From b4f2ce301d564a6b1e84c58b90f4fbefc10bfffd Mon Sep 17 00:00:00 2001
From: Denis <93516910+Bussun@users.noreply.github.com>
Date: Fri, 28 Jan 2022 22:58:37 +0100
Subject: [PATCH] all the code I find seems to be based on thakis code

too bad I just can't figure it out by myself, I'd rewrite it if I could.
I'm pretty sure it's just one little stupid thing that makes it decode it incorrectly for sm3das
---
 BMGEditor/FS/Yaz0Stream.cs | 75 +++++++++++++++++++++++++++-----------
 1 file changed, 54 insertions(+), 21 deletions(-)

diff --git a/BMGEditor/FS/Yaz0Stream.cs b/BMGEditor/FS/Yaz0Stream.cs
index a49b916..b7b8b17 100644
--- a/BMGEditor/FS/Yaz0Stream.cs
+++ b/BMGEditor/FS/Yaz0Stream.cs
@@ -33,36 +33,69 @@ namespace BMGEditor
                 return;
 
             Int32 decompSize = (data[4] << 24 | data[5] << 16 | data[6] << 8 | data[7]);
-            byte[] output = new byte[decompSize];
-            int Offs = 16;
-            int dstoffs = 0;
-            while (true)
+            uint sourcePos = 16; //start at 0x10
+            uint writePos = 0; //destination to write data to
+            byte[] decodedData = new byte[decompSize + 1];
+            byte groupHead = 0;
+            byte lastGroupHead = 0;
+            uint validBitCount = 0;
+
+            while (writePos < decompSize)
             {
-                byte header = data[Offs++];
-                for (int i = 0; i < 8; i++)
+                //System.Windows.Forms.MessageBox.Show(groupHead.ToString("X") + "|" + validBitCount);
+                if (validBitCount == 0) //new group header 
                 {
-                    if ((header & 0x80) != 0) output[dstoffs++] = data[Offs++];
+                    groupHead = data[sourcePos];
+                    lastGroupHead = groupHead;
+                    ++sourcePos;
+                    validBitCount = 8; //reset count
+                }
+                if ((groupHead & 0x80) != 0) //straight copy as long as groupheader maintains left most bit as 1 (1000 0000)
+                {
+                    decodedData[writePos] = data[sourcePos];
+                    writePos++;
+                    sourcePos++;
+                }
+                else
+                {
+                    byte b1 = data[sourcePos]; //byte 1 
+                    byte b2 = data[sourcePos + 1]; //byte 2
+                    sourcePos += 2; //move past those two bytes
+                    uint dist = (uint)((b1 & 0xF) << 8) | b2; //distance
+                    if (Tests.isBE)
+                        dist = (UInt32)((dist >> 24) | ((dist & 0xFF0000) >> 8) | ((dist & 0xFF00) << 8) | (dist << 24));
+
+                    uint copySource = writePos - (dist + 1); //copy 
+                    uint numBytes = (uint)b1 >> 4; //how many bytes to copy
+
+                    //if (sourcePos-2 > 0x13C0) //debug decode
+                    //System.Windows.Forms.MessageBox.Show("lastGroupHead: 0x" + lastGroupHead.ToString("X") + "\n" + "b1: 0x" + b1.ToString("X") + "\n" + "b2: 0x" + b2.ToString("X") + "\n" + "sourcePos: 0x" + sourcePos.ToString("X") + "\n" + "dist: " + dist + "\n" + "copySource: 0x" + copySource.ToString("X") + "\n" + "writePos: 0x" + writePos.ToString("X") + "\n" + "numBytes: " + numBytes);
+
+                    if (numBytes == 0) //If the first 4 bits of the first byte is 0...
+                    {
+                        numBytes = data[sourcePos] + (uint)0x12;
+                        sourcePos++;
+                    }
                     else
                     {
-                        byte b = data[Offs++];
-                        int offs = ((b & 0xF) << 8 | data[Offs++]) + 1;
-                        int length = (b >> 4) + 2;
-                        if (length == 2) length = data[Offs++] + 0x12;
-                        for (int j = 0; j < length; j++)
-                        {
-                            output[dstoffs] = output[dstoffs - offs];
-                            dstoffs++;
-                        }
+                        numBytes += 2;
                     }
-                    if (dstoffs >= decompSize)
+                    for (int i = 0; i < numBytes; ++i)
                     {
-                        Array.Resize(ref data, decompSize);
-                        output.CopyTo(data, 0);
-                        return;
+                        decodedData[writePos] = decodedData[copySource];
+                        copySource++;
+                        writePos++;
                     }
-                    header <<= 1;
                 }
+                groupHead <<= 1; //left shift!!!
+                validBitCount -= 1; //group header validation count of the binary position
             }
+
+            Array.Resize(ref data, decompSize + 1);
+            decodedData.CopyTo(data, 0);
+            FileStream test = File.OpenWrite("./test.bin");
+            test.Write(data);
+            return;
         }
     }
 }