context: #region Copyright & License Information
  /*
   * Copyright 2007-2020 The OpenRA Developers (see AUTHORS)
   * This file is part of OpenRA, which is free software. It is made
   * available to you under the terms of the GNU General Public License
   * as published by the Free Software Foundation, either version 3 of
   * the License, or (at your option) any later version. For more
   * information, see COPYING.
   */
  #endregion
  
  using System;
  using System.Collections.Generic;
  using System.IO;
  using System.Linq;
  using System.Net;
  using System.Text;
  using ICSharpCode.SharpZipLib.Checksum;
  using ICSharpCode.SharpZipLib.Zip.Compression.Streams;
  using OpenRA.Primitives;
  
  namespace OpenRA.FileFormats
  {
  	public class Png
  	{
  		static readonly byte[] Signature = { 0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a };
  
  		public int Width { get; set; }
  		public int Height { get; set; }
  		public Color[] Palette { get; set; }
  		public byte[] Data { get; set; }
  		public Dictionary<string, string> EmbeddedData = new Dictionary<string, string>();
  
  		public Png(Stream s)
  		{
  			if (!Verify(s))
  				throw new InvalidDataException("PNG Signature is bogus");
  
  			s.Position += 8;
  			var headerParsed = false;
  			var isPaletted = false;
  			var is24Bit = false;
  			var data = new List<byte>();
  
  			while (true)
  			{
  				var length = IPAddress.NetworkToHostOrder(s.ReadInt32());
  				var type = Encoding.UTF8.GetString(s.ReadBytes(4));
  				var content = s.ReadBytes(length);
  				/*var crc = */s.ReadInt32();
  
  				if (!headerParsed && type != "IHDR")
  					throw new InvalidDataException("Invalid PNG file - header does not appear first.");
  
  				using (var ms = new MemoryStream(content))
  				{
  					switch (type)
  					{
  						case "IHDR":
  						{
  							if (headerParsed)
  								throw new InvalidDataException("Invalid PNG file - duplicate header.");
  							Width = IPAddress.NetworkToHostOrder(ms.ReadInt32());
  							Height = IPAddress.NetworkToHostOrder(ms.ReadInt32());
  
  							var bitDepth = ms.ReadUInt8();
  							var colorType = (PngColorType)ms.ReadByte();
  							isPaletted = IsPaletted(bitDepth, colorType);
  							is24Bit = colorType == PngColorType.Color;
  
  							var dataLength = Width * Height;
  							if (!isPaletted)
  								dataLength *= 4;
  
  							Data = new byte[dataLength];
  
  							var compression = ms.ReadByte();
  							/*var filter = */ms.ReadByte();
  							var interlace = ms.ReadByte();
  
  							if (compression != 0)
  								throw new InvalidDataException("Compression method not supported");
  
  							if (interlace != 0)
  								throw new InvalidDataException("Interlacing not supported");
  
  							headerParsed = true;
  
  							break;
  						}
  
  						case "PLTE":
  						{
  							Palette = new Color[256];
  							for (var i = 0; i < length / 3; i++)
  							{
  								var r = ms.ReadByte(); var g = ms.ReadByte(); var b = ms.ReadByte();
  								Palette[i] = Color.FromArgb(r, g, b);
  							}
  
  							break;
  						}
  
  						case "tRNS":
  						{
  							if (Palette == null)
  								throw new InvalidDataException("Non-Palette indexed PNG are not supported.");
  
  							for (var i = 0; i < length; i++)
  								Palette[i] = Color.FromArgb(ms.ReadByte(), Palette[i]);
  
  							break;
  						}
  
  						case "IDAT":
  						{
  							data.AddRange(content);
  
  							break;
  						}
  
  						case "tEXt":
  						{
  							var key = ms.ReadASCIIZ();
  							EmbeddedData.Add(key, ms.ReadASCII(length - key.Length - 1));
  
  							break;
  						}
  
  						case "IEND":
  						{
  							using (var ns = new MemoryStream(data.ToArray()))
  							{
  								using (var ds = new InflaterInputStream(ns))
  								{
  									var pxStride = isPaletted ? 1 : is24Bit ? 3 : 4;
  									var srcStride = Width * pxStride;
  									var destStride = Width * (isPaletted ? 1 : 4);
  
  									var prevLine = new byte[srcStride];
  									for (var y = 0; y < Height; y++)
  									{
  										var filter = (PngFilter)ds.ReadByte();
  										var line = ds.ReadBytes(srcStride);
  
  										for (var i = 0; i < srcStride; i++)
  											line[i] = i < pxStride
  												? UnapplyFilter(filter, line[i], 0, prevLine[i], 0)
  												: UnapplyFilter(filter, line[i], line[i - pxStride], prevLine[i], prevLine[i - pxStride]);
  
  										if (is24Bit)
  										{
  											// Fold alpha channel into RGB data
  											for (var i = 0; i < line.Length / 3; i++)
  											{
  												Array.Copy(line, 3 * i, Data, y * destStride + 4 * i, 3);
  												Data[y * destStride + 4 * i + 3] = 255;
  											}
  										}
  										else
  											Array.Copy(line, 0, Data, y * destStride, line.Length);
  
  										prevLine = line;
  									}
  								}
  							}
  
  							if (isPaletted && Palette == null)
  								throw new InvalidDataException("Non-Palette indexed PNG are not supported.");
  
  							return;
  						}
  					}
  				}
  			}
  		}
  
  		public Png(byte[] data, int width, int height, Color[] palette = null,
  			Dictionary<string, string> embeddedData = null)
  		{
  			var expectLength = width * height;
  			if (palette == null)
  				expectLength *= 4;
  
  			if (data.Length != expectLength)
  				throw new InvalidDataException("Input data does not match expected length");
  
  			Width = width;
  			Height = height;
  
  			Palette = palette;
  			Data = data;
  
  			if (embeddedData != null)
  				EmbeddedData = embeddedData;
  		}
  
  		public static bool Verify(Stream s)
  		{
  			var pos = s.Position;
  			var isPng = Signature.Aggregate(true, (current, t) => current && s.ReadUInt8() == t);
  			s.Position = pos;
  			return isPng;
  		}
  
  		static byte UnapplyFilter(PngFilter f, byte x, byte a, byte b, byte c)
  		{
  			switch (f)
  			{
  				case PngFilter.None: return x;
  
gt: 				case PngFilter.Sub: return (byte)(x + a);