// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Collections.Immutable;
using System.IO;
using System.IO.MemoryMappedFiles;
using System.Runtime.CompilerServices;
using System.Text;

namespace ICSharpCode.Decompiler
{
	/// <summary>
	/// Class for dealing with .NET 5 single-file bundles.
	/// 
	/// Based on code from Microsoft.NET.HostModel.
	/// </summary>
	public static class SingleFileBundle
	{
		/// <summary>
		/// Check if the memory-mapped data is a single-file bundle
		/// </summary>
		public static unsafe bool IsBundle(MemoryMappedViewAccessor view, out long bundleHeaderOffset)
		{
			var buffer = view.SafeMemoryMappedViewHandle;
			byte* ptr = null;
			buffer.AcquirePointer(ref ptr);
			try
			{
				return IsBundle(ptr, checked((long)buffer.ByteLength), out bundleHeaderOffset);
			}
			finally
			{
				buffer.ReleasePointer();
			}
		}

		/// <summary>
		/// The 32-byte bundle signature: SHA-256 of ".net core bundle".
		/// </summary>
		internal static ReadOnlySpan<byte> BundleSignature => new byte[] {
			0x8b, 0x12, 0x02, 0xb9, 0x6a, 0x61, 0x20, 0x38,
			0x72, 0x7b, 0x93, 0x02, 0x14, 0xd7, 0xa0, 0x32,
			0x13, 0xf5, 0xb9, 0xe6, 0xef, 0xae, 0x33, 0x18,
			0xee, 0x3b, 0x2d, 0xce, 0x24, 0xb3, 0x6a, 0xae
		};

		public static unsafe bool IsBundle(byte* data, long size, out long bundleHeaderOffset)
		{
			ReadOnlySpan<byte> bundleSignature = BundleSignature;

			// 'end' is the last position at which a full signature still fits, so it is a valid
			// candidate itself: a memory-mapped view reports the exact file length on Unix (Windows
			// rounds it up to a page), and the signature may occupy the final bytes of the file.
			byte* end = data + (size - bundleSignature.Length);
			for (byte* ptr = data; ptr <= end; ptr++)
			{
				if (*ptr == 0x8b && bundleSignature.SequenceEqual(new ReadOnlySpan<byte>(ptr, bundleSignature.Length)))
				{
					// A genuine bundle stores the 8-byte header offset immediately before the
					// signature, so the signature never appears within the first sizeof(long)
					// bytes of the file. Without this guard, a crafted file with the signature at
					// offset 0..7 reads before the start of the (page-aligned, memory-mapped)
					// buffer, faulting on the preceding unmapped page.
					if (ptr - data >= sizeof(long))
					{
						bundleHeaderOffset = Unsafe.ReadUnaligned<long>(ptr - sizeof(long));
						if (bundleHeaderOffset > 0 && bundleHeaderOffset < size)
						{
							return true;
						}
					}
				}
			}

			bundleHeaderOffset = 0;
			return false;
		}

		public struct Header
		{
			public uint MajorVersion;
			public uint MinorVersion;
			public int FileCount;
			public string BundleID;

			// Fields introduced with v2:
			public long DepsJsonOffset;
			public long DepsJsonSize;
			public long RuntimeConfigJsonOffset;
			public long RuntimeConfigJsonSize;
			public ulong Flags;

			public ImmutableArray<Entry> Entries;
		}

		/// <summary>
		/// FileType: Identifies the type of file embedded into the bundle.
		///
		/// The bundler differentiates a few kinds of files via the manifest,
		/// with respect to the way in which they'll be used by the runtime.
		/// </summary>
		public enum FileType : byte
		{
			Unknown,           // Type not determined.
			Assembly,          // IL and R2R Assemblies
			NativeBinary,      // NativeBinaries
			DepsJson,          // .deps.json configuration file
			RuntimeConfigJson, // .runtimeconfig.json configuration file
			Symbols            // PDB Files
		};

		public struct Entry
		{
			public long Offset;
			public long Size;
			public long CompressedSize; // 0 if not compressed, otherwise the compressed size in the bundle
			public FileType Type;
			public string RelativePath; // Path of an embedded file, relative to the Bundle source-directory.
		}

		/// <summary>
		/// Reads the manifest header from the memory mapping.
		/// </summary>
		public static Header ReadManifest(MemoryMappedViewAccessor view, long bundleHeaderOffset)
		{
			using var stream = view.AsStream();
			stream.Seek(bundleHeaderOffset, SeekOrigin.Begin);
			return ReadManifest(stream);
		}

		/// <summary>
		/// Reads the manifest header from the stream.
		/// </summary>
		public static Header ReadManifest(Stream stream)
		{
			var header = new Header();
			using var reader = new BinaryReader(stream, Encoding.UTF8, leaveOpen: true);
			header.MajorVersion = reader.ReadUInt32();
			header.MinorVersion = reader.ReadUInt32();

			// Major versions 3, 4 and 5 were skipped to align bundle versioning with .NET versioning scheme
			if (header.MajorVersion < 1 || header.MajorVersion > 6)
			{
				throw new InvalidDataException($"Unsupported manifest version: {header.MajorVersion}.{header.MinorVersion}");
			}
			header.FileCount = reader.ReadInt32();
			// FileCount is used below to pre-size the entry array. Each entry occupies at least
			// one byte in the stream, so a count larger than the bytes that remain cannot be
			// honest; reject it instead of attempting a huge allocation for a crafted manifest.
			long remainingBytes = stream.Length - stream.Position;
			if (header.FileCount < 0 || header.FileCount > remainingBytes)
			{
				throw new InvalidDataException($"Invalid bundle manifest: FileCount {header.FileCount} exceeds available data.");
			}
			header.BundleID = reader.ReadString();
			if (header.MajorVersion >= 2)
			{
				header.DepsJsonOffset = reader.ReadInt64();
				header.DepsJsonSize = reader.ReadInt64();
				header.RuntimeConfigJsonOffset = reader.ReadInt64();
				header.RuntimeConfigJsonSize = reader.ReadInt64();
				header.Flags = reader.ReadUInt64();
			}
			var entries = ImmutableArray.CreateBuilder<Entry>(header.FileCount);
			for (int i = 0; i < header.FileCount; i++)
			{
				entries.Add(ReadEntry(reader, header.MajorVersion));
			}
			header.Entries = entries.MoveToImmutable();
			return header;
		}

		private static Entry ReadEntry(BinaryReader reader, uint bundleMajorVersion)
		{
			Entry entry;
			entry.Offset = reader.ReadInt64();
			entry.Size = reader.ReadInt64();
			entry.CompressedSize = bundleMajorVersion >= 6 ? reader.ReadInt64() : 0;
			entry.Type = (FileType)reader.ReadByte();
			entry.RelativePath = reader.ReadString();
			return entry;
		}
	}
}