# AGENTS.md - Developer & Agent Guide for snakePlus

This document provides a comprehensive overview of the **snakePlus** codebase for AI agents and developers. It details the project architecture, target platform, hardware constraints, build workflows, code organization, and development guidelines.

---

## 1. Project Overview

- **Project Name:** Snake Plus (`snakePlus`)
- **Subtitle:** *Snake with a twist*
- **Author:** Phillip Kühne
- **Platform:** Nintendo DS (NDS) Homebrew
- **Target CPU:** ARM946E-S (`armv5te`, `-mtune=arm946e-s`, `-marm`, `-mthumb-interwork`)
- **ROM Output:** [`snakePlus.nds`](snakePlus.nds) & [`snakePlus.elf`](snakePlus.elf)
- **Primary Toolchain:** [devkitPro](https://devkitpro.org) / `devkitARM`, `libnds`, `maxmod9`, `grit`, `mmutil`, `bin2s`, `ndstool`
- **Build System:** CMake (>= 3.13) with Ninja generator

---

## 2. Directory Structure & File Map

```
snakePlus/
├── CMakeLists.txt              # Primary CMake cross-compilation & asset build rules
├── compile_commands.json       # Exported compilation database for clangd / clang-tidy
├── icon.bmp                    # 32x32 banner icon embedded into the .nds ROM
├── melonDS.ini                 # Local configuration for melonDS emulator
│
├── include/                    # C++ Header declarations
│   ├── consts.h                # Grid dimensions, scores, tick rates, legacy bitmaps
│   ├── game_audio.h            # AudioContext struct, MaxMod sound & BGM declarations
│   ├── game_bg.h               # BackgroundContext struct, animated dual-screen BG declarations
│   ├── game_input.h            # InputContext struct, button scanning & menu logic
│   ├── game_logic.h            # Snake mechanics, collision detection, apple spawning
│   ├── game_render.h           # RenderAssets struct, dual-screen console/OAM draw routines
│   └── game_state.h            # Core GameState struct, Scene enum, coordinate containers
│
├── source/                     # C++ Implementation files
│   ├── main.cpp                # Hardware setup, VBlank IRQ, timer ticks, game bootstrap
│   ├── game_audio.cpp          # MaxMod soundbank loading, BGM loop, SFX dispatch
│   ├── game_bg.cpp             # Dual-screen procedural tilemaps, scrolling & VBlank palette cycling
│   ├── game_input.cpp          # Key mapping, inverted control logic, reverse-denial, menu navigation
│   ├── game_logic.cpp          # Position update, edge wrapping, collision resolution, RNG
│   └── game_render.cpp         # OAM sprite rendering, console text HUD & menu screens
│
├── gfx/                        # Graphical assets
│   ├── tilemap.png             # 8bpp tile sheet (snake head/body/tail, apples)
│   ├── tilemap.grit            # Grit conversion configuration (-gt, -gB8, -mRtf, etc.)
│   └── tilemap.xcf             # GIMP source image
│
├── sounds/                     # Audio assets (built into soundbank by mmutil)
│   ├── 0rigin.xm               # FastTracker II background music track (MOD_0RIGIN)
│   ├── 273792__pacomav__plop.wav                                  # Movement plop SFX
│   ├── 332629__treasuresounds__item-pickup.wav                    # Apple pickup SFX
│   ├── Boom.wav                                                   # Rotten / Cursed apple boom SFX
│   ├── denied.wav                                                  # Reverse-direction denied thud SFX (synthesized)
│   ├── 167126__crisstanza__unpause.wav                            # Unpause SFX
│   ├── 167127__crisstanza__pause.wav                              # Pause SFX
│   └── 73750__timbre__remix-of-benboncan-sad-trombone-more-wah-bright-de-clicked.wav # Fail SFX
│
├── tools/                      # Tooling & automation scripts
│   └── clang_tidy_watch.sh     # File watcher script running clang-tidy on file change
│
├── .vscode/                    # VS Code configuration
│   ├── launch.json             # GDB debugger launch for melonDS (port 3333)
│   ├── settings.json           # clangd and editor settings
│   └── tasks.json              # CMake build, run, clean, and clang-tidy tasks
│
├── .clang-tidy                 # Clang-tidy rules and naming conventions
├── .clangd                     # Clangd LSP configuration and system include paths
└── .gitignore                  # Git ignore rules for build artifacts and caches
```

