yaze/docs/internal/zscream_analysis.md

ZScreamDungeon Codebase Analysis for YAZE Feature Parity

Date: November 22, 2025 Source Codebase: ZScreamDungeon (C#) Target Project: yaze (C++)

1. Executive Summary

The ZScreamDungeon codebase provides a comprehensive roadmap for implementing feature parity in the 'yaze' project. The core architecture revolves around direct manipulation of a byte array containing the game's ROM data. Data is read from the ROM in a structured way, with Constants.cs providing the necessary memory addresses.

The system uses a custom object factory pattern to parse variable-length object streams from the ROM and instantiate specific C# classes for rendering. Graphics are decompressed on startup and cached, then composited into a final image buffer during the room loading process.

2. Core Architecture

• ROM Handling: The entire ROM is loaded into a static byte[] array in ROM.cs. All reads/writes happen directly on this buffer.
• Address Mapping: Constants.cs serves as the "Rosetta Stone," mapping high-level concepts (headers, sprite pointers, etc.) to physical ROM addresses.
• Room Loading: Rooms are not contiguous blocks. Room.cs orchestrates loading from multiple scattered tables (headers, object data, sprites) based on the room index.

3. Key Data Structures

Rooms (Room.cs)

A room is composed of:

• Header (14 bytes): Contains palettes, blocksets, effect flags, and collision modes.
• Object Data: A variable-length stream of bytes. This is not a fixed array. It uses a custom packing format where the first byte determines the object type (Type 1, 2, or 3) and subsequent bytes define position and size.
• Sprite Data: A separate list of sprites, typically 3 bytes per entry (ID, X, Y), loaded via addSprites().

Room Objects (Room_Object.cs)

• Base class for all interactive elements.
• Factory Pattern: The loadTilesObjects() method in Room.cs parses the byte stream and calls addObject(), which switches on the object ID to instantiate the correct subclass (e.g., Room_Object_Door, Room_Object_Chest).

Graphics (GFX.cs)

• Storage: Graphics are stored compressed in the ROM.
• Decompression: Uses a custom LZ-style algorithm (std_nintendo_decompress).
• Decoding: Raw bitplanes (2bpp/3bpp) are decoded into a 4bpp format usable by the editor's renderer.

4. Rendering Pipeline

1. Initialization: On startup, all graphics packs are decompressed and decoded into a master allgfxBitmap.
2. Room Load: When entering a room, the specific graphics needed for that room (based on the header) are copied into currentgfx16Bitmap.
3. Object Rasterization:
   • Room.cs iterates through all loaded Room_Object instances.
   • It calls obj.Draw().
   • Crucially, Draw() does not render to the screen. It writes tile IDs, palette indices, and flip flags into two 2D arrays: GFX.tilesBg1Buffer and GFX.tilesBg2Buffer (representing the SNES background layers).
4. Final Composition:
   • GFX.DrawBG1() and GFX.DrawBG2() iterate through the buffers.
   • They look up the actual pixel data in currentgfx16Bitmap based on the tile IDs.
   • The pixels are written to the final Bitmap displayed in the UI.

5. Implementation Recommendations for YAZE

To achieve feature parity, yaze should:

1. Replicate the Parsing Logic: The loadTilesObjects() method in Room.cs is the critical path. Porting this logic to C++ is essential for correctly interpreting the room object data stream.
2. Port the Decompressor: The std_nintendo_decompress algorithm in Decompress.cs must be ported to C++ to read graphics and map data.
3. Adopt the Buffer-Based Rendering: Instead of trying to render objects directly to a texture, use an intermediate "Tilemap Buffer" (similar to tilesBg1Buffer). This accurately simulates the SNES PPU architecture where objects are just collections of tilemap entries.
4. Constants Mirroring: Create a C++ header that mirrors Constants.cs. Do not try to derive these addresses algorithmically; the hardcoded values are necessary for compatibility.

6. Key File References

File Path (Relative to ZScreamDungeon)	Key Responsibility	Important Symbols
ZeldaFullEditor/ROM.cs	Raw ROM access	ROM.DATA, ReadByte, WriteByte
ZeldaFullEditor/Constants.cs	Address definitions	room_header_pointer, room_object_pointer
ZeldaFullEditor/Rooms/Room.cs	Room parsing & orchestration	loadTilesObjects(), addSprites(), addObject()
ZeldaFullEditor/Rooms/Room_Object.cs	Object base class	Draw(), draw_tile()
ZeldaFullEditor/GFX.cs	Graphics pipeline	CreateAllGfxData(), DrawBG1(), tilesBg1Buffer
ZeldaFullEditor/ZCompressLibrary/Decompress.cs	Data decompression	std_nintendo_decompress()