/*---------------------------------------------------------------------------------------------
 *  Copyright (c) Microsoft Corporation. All rights reserved.
 *  Licensed under the MIT License. See License.txt in the project root for license information.
 *--------------------------------------------------------------------------------------------*/
using System;
using System.IO;

namespace Microsoft.Unity.VisualStudio.Editor
{
	public sealed class Image : IDisposable
	{

		long position;
		Stream stream;

		Image(Stream stream)
		{
			this.stream = stream;
			this.position = stream.Position;
			this.stream.Position = 0;
		}

		bool Advance(int length)
		{
			if (stream.Position + length >= stream.Length)
				return false;

			stream.Seek(length, SeekOrigin.Current);
			return true;
		}

		bool MoveTo(uint position)
		{
			if (position >= stream.Length)
				return false;

			stream.Position = position;
			return true;
		}

		void IDisposable.Dispose()
		{
			stream.Position = position;
		}

		ushort ReadUInt16()
		{
			return (ushort)(stream.ReadByte()
				| (stream.ReadByte() << 8));
		}

		uint ReadUInt32()
		{
			return (uint)(stream.ReadByte()
				| (stream.ReadByte() << 8)
				| (stream.ReadByte() << 16)
				| (stream.ReadByte() << 24));
		}

		bool IsManagedAssembly()
		{
			if (stream.Length < 318)
				return false;
			if (ReadUInt16() != 0x5a4d)
				return false;
			if (!Advance(58))
				return false;
			if (!MoveTo(ReadUInt32()))
				return false;
			if (ReadUInt32() != 0x00004550)
				return false;
			if (!Advance(20))
				return false;
			if (!Advance(ReadUInt16() == 0x20b ? 222 : 206))
				return false;

			return ReadUInt32() != 0;
		}

		public static bool IsAssembly(string file)
		{
			if (file == null)
				throw new ArgumentNullException("file");

			using (var stream = new FileStream(file, FileMode.Open, FileAccess.Read, FileShare.Read))
				return IsAssembly(stream);
		}

		public static bool IsAssembly(Stream stream)
		{
			if (stream == null)
				throw new ArgumentNullException(nameof(stream));
			if (!stream.CanRead)
				throw new ArgumentException(nameof(stream));
			if (!stream.CanSeek)
				throw new ArgumentException(nameof(stream));

			using (var image = new Image(stream))
				return image.IsManagedAssembly();
		}
	}
}