---

## 3. Game Mechanics & State Flow

### 3.1 Grid & Screen Setup
- The playfield is a **32 × 24 grid** (`X_MAX = 31`, `Y_MAX = 23`).
- Each cell corresponds to an **8 × 8 pixel** sprite tile.
- Moving outside the boundary wraps around to the opposing edge.
- **Dual-Screen Layout:**
  - **Top Screen:** Console text display (Score, status effects like "CONTROLS INVERTED!", pause state, debug internals HUD).
  - **Bottom Screen:** Sub-engine 2D graphics (`oamSub`), rendering the snake and apples using 8x8 256-color sprites.
  - Can be swapped in real-time by pressing the **R** trigger (`lcdSwap()`).

### 3.2 Scenes & State Machine
The game operates in one of two scenes ([`Scene`](include/game_state.h#L9-L12)):
1. **`Scene::Menu`**:
   - Menu options: **Start Game**, **Hard Mode: ON/OFF**, **Music: ON/OFF**.
   - Navigated via D-Pad Up/Down; toggled/started via **A** or **START**.
2. **`Scene::Playing`**:
   - Driven by hardware **Timer 0** executing [`do_tick()`](source/main.cpp#L127-L163).
   - Interrupt handler [`handle_vblank()`](source/main.cpp#L117-L125) handles VBlank frame counting, OAM updates (`oamUpdate(&oamSub)`), and top-screen stat refreshes.

### 3.3 Game Elements & Apples
- **Snake Body:** Represented as `std::deque<std::array<int, 3>>` where each entry is `{x, y, direction}`. Alternating segments use regular and small sprite tiles. Head and tail sprites flip dynamically according to direction.
- **Immediate Head Rotation Feedback:** When the player presses a D-Pad direction while moving, `handle_input_frame()` validates that the input is not a reverse into the neck. If valid, `state.snakeDirection` is immediately updated and `draw_game()` is called on the main thread before the next movement tick. In `draw_snake_head()`, the head sprite tile pointer (`gfx_head_v` vs `gfx_head_h`) and hardware flip flags (`hflip`, `vflip`) are selected directly from `state.snakeDirection` and written to shadow OAM (`oamSub`). On the subsequent 60 FPS VBlank (~16.6 ms), `handle_vblank()` commits shadow OAM to hardware registers via `oamUpdate(&oamSub)`, providing instantaneous visual feedback that the snake has turned to face the new heading before the slower Timer 0 tick (5.0–17.0 Hz) advances grid coordinates via `update_snake()`.
- **Normal Apple (Red):** Spawns if count < 1. Awards +10 score, increases length by 1, plays pickup sound. Flashes/blinks every 4 ticks.
- **Rotten Apple (Bruised Amber & Toxic Green Mold):** Spawns in Hard Mode (periodic chance up to 20 on screen). Decreases length by 1, plays explosion SFX. If length drops below 2, triggers Game Over. Rendered using a dedicated handcrafted 8x8 sprite with bruised golden-amber decaying skin, sunken dark necrotic rot cavities, a withered dead twig stem, and festering green mold colonies with radiant toxic spore pustules. Uses high-contrast dedicated palette entries (`ROTTEN_PAL_OUTLINE`, `ROTTEN_PAL_SKIN`, `ROTTEN_PAL_DECAY`, `ROTTEN_PAL_MOLD`, `ROTTEN_PAL_SPORE`, `ROTTEN_PAL_STEM`) specifically tuned for high visibility and contrast against the Hard Mode crimson background. Its mold and spore pustules organically breathe and fester with toxic bioluminescence every frame at 60 FPS in [`handle_vblank()`](source/main.cpp), and sprites alternate horizontal orientation based on grid coordinates (`hflip`) in [`draw_apples_rotten()`](source/game_render.cpp) for natural visual variety across the playfield. Despawns after `BAD_APPLE_LIFETIME_TICKS` (80 ticks).
- **Cursed Apple (Electric Neon Purple & Radioactive Green):** Spawns occasionally. Inverts player steering controls (Up ↔ Down, Left ↔ Right) for `CURSE_DURATION_TICKS` (40 ticks). Rendered using a dedicated 8x8 sprite with a glowing core rune and toxic green horn, using high-contrast dedicated palette entries (`CURSE_PAL_OUTLINE`, `CURSE_PAL_FILL`, `CURSE_PAL_GLOW`, `CURSE_PAL_CORE`, `CURSE_PAL_LEAF`). Its palette dynamically breathes and pulses with radioactive energy every frame at 60 FPS in [`handle_vblank()`](source/main.cpp#L134-L153), and its sprite sways/twitches with mystical instability in [`draw_apples_cursed()`](source/game_render.cpp#L139-L151). Despawns after `CURSED_APPLE_LIFETIME_TICKS` (90 ticks), clearing the way for new cursed apples to spawn if previously avoided.
- **Reverse-Direction Denial:** Pressing the D-Pad in the exact opposite direction (i.e. into the snake's own neck) is silently denied instead of causing an immediate Game Over. The game computes the would-be next head position (including edge-wrapping) and compares it to the neck segment; if they match, the direction change is rejected and a subtle "denied" thud SFX plays.

