250 lines
17 KiB
Markdown
250 lines
17 KiB
Markdown
# AGENTS.md - Developer & Agent Guide for snakePlus
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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.
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---
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## 1. Project Overview
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- **Project Name:** Snake Plus (`snakePlus`)
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- **Subtitle:** *Snake with a twist*
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- **Author:** Phillip Kühne
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- **Platform:** Nintendo DS (NDS) Homebrew
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- **Target CPU:** ARM946E-S (`armv5te`, `-mtune=arm946e-s`, `-marm`, `-mthumb-interwork`)
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- **ROM Output:** [`snakePlus.nds`](snakePlus.nds) & [`snakePlus.elf`](snakePlus.elf)
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- **Primary Toolchain:** [devkitPro](https://devkitpro.org) / `devkitARM`, `libnds`, `maxmod9`, `grit`, `mmutil`, `bin2s`, `ndstool`
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- **Build System:** CMake (>= 3.13) with Ninja generator
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---
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## 2. Directory Structure & File Map
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```
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snakePlus/
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├── CMakeLists.txt # Primary CMake cross-compilation & asset build rules
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├── compile_commands.json # Exported compilation database for clangd / clang-tidy
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├── icon.bmp # 32x32 banner icon embedded into the .nds ROM
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├── melonDS.ini # Local configuration for melonDS emulator
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│
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├── include/ # C++ Header declarations
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│ ├── consts.h # Grid dimensions, scores, tick rates, legacy bitmaps
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│ ├── game_audio.h # AudioContext struct, MaxMod sound & BGM declarations
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│ ├── game_bg.h # BackgroundContext struct, animated dual-screen BG declarations
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│ ├── game_input.h # InputContext struct, button scanning & menu logic
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│ ├── game_logic.h # Snake mechanics, collision detection, apple spawning
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│ ├── game_render.h # RenderAssets struct, dual-screen console/OAM draw routines
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│ └── game_state.h # Core GameState struct, Scene enum, coordinate containers
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│
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├── source/ # C++ Implementation files
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│ ├── main.cpp # Hardware setup, VBlank IRQ, timer ticks, game bootstrap
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│ ├── game_audio.cpp # MaxMod soundbank loading, BGM loop, SFX dispatch
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│ ├── game_bg.cpp # Dual-screen procedural tilemaps, scrolling & VBlank palette cycling
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│ ├── game_input.cpp # Key mapping, inverted control logic, reverse-denial, menu navigation
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│ ├── game_logic.cpp # Position update, edge wrapping, collision resolution, RNG
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│ └── game_render.cpp # OAM sprite rendering, console text HUD & menu screens
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│
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├── gfx/ # Graphical assets
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│ ├── tilemap.png # 8bpp tile sheet (snake head/body/tail, apples)
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│ ├── tilemap.grit # Grit conversion configuration (-gt, -gB8, -mRtf, etc.)
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│ └── tilemap.xcf # GIMP source image
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│
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├── sounds/ # Audio assets (built into soundbank by mmutil)
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│ ├── 0rigin.xm # FastTracker II background music track (MOD_0RIGIN)
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│ ├── 273792__pacomav__plop.wav # Movement plop SFX
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│ ├── 332629__treasuresounds__item-pickup.wav # Apple pickup SFX
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│ ├── Boom.wav # Rotten / Cursed apple boom SFX
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│ ├── denied.wav # Reverse-direction denied thud SFX (synthesized)
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│ ├── 167126__crisstanza__unpause.wav # Unpause SFX
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│ ├── 167127__crisstanza__pause.wav # Pause SFX
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│ └── 73750__timbre__remix-of-benboncan-sad-trombone-more-wah-bright-de-clicked.wav # Fail SFX
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│
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├── tools/ # Tooling & automation scripts
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│ └── clang_tidy_watch.sh # File watcher script running clang-tidy on file change
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│
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├── .vscode/ # VS Code configuration
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│ ├── launch.json # GDB debugger launch for melonDS (port 3333)
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│ ├── settings.json # clangd and editor settings
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│ └── tasks.json # CMake build, run, clean, and clang-tidy tasks
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│
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├── .clang-tidy # Clang-tidy rules and naming conventions
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├── .clangd # Clangd LSP configuration and system include paths
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└── .gitignore # Git ignore rules for build artifacts and caches
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```
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---
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## 3. Game Mechanics & State Flow
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### 3.1 Grid & Screen Setup
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- The playfield is a **32 × 24 grid** (`X_MAX = 31`, `Y_MAX = 23`).
