tex);
                                } catch (Exception e) {
                                        Log.Exception (e);
                                }
                        }
                }

                private void AddIcon (string _name, byte[] _sourceBytes, Texture2D _tex, Dictionary<string, List<Color>> _tintedIcons, LoadingStats _stats) {
                        _stats?.MswEncoding.Start ();
                        icons [$"{_name}__FFFFFF"] = _sourceBytes;
                        _stats?.MswEncoding.Stop ();

                        if (_stats != null) {
                                _stats.Files++;
                        }

                        if (!_tintedIcons.TryGetValue (_name, out List<Color> tintsList)) {
                                return;
                        }

                        foreach (Color c in tintsList) {
                                string tintedName = $"{_name}__{c.ToHexCode ()}";
                                if (icons.ContainsKey (tintedName)) {
                                        continue;
                                }

                                Texture2D tintedTex = new Texture2D (_tex.width, _tex.height, TextureFormat.ARGB32, false);

                                _stats?.MswTinting.Start ();
                                TextureUtils.ApplyTint (_tex, tintedTex, c);
                                _stats?.MswTinting.Stop ();

                                _stats?.MswEncoding.Start ();
                                icons [tintedName] = tintedTex.EncodeToPNG ();
                                _stats?.MswEncoding.Stop ();

                                Object.Destroy (tintedTex);

                                if (_stats != null) {
                                        _stats.Tints++;
                                }
                        }
                }
                
        }
}