### 3.4 Game Modes & Progressive Speed Mechanics
- **Normal Mode:** Starts with length 4, base timer frequency 5.0 Hz (`SPEED_BASE_NORMAL_MHZ = 5000`), scaling up to 13.0 Hz (`SPEED_MAX_NORMAL_MHZ = 13000`).
- **Hard Mode:** Starts with length 2, base timer frequency 5.0 Hz (`SPEED_BASE_HARD_MHZ = 5000`, matching Normal Mode's starting speed), scaling up to 17.0 Hz (`SPEED_MAX_HARD_MHZ = 17000`), spawns rotten apples, and spawns temporary fatal barriers.
- **Temporary Fatal Barriers (Hard Mode):** Straight lines of blocks appear dynamically across the playfield without overlapping the snake or apples. Barriers only begin spawning after a few apples have been eaten (`BARRIER_START_APPLES = 3`). Their maximum possible length begins at 4 blocks (`BARRIER_BASE_MAX_LENGTH`) and gradually scales up to 10 blocks (`BARRIER_MAX_LENGTH`) later on in the game. At later stages of the game, multiple barriers can spawn simultaneously in a single spawn event (up to 2 at once starting at 16 apples, and up to 3 at once starting at 30 apples), synchronizing their warning and activation phases. Before becoming lethal, each barrier announces itself with a flashing warning reticle frame (`BARRIER_WARN_FRAMES = 120` frames / 2.0 s) whose pulse frequency accelerates and warning color intensifies from dim orange to incandescent yellow-white as solidification approaches. Once active, barriers turn into solid hazard-striped energy walls (`BARRIER_ACTIVE_FRAMES = 240` frames / 4.0 s) fatal to the snake's head on contact. As apples are eaten, spawn intervals accelerate from 9.0 s down to 2.0 s (`BARRIER_INTERVAL_MAX_FRAMES` to `BARRIER_INTERVAL_MIN_FRAMES`) and max concurrent barriers scale up from 1 to 4 (`BARRIER_MAX_COUNT`).
- **Progressive Speed Scaling:** Each consumed regular apple increases `state.applesEaten`. Game speed scales linearly with consumed apples up to `SPEED_APPLES_CAP = 40`, granting smooth +200 mHz acceleration per apple (or +300 mHz in Hard Mode). Timer 0 reload registers are recomputed on the fly (`calculate_timer_reload()`).
- **Progressive Background Intensity Scaling:** Visual effects intensify at the exact same rate (`calculate_intensity_31()`):
  - Hardware drift scroll speeds accelerate on both top and bottom screens.
  - Celestial node oscillation & twinkling breathing frequencies increase via fixed-point phase accumulation (`anim_phase_fp`).
  - Color palettes dynamically interpolate from calm cyber tones towards blazing electric plasma cyan (Normal) or volcanic hyper-crimson / solar white-gold (Hard).
  - Deep-space backdrop colors illuminate in tandem with the intensity level.
  - Apple pickup shockwave pulses flare with heightened luminance.