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- Each cell corresponds to an **8 × 8 pixel** sprite tile.
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- Moving outside the boundary wraps around to the opposing edge.
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- **Dual-Screen Layout:**
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- **Top Screen:** Console text display (Score, status effects like "CONTROLS INVERTED!", pause state, debug internals HUD).
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- **Bottom Screen:** Sub-engine 2D graphics (`oamSub`), rendering the snake and apples using 8x8 256-color sprites.
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- Can be swapped in real-time by pressing the **R** trigger (`lcdSwap()`).
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### 3.2 Scenes & State Machine
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The game operates in one of two scenes ([`Scene`](include/game_state.h#L9-L12)):
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1. **`Scene::Menu`**:
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- Menu options: **Start Game**, **Hard Mode: ON/OFF**, **Music: ON/OFF**.
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- Navigated via D-Pad Up/Down; toggled/started via **A** or **START**.
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2. **`Scene::Playing`**:
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- Driven by hardware **Timer 0** executing [`do_tick()`](source/main.cpp#L127-L163).
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- Interrupt handler [`handle_vblank()`](source/main.cpp#L117-L125) handles VBlank frame counting, OAM updates (`oamUpdate(&oamSub)`), and top-screen stat refreshes.
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### 3.3 Game Elements & Apples
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- **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.
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- **Normal Apple (Red):** Spawns if count < 1. Awards +10 score, increases length by 1, plays pickup sound. Flashes/blinks every 4 ticks.
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- **Rotten Apple (Brown/Green):** 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.
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- **Cursed Apple (Purple):** Spawns occasionally. Inverts player steering controls (Up ↔ Down, Left ↔ Right) for `CURSE_DURATION_TICKS` (40 ticks). The sprite is procedurally constructed at startup in [`main.cpp:91-112`](source/main.cpp#L91-L112) by copying the rotten apple tile pixels and mapping them to dedicated palette entries (`CURSE_PAL_SHADOW`, `CURSE_PAL_OUTLINE`, `CURSE_PAL_FILL`, `CURSE_PAL_GLOW`).
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- **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.
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### 3.4 Game Modes & Progressive Speed Mechanics
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- **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`).
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- **Hard Mode:** Starts with length 2, base timer frequency 9.0 Hz (`SPEED_BASE_HARD_MHZ = 9000`), scaling up to 17.0 Hz (`SPEED_MAX_HARD_MHZ = 17000`), and spawns rotten apples.
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- **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. Timer 0 reload registers are recomputed on the fly (`calculate_timer_reload()`).
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- **Progressive Background Intensity Scaling:** Visual effects intensify at the exact same rate (`calculate_intensity_31()`):
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- Hardware drift scroll speeds accelerate on both top and bottom screens.
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- Celestial node oscillation & twinkling breathing frequencies increase via fixed-point phase accumulation (`anim_phase_fp`).
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- Color palettes dynamically interpolate from calm cyber tones towards blazing electric plasma cyan (Normal) or volcanic hyper-crimson / solar white-gold (Hard).
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- Deep-space backdrop colors illuminate in tandem with the intensity level.
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- Apple pickup shockwave pulses flare with heightened luminance.
