source: TFP-WebServer/MapRendering/src/MapTileCache.cs@ 501

using System;
using System.Diagnostics.CodeAnalysis;
using System.IO;
using UnityEngine;
using UnityEngine.Profiling;
using Webserver.FileCache;
using Object = UnityEngine.Object;

namespace MapRendering {
        // Special "cache" for map tile folder as both map rendering and webserver access files in there.
        // Only map rendering tiles are cached. Writing is done by WriteThrough.
        public class MapTileCache : AbstractCache {
                private readonly byte[] transparentTile;
                private CurrentZoomFile[] cache;

                public MapTileCache (int _tileSize) {
                        Texture2D tex = new Texture2D (_tileSize, _tileSize);
                        Color nullColor = new Color (0, 0, 0, 0);
                        for (int x = 0; x < _tileSize; x++) {
                                for (int y = 0; y < _tileSize; y++) {
                                        tex.SetPixel (x, y, nullColor);
                                }
                        }

                        transparentTile = tex.EncodeToPNG ();
                        Object.Destroy (tex);
                }

                // SetZoomCount only called before processing happens in MapRenderer.ctor, no locking required
                [SuppressMessage ("ReSharper", "InconsistentlySynchronizedField")]
                public void SetZoomCount (int _count) {
                        cache = new CurrentZoomFile[_count];
                        for (int i = 0; i < cache.Length; i++) {
                                cache [i] = new CurrentZoomFile ();
                        }
                }

                public byte[] LoadTile (int _zoomlevel, string _filename) {
                        try {
                                lock (cache) {
                                        CurrentZoomFile cacheEntry = cache [_zoomlevel];

                                        if (cacheEntry.filename != null && cacheEntry.filename.Equals (_filename)) {
                                                return cacheEntry.pngData;
                                        }

                                        cacheEntry.filename = _filename;

                                        if (!File.Exists (_filename)) {
                                                cacheEntry.pngData = null;
                                                return null;
                                        }

                                        Profiler.BeginSample ("ReadPng");
                                        cacheEntry.pngData = ReadAllBytes (_filename);
                                        Profiler.EndSample ();

                                        return cacheEntry.pngData;
                                }
                        } catch (Exception e) {
                                Log.Warning ($"Error in MapTileCache.LoadTile: {e}");
                        }

                        return null;
                }

                public void SaveTile (int _zoomlevel, byte[] _contentPng) {
                        try {
                                lock (cache) {
                                        CurrentZoomFile cacheEntry = cache [_zoomlevel];

                                        string file = cacheEntry.filename;
                                        if (string.IsNullOrEmpty (file)) {
                                                return;
                                        }
                                        
                                        cacheEntry.pngData = _contentPng;

                                        Profiler.BeginSample ("WritePng");
                                        using (Stream stream = new FileStream (file, FileMode.Create, FileAccess.ReadWrite, FileShare.None,
                                                4096)) {
                                                stream.Write (_contentPng, 0, _contentPng.Length);
                                        }
                                        Profiler.EndSample ();
                                }
                        } catch (Exception e) {
                                Log.Warning ($"Error in MapTileCache.SaveTile: {e}");
                        }
                }

                public void ResetTile (int _zoomlevel) {
                        try {
                                lock (cache) {
                                        cache [_zoomlevel].filename = null;
                                        cache [_zoomlevel].pngData = null;
                                }
                        } catch (Exception e) {
                                Log.Warning ($"Error in MapTileCache.ResetTile: {e}");
                        }
                }

                public override byte[] GetFileContent (string _filename) {
                        try {
                                lock (cache) {
                                        foreach (CurrentZoomFile czf in cache) {
                                                if (czf.filename != null && czf.filename.Equals (_filename)) {
                                                        return czf.pngData;
                                                }
                                        }

                                        return !File.Exists (_filename) ? transparentTile : ReadAllBytes (_filename);
                                }
                        } catch (Exception e) {
                                Log.Warning ($"Error in MapTileCache.GetFileContent: {e}");
                        }

                        return null;
                }

                public override (int, int) Invalidate () {
                        return (0, 0);
                }

                private static byte[] ReadAllBytes (string _path) {
                        using FileStream fileStream = new FileStream(_path, FileMode.Open, FileAccess.Read, FileShare.Read, 4096);
                        
                        int bytesRead = 0;
                        int bytesLeft = (int) fileStream.Length;
                        byte[] result = new byte[bytesLeft];
                        while (bytesLeft > 0) {
                                int readThisTime = fileStream.Read (result, bytesRead, bytesLeft);
                                if (readThisTime == 0) {
                                        throw new IOException ("Unexpected end of stream");
                                }

                                bytesRead += readThisTime;
                                bytesLeft -= readThisTime;
                        }

                        return result;
                }


                private class CurrentZoomFile {
                        public string filename;
                        public byte[] pngData;
                }
        }
}