---

## 4. Input & Control Mapping

| Context | Button | Action |
|---|---|---|
| **Menu** | D-Pad Up / Down | Change selected menu item |
| **Menu** | A / START | Select / Toggle option / Start game |
| **Playing** | D-Pad | Steer snake (inverted if cursed; reversing into neck is denied with subtle SFX) |
| **Playing** | START | Pause / Unpause |
| **Playing** | R | Swap LCD displays (`lcdSwap()`) |
| **Playing** | SELECT | Toggle debug internals HUD overlay |
| **Playing** | START + L | Exit main loop / Quit |
| **Paused** | B | Return to Main Menu |
| **Paused** | X | Toggle background music on/off |
| **Paused** | Y | Toggle Normal/Hard mode and restart |
| **Game Over** | START | Restart game |
| **Game Over** | B | Return to Main Menu |

---

## 5. Build System & Toolchain

### Prerequisites
- `DEVKITPRO` environment variable pointing to the devkitPro root (e.g. `/opt/devkitpro`).
- `devkitARM` toolchain with GCC cross-compiler (`arm-none-eabi-gcc`, `arm-none-eabi-g++`).
- Tools on PATH or in `$DEVKITPRO/tools/bin`: `ndstool`, `grit`, `mmutil`, `bin2s`.
- Build tools: `cmake` (>= 3.13), `ninja`.
- Emulator: `melonDS`.

### Build Commands
To configure and build from the command line:

```bash
# Configure with Ninja generator
cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_EXPORT_COMPILE_COMMANDS=ON

# Copy compile_commands.json to root for clangd
cmake -E copy_if_different build/compile_commands.json compile_commands.json

# Build the NDS ROM image
cmake --build build
```

### Running & Emulation
```bash
# Run using the custom CMake target (launches melonDS)
cmake --build build --target run

# Or launch directly with melonDS
melonDS snakePlus.nds
```

### VS Code Integration
The repository includes preconfigured tasks in [`.vscode/tasks.json`](.vscode/tasks.json):
- **`Run new build`** (Default Build Task, Ctrl+Shift+B): Configures, compiles, and runs in `melonDS`.
- **`cmake build`**: Compiles the project.
- **`cmake clean`**: Cleans build artifacts.
- **`clang-tidy (all)`**: Runs `run-clang-tidy` across all source and header files.
- **`clang-tidy (watch changed)`**: Runs the background watcher [`tools/clang_tidy_watch.sh`](tools/clang_tidy_watch.sh).
- **`Copy new build to SD`**: Copies ROM to external SD card mount.

---

## 6. Architecture & Implementation Details

### 6.1 Hardware Subsystems ([`source/main.cpp`](source/main.cpp))
- **Video Modes:** Both Main and Sub 2D engines are initialized into `MODE_0_2D`.
- **VRAM Allocation:**
  - `VRAM_A`: Assigned to Main Engine Background (`VRAM_A_MAIN_BG`).
  - `VRAM_C`: Assigned to Sub Engine Background (`VRAM_C_SUB_BG`).
  - `VRAM_D`: Assigned to Sub Engine Sprites (`VRAM_D_SUB_SPRITE`).
- **Consoles:** Standard text consoles are attached to both screens for debug / stats / menus.
- **OAM:** Sub-engine OAM is initialized via `oamInit(&oamSub, SpriteMapping_1D_32, false)`. Sprites are allocated with `oamAllocateGfx(&oamSub, SpriteSize_8x8, SpriteColorFormat_256Color)`.
- **Timer & IRQ:** Hardware Timer 0 triggers `do_tick` using `TIMER_FREQ_1024()`. Interrupt `IRQ_VBLANK` runs `handle_vblank()`.