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---
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## 4. Input & Control Mapping
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| Context | Button | Action |
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| **Menu** | D-Pad Up / Down | Change selected menu item |
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| **Menu** | A / START | Select / Toggle option / Start game |
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| **Playing** | D-Pad | Steer snake (inverted if cursed; reversing into neck is denied with subtle SFX) |
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| **Playing** | START | Pause / Unpause |
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| **Playing** | R | Swap LCD displays (`lcdSwap()`) |
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| **Playing** | SELECT | Toggle debug internals HUD overlay |
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| **Playing** | START + L | Exit main loop / Quit |
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| **Paused** | B | Return to Main Menu |
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| **Paused** | X | Toggle background music on/off |
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| **Paused** | Y | Toggle Normal/Hard mode and restart |
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| **Game Over** | START | Restart game |
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| **Game Over** | B | Return to Main Menu |
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---
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## 5. Build System & Toolchain
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### Prerequisites
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- `DEVKITPRO` environment variable pointing to the devkitPro root (e.g. `/opt/devkitpro`).
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- `devkitARM` toolchain with GCC cross-compiler (`arm-none-eabi-gcc`, `arm-none-eabi-g++`).
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- Tools on PATH or in `$DEVKITPRO/tools/bin`: `ndstool`, `grit`, `mmutil`, `bin2s`.
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- Build tools: `cmake` (>= 3.13), `ninja`.
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- Emulator: `melonDS`.
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### Build Commands
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To configure and build from the command line:
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```bash
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# Configure with Ninja generator
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cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
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# Copy compile_commands.json to root for clangd
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cmake -E copy_if_different build/compile_commands.json compile_commands.json
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# Build the NDS ROM image
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cmake --build build
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```
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### Running & Emulation
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```bash
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# Run using the custom CMake target (launches melonDS)
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cmake --build build --target run
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# Or launch directly with melonDS
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melonDS snakePlus.nds
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```
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### VS Code Integration
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The repository includes preconfigured tasks in [`.vscode/tasks.json`](.vscode/tasks.json):
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- **`Run new build`** (Default Build Task, Ctrl+Shift+B): Configures, compiles, and runs in `melonDS`.
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- **`cmake build`**: Compiles the project.
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- **`cmake clean`**: Cleans build artifacts.
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- **`clang-tidy (all)`**: Runs `run-clang-tidy` across all source and header files.
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- **`clang-tidy (watch changed)`**: Runs the background watcher [`tools/clang_tidy_watch.sh`](tools/clang_tidy_watch.sh).
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- **`Copy new build to SD`**: Copies ROM to external SD card mount.
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---
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## 6. Architecture & Implementation Details
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### 6.1 Hardware Subsystems ([`source/main.cpp`](source/main.cpp))
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- **Video Modes:** Both Main and Sub 2D engines are initialized into `MODE_0_2D`.
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- **VRAM Allocation:**
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- `VRAM_A`: Assigned to Main Engine Background (`VRAM_A_MAIN_BG`).
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- `VRAM_C`: Assigned to Sub Engine Background (`VRAM_C_SUB_BG`).
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- `VRAM_D`: Assigned to Sub Engine Sprites (`VRAM_D_SUB_SPRITE`).
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- **Consoles:** Standard text consoles are attached to both screens for debug / stats / menus.
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- **OAM:** Sub-engine OAM is initialized via `oamInit(&oamSub, SpriteMapping_1D_32, false)`. Sprites are allocated with `oamAllocateGfx(&oamSub, SpriteSize_8x8, SpriteColorFormat_256Color)`.
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- **Timer & IRQ:** Hardware Timer 0 triggers `do_tick` using `TIMER_FREQ_1024()`. Interrupt `IRQ_VBLANK` runs `handle_vblank()`.
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### 6.2 Asset Pipeline
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- **Graphics Pipeline:**
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- [`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`.
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- 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), 11 (rotten apple). All 12 sprite tiles are preloaded once into VRAM at startup; directional and blinking variants are rendered via pointer selection and hardware flipping flags without mid-frame DMA transfers.
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- Active OAM sprites are clamped at hardware limit (128) and trailing unused sprites are hidden every frame via `oamClear(&oamSub, oamId, 128 - oamId)`.
