From 294dbff40a068fed56b6470c77df4aaeaf5ff32c Mon Sep 17 00:00:00 2001
From: Pavel Djundik <xPaw@users.noreply.github.com>
Date: Tue, 13 Aug 2024 17:48:06 +0300
Subject: [PATCH] Optimize calculating adler hashes to not allocate entire
 chunk in memory

---
 DepotDownloader/ContentDownloader.cs | 16 ++++++++---
 DepotDownloader/Util.cs              | 40 +++++++---------------------
 2 files changed, 21 insertions(+), 35 deletions(-)

diff --git a/DepotDownloader/ContentDownloader.cs b/DepotDownloader/ContentDownloader.cs
index 65248504..726df929 100644
--- a/DepotDownloader/ContentDownloader.cs
+++ b/DepotDownloader/ContentDownloader.cs
@@ -1059,10 +1059,18 @@ namespace DepotDownloader
                             {
                                 fsOld.Seek((long)match.OldChunk.Offset, SeekOrigin.Begin);
 
-                                var tmp = new byte[match.OldChunk.UncompressedLength];
-                                fsOld.Read(tmp, 0, tmp.Length);
+                                uint a = 0, b = 0;
 
-                                var adler = Util.AdlerHash(tmp);
+                                for (var i = 0; i < match.OldChunk.UncompressedLength; i++)
+                                {
+                                    var c = (uint)fsOld.ReadByte();
+
+                                    // adler hash
+                                    a = (a + c) % 65521;
+                                    b = (b + a) % 65521;
+                                }
+
+                                var adler = BitConverter.GetBytes(a | (b << 16));
                                 if (!adler.SequenceEqual(match.OldChunk.Checksum))
                                 {
                                     neededChunks.Add(match.NewChunk);
@@ -1191,7 +1199,7 @@ namespace DepotDownloader
             var depot = depotFilesData.depotDownloadInfo;
             var depotDownloadCounter = depotFilesData.depotCounter;
 
-            var chunkID = Util.EncodeHexString(chunk.ChunkID);
+            var chunkID = Convert.ToHexString(chunk.ChunkID).ToLowerInvariant();
 
             var data = new DepotManifest.ChunkData
             {
diff --git a/DepotDownloader/Util.cs b/DepotDownloader/Util.cs
index 3cda8dab..5157d92e 100644
--- a/DepotDownloader/Util.cs
+++ b/DepotDownloader/Util.cs
@@ -78,26 +78,23 @@ namespace DepotDownloader
         public static List<ProtoManifest.ChunkData> ValidateSteam3FileChecksums(FileStream fs, ProtoManifest.ChunkData[] chunkdata)
         {
             var neededChunks = new List<ProtoManifest.ChunkData>();
-            int read;
 
             foreach (var data in chunkdata)
             {
-                var chunk = new byte[data.UncompressedLength];
                 fs.Seek((long)data.Offset, SeekOrigin.Begin);
-                read = fs.Read(chunk, 0, (int)data.UncompressedLength);
 
-                byte[] tempchunk;
-                if (read < data.UncompressedLength)
-                {
-                    tempchunk = new byte[read];
-                    Array.Copy(chunk, 0, tempchunk, 0, read);
-                }
-                else
+                uint a = 0, b = 0;
+
+                for (var i = 0; i < data.UncompressedLength; i++)
                 {
-                    tempchunk = chunk;
+                    var c = (uint)fs.ReadByte();
+
+                    // adler hash
+                    a = (a + c) % 65521;
+                    b = (b + a) % 65521;
                 }
 
-                var adler = AdlerHash(tempchunk);
+                var adler = BitConverter.GetBytes(a | (b << 16));
                 if (!adler.SequenceEqual(data.Checksum))
                 {
                     neededChunks.Add(data);
@@ -107,18 +104,6 @@ namespace DepotDownloader
             return neededChunks;
         }
 
-        public static byte[] AdlerHash(byte[] input)
-        {
-            uint a = 0, b = 0;
-            for (var i = 0; i < input.Length; i++)
-            {
-                a = (a + input[i]) % 65521;
-                b = (b + a) % 65521;
-            }
-
-            return BitConverter.GetBytes(a | (b << 16));
-        }
-
         public static byte[] DecodeHexString(string hex)
         {
             if (hex == null)
@@ -133,13 +118,6 @@ namespace DepotDownloader
             return bytes;
         }
 
-        public static string EncodeHexString(byte[] input)
-        {
-            return input.Aggregate(new StringBuilder(),
-                (sb, v) => sb.Append(v.ToString("x2"))
-            ).ToString();
-        }
-
         /// <summary>
         /// Decrypts using AES/ECB/PKCS7
         /// </summary>