### 6.2 Asset Pipeline
- **Graphics Pipeline:**
  - [`gfx/tilemap.png`](gfx/tilemap.png) + [`gfx/tilemap.grit`](gfx/tilemap.grit) are processed by `grit` into `${CMAKE_CURRENT_BINARY_DIR}/tilemap.s` and `tilemap.h`.
  - Sprites are referenced by index: tile 1 (segment), 2 (small segment), 3 (head horizontal), 4 (head vertical), 5/6 (apple flash frames), 7/8/9/10 (tail variations). Special entities (Rotten Apple, Cursed Apple, Barrier solid, and Barrier warning reticle) use dedicated handcrafted 8x8 256-color tile data copied directly into VRAM at startup. All sprites are preloaded once into VRAM; directional, orientation, and blinking variants are rendered via pointer selection and hardware flipping flags without mid-frame DMA transfers.
  - Active OAM sprites are clamped at hardware limit (128) and trailing unused sprites are hidden every frame via `oamClear(&oamSub, oamId, 128 - oamId)`.
  - **Decoupled Steering & Render Flow:** Steering inputs update `state.snakeDirection` and invoke `draw_game()` immediately on the main thread, updating the snake head's sprite pointer and flipping flags in shadow OAM (`oamSub`). Display updates are synchronized via VBlank (`handle_vblank()`), which flushes `oamSub` to hardware registers with `oamUpdate(&oamSub)` at 60 FPS (~16.6 ms) independently of the slower Timer 0 movement ticks (5.0–17.0 Hz).
  - Console text HUD rendering in `draw_stats()` is throttled via dirty-flag caching (`HudCache`), eliminating redundant `iprintf` calls on VBlank.
- **Audio Pipeline:**
  - Audio files in [`sounds/`](sounds/) are compiled by `mmutil` into `soundbank.bin` and header `soundbank.h`.
  - `bin2s` embeds `soundbank.bin` into assembly source `soundbank_bin.S` and header `soundbank_bin.h`.
  - Audio runs via `libnds`'s `maxmod9` (`mm9`).

### 6.3 Background Pipeline ([`include/game_bg.h`](include/game_bg.h), [`source/game_bg.cpp`](source/game_bg.cpp))
- **Dual-Engine Tiled Layers:** Both Main and Sub 2D engines allocate Background Layer 2 (`BgType_Text4bpp`, `BgSize_T_256x256`, `mapBase = 28`, `tileBase = 1`).
  - **Main Engine:** BG2 renders a dynamic cyber horizon, corner brackets, and starfield behind the top-screen console HUD and menu options (`bgSetPriority(topScreen.bgId, 0)`, `bgSetPriority(bgMainId, 3)`).
  - **Sub Engine:** BG2 renders an animated retro cyber-grid with 8x8 cell alignment dots, ambient celestial nodes, and circuit pathways behind the snake and apple sprites (`bgSetPriority(bottomScreen.bgId, 1)`, `bgSetPriority(bgSubId, 3)`, sprites at priority 0).
- **Smooth 60 FPS Hardware Scrolling:** Both background layers use fixed-point 8.8 subpixel accumulators with `bgSetScroll()` and `bgUpdate()` during VBlank for silky smooth drift at 0% CPU cost. Drift speed accelerates dynamically as apples are consumed.
- **Dynamic VBlank Palette Animation & Progressive Intensity:**
  - **Normal Mode:** Neon cyan / teal / midnight blue theme with oscillating sine-wave twinkling stars and breathing grid nodes, transitioning smoothly towards blazing electric plasma cyan at higher apple counts.
  - **Hard Mode:** Fiery amber / crimson cyber grid with faster pulse rate, transitioning towards volcanic hyper-crimson and incandescent solar white-gold.
  - **Cursed Mode:** Radioactive violet / magenta glitch strobe and high-speed warp drift during inverted controls (further intensified with apple count).
  - **Apple Pickup Wave:** Eating an apple triggers a glowing shockwave pulse (`bg_trigger_pulse()`) that ripples through the grid with luminance scaled by current intensity.
  - **Deep-Space Backdrop Glow:** Screen backdrops illuminate in sync with intensity levels.
  - **Game Over:** Palette dims to a somber dark charcoal / muted red tone with gentle drift deceleration.

