From a345ff5b7072df1832427a54e9e3e5ec1a65e098 Mon Sep 17 00:00:00 2001
From: Daniel Crenna <daniel.crenna@gmail.com>
Date: Sun, 1 Sep 2019 18:54:56 -0400
Subject: [PATCH] implement build independent hash

per https://groups.google.com/forum/#!topic/mono-cecil/AGq0LfBdqjo
---
 Mono.Cecil.PE/Image.cs         |  2 ++
 Mono.Cecil.PE/ImageReader.cs   |  4 +++-
 Mono.Cecil/AssemblyReader.cs   |  1 +
 Mono.Cecil/ModuleDefinition.cs | 26 ++++++++++++++++++++++++++
 4 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/Mono.Cecil.PE/Image.cs b/Mono.Cecil.PE/Image.cs
index 109206187..1e8253cc1 100644
--- a/Mono.Cecil.PE/Image.cs
+++ b/Mono.Cecil.PE/Image.cs
@@ -23,6 +23,7 @@ sealed class Image : IDisposable {
 
 		public Disposable<Stream> Stream;
 		public string FileName;
+		public int FileChecksumPosition;
 
 		public ModuleKind Kind;
 		public string RuntimeVersion;
@@ -40,6 +41,7 @@ sealed class Image : IDisposable {
 
 		public uint EntryPointToken;
 		public uint Timestamp;
+		public int TimestampPosition;
 		public ModuleAttributes Attributes;
 
 		public DataDirectory Win32Resources;
diff --git a/Mono.Cecil.PE/ImageReader.cs b/Mono.Cecil.PE/ImageReader.cs
index 9731ba90b..cd1c36a03 100644
--- a/Mono.Cecil.PE/ImageReader.cs
+++ b/Mono.Cecil.PE/ImageReader.cs
@@ -72,6 +72,7 @@ void ReadImage ()
 			ushort sections = ReadUInt16 ();
 
 			// TimeDateStamp		4
+			image.TimestampPosition = (int)BaseStream.Position;
 			image.Timestamp = ReadUInt32 ();
 			// PointerToSymbolTable	4
 			// NumberOfSymbols		4
@@ -130,7 +131,7 @@ void ReadOptionalHeaders (out ushort subsystem, out ushort dll_characteristics)
 
 			// ImageBase			4 || 8
 			// SectionAlignment		4
-			// FileAlignement		4
+			// FileAlignment		4
 			// OSMajor				2
 			// OSMinor				2
 			// UserMajor			2
@@ -147,6 +148,7 @@ void ReadOptionalHeaders (out ushort subsystem, out ushort dll_characteristics)
 			// HeaderSize			4
 			// FileChecksum			4
 			Advance (16);
+			image.FileChecksumPosition = (int)(BaseStream.Position - 4);
 
 			// SubSystem			2
 			subsystem = ReadUInt16 ();
diff --git a/Mono.Cecil/AssemblyReader.cs b/Mono.Cecil/AssemblyReader.cs
index 1728338c6..6377306e5 100644
--- a/Mono.Cecil/AssemblyReader.cs
+++ b/Mono.Cecil/AssemblyReader.cs
@@ -593,6 +593,7 @@ public ModuleDefinition Populate (ModuleDefinition module)
 			Advance (2); // Generation
 
 			module.Name = ReadString ();
+			module.MvidPosition = position;
 			module.Mvid = ReadGuid ();
 
 			return module;
diff --git a/Mono.Cecil/ModuleDefinition.cs b/Mono.Cecil/ModuleDefinition.cs
index 707a1358f..4f62d5c3a 100644
--- a/Mono.Cecil/ModuleDefinition.cs
+++ b/Mono.Cecil/ModuleDefinition.cs
@@ -11,6 +11,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Security.Cryptography;
 using System.Threading;
 using SR = System.Reflection;
 
@@ -252,6 +253,7 @@ public sealed class ModuleDefinition : ModuleReference, ICustomAttributeProvider
 		ModuleAttributes attributes;
 		ModuleCharacteristics characteristics;
 		Guid mvid;
+		internal int MvidPosition;
 
 		internal ushort linker_version = 8;
 		internal ushort subsystem_major = 4;
@@ -1159,6 +1161,30 @@ public void Write (Stream stream, WriterParameters parameters)
 		}
 	}
 
+	public static class ModuleDefinitionExtensions {
+
+		// See: https://groups.google.com/forum/#!topic/mono-cecil/AGq0LfBdqjo
+		public static byte [] GetBuildIndependentHash (this ModuleDefinition moduleDefinition, HashAlgorithm hashAlgorithm)
+		{
+			var buffer = File.ReadAllBytes (moduleDefinition.Image.FileName);
+
+			// Overwrite parts of the assembly file content byte array (only in memory of course)
+			// that change on every build with an empty byte array, and thus make it comparable 
+
+			Buffer.BlockCopy (new byte [sizeof (uint)], 0, buffer, moduleDefinition.Image.TimestampPosition, sizeof(uint));
+			Buffer.BlockCopy (new byte [sizeof (uint)], 0, buffer, moduleDefinition.Image.FileChecksumPosition, sizeof(uint));
+			Buffer.BlockCopy (new byte [moduleDefinition.Image.GuidHeap.IndexSize], 0, buffer, moduleDefinition.MvidPosition, moduleDefinition.Image.GuidHeap.IndexSize);
+
+			return hashAlgorithm.ComputeHash (buffer);
+		}
+
+		internal static byte [] GetBuildDependentHash (this ModuleDefinition moduleDefinition, HashAlgorithm hashAlgorithm)
+		{
+			var buffer = File.ReadAllBytes (moduleDefinition.Image.FileName);
+			return hashAlgorithm.ComputeHash (buffer);
+		}
+	}
+
 	static partial class Mixin {
 
 		public enum Argument {