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- Console text HUD rendering in `draw_stats()` is throttled via dirty-flag caching (`HudCache`), eliminating redundant `iprintf` calls on VBlank.
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- **Audio Pipeline:**
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- Audio files in [`sounds/`](sounds/) are compiled by `mmutil` into `soundbank.bin` and header `soundbank.h`.
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- `bin2s` embeds `soundbank.bin` into assembly source `soundbank_bin.S` and header `soundbank_bin.h`.
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- Audio runs via `libnds`'s `maxmod9` (`mm9`).
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### 6.3 Background Pipeline ([`include/game_bg.h`](include/game_bg.h), [`source/game_bg.cpp`](source/game_bg.cpp))
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- **Dual-Engine Tiled Layers:** Both Main and Sub 2D engines allocate Background Layer 2 (`BgType_Text4bpp`, `BgSize_T_256x256`, `mapBase = 28`, `tileBase = 1`).
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- **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)`).
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- **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).
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- **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.
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- **Dynamic VBlank Palette Animation & Progressive Intensity:**
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- **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.
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- **Hard Mode:** Fiery amber / crimson cyber grid with faster pulse rate, transitioning towards volcanic hyper-crimson and incandescent solar white-gold.
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- **Cursed Mode:** Radioactive violet / magenta glitch strobe and high-speed warp drift during inverted controls (further intensified with apple count).
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- **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.
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- **Deep-Space Backdrop Glow:** Screen backdrops illuminate in sync with intensity levels.
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- **Game Over:** Palette dims to a somber dark charcoal / muted red tone with gentle drift deceleration.
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### 6.4 Static Analysis & Tooling
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- **Clang-Tidy:** Configured in [`.clang-tidy`](.clang-tidy). Enforces modern C++ guidelines, bugprone checks, readability conventions, and snake_case / CamelCase identifiers.
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- **Clangd:** Configured in [`.clangd`](.clangd). Removes `-mthumb-interwork` (which clang warns against) and explicitly includes devkitARM GCC 16.1.0 system headers.
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---
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## 7. Guidelines for AI Agents Modifying the Codebase
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1. **Embedded / Bare-Metal Constraints:**
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- The ARM9 CPU lacks an OS runtime. Avoid throwing exceptions or relying on RTTI (`-fno-exceptions -fno-rtti` are compiler flags).
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- Prefer lightweight containers (`std::array`, `std::deque`, `std::vector` with controlled growth).
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- Be mindful of OAM hardware limitations (Sub engine supports up to 128 sprites; current game allocates sprite slots per apple and snake segment).
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2. **Interrupt & Concurrency Safety:**
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- 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.
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3. **Hardware Registers & Memory Banking:**
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- Graphic tiles and palettes must be transferred using DMA (`dmaCopy`) or direct memory copies into allocated VRAM / OAM memory banks.
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- Do not modify VRAM allocation configurations without ensuring compatibility with console text generators and OAM sub-engines.
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4. **Toolchain Flags:**
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- Keep `-mthumb-interwork`, `-march=armv5te`, `-mtune=arm946e-s`, and specs `-specs=${DEVKITPRO_ROOT}/calico/share/ds9.specs` intact in [`CMakeLists.txt`](CMakeLists.txt).
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5. **Code Style:**
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- Follow the naming and check options specified in [`.clang-tidy`](.clang-tidy) (functions/variables in `snake_case`, classes/structs in `CamelCase`).
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- Run `cmake --build build` to verify clean compilation without warnings or errors.
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6. **Commit Message Attribution:**
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- 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:
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```
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Co-authored-by: <Agent Name> <agent-email-or-url>
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```
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- Example commit message format:
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```
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feat: add cursed apple spawn rate tuning
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Expose CURSE_SPAWN_CHANCE constant in consts.h so it can be adjusted
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without touching game logic.
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Co-authored-by: <Agent Name> <agent-email-or-url>
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```
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