### 6.4 Static Analysis & Tooling
- **Clang-Tidy:** Configured in [`.clang-tidy`](.clang-tidy). Enforces modern C++ guidelines, bugprone checks, readability conventions, and snake_case / CamelCase identifiers.
- **Clangd:** Configured in [`.clangd`](.clangd). Removes `-mthumb-interwork` (which clang warns against) and explicitly includes devkitARM GCC 16.1.0 system headers.

---

## 7. Guidelines for AI Agents Modifying the Codebase

1. **Embedded / Bare-Metal Constraints:**
   - The ARM9 CPU lacks an OS runtime. Avoid throwing exceptions or relying on RTTI (`-fno-exceptions -fno-rtti` are compiler flags).
   - Prefer lightweight containers (`std::array`, `std::deque`, `std::vector` with controlled growth).
   - Be mindful of OAM hardware limitations (Sub engine supports up to 128 sprites; current game allocates sprite slots per apple and snake segment).
2. **Interrupt & Concurrency Safety:**
   - Variables accessed across both the main loop, timer ticks, and VBlank IRQ (such as `ticks`, `frame`, `snakelength`, `snakeDirection`, `internals`) must maintain appropriate `volatile` qualifiers or thread-safe synchronization.
   - **IRQ Stack Limit (256 bytes):** In `calico/lib/ds9.ld`, the ARM9 IRQ stack (`sp_irq`) has a total budget of only 256 bytes (`__sp_irq = __sp_usr + 0x100`), growing downwards directly into the user stack area. Therefore:
     - **Zero dynamic allocations in ISRs:** Never perform dynamic heap allocation (`malloc`, `operator new`, vector reallocation) inside hardware interrupt handlers (`do_tick`, `handle_vblank`). Dynamic vectors like `bad_apples`, `cursed_apples`, and `apples` must pre-reserve capacity upfront on the main thread in `setup()` / `init_game()`.
     - **Context objects in `.bss`:** Large structs such as `InputContext` must be allocated in `.bss` (global or static) rather than on `main()`'s DTCM stack frame to avoid stack-smash corruption if an interrupt handler pushes registers.
     - **Safe state re-initialization:** In `init_game()`, call `timerStop(0)` before resetting game state or clearing containers, and only call `timerStart(0, ...)` at the very end when all state members and HUD caches are cleanly initialized.
3. **Hardware Registers & Memory Banking:**
   - Graphic tiles and palettes must be transferred using DMA (`dmaCopy`) or direct memory copies into allocated VRAM / OAM memory banks.
   - Do not modify VRAM allocation configurations without ensuring compatibility with console text generators and OAM sub-engines.
4. **Toolchain Flags:**
   - Keep `-mthumb-interwork`, `-march=armv5te`, `-mtune=arm946e-s`, and specs `-specs=${DEVKITPRO_ROOT}/calico/share/ds9.specs` intact in [`CMakeLists.txt`](CMakeLists.txt).
5. **Code Style:**
   - Follow the naming and check options specified in [`.clang-tidy`](.clang-tidy) (functions/variables in `snake_case`, classes/structs in `CamelCase`).
   - Run `cmake --build build` to verify clean compilation without warnings or errors.
6. **Commit Message Attribution:**
   - Every commit message generated by an AI agent **must** include a `Co-authored-by` trailer, separated from the subject/body by a blank line. Each agent should substitute its own name and contact address:
     ```
     Co-authored-by: <Agent Name> <agent-email-or-url>
     ```
   - Example commit message format:
     ```
     feat: add cursed apple spawn rate tuning

     Expose CURSE_SPAWN_CHANCE constant in consts.h so it can be adjusted
     without touching game logic.

     Co-authored-by: <Agent Name> <agent-email-or-url>
     ```