kiln/ARCHITECTURE.md

viberogue — Architecture & Design Notes

What this is

viberogue is a game-making system, not just a game. The model is ZZT (1991, Epic MegaGames) — a DOS game that shipped with a built-in editor and a simple scripting language (ZZT-OOP), which let players create and share their own worlds. The goal here is something similar: a runtime + authoring environment where game worlds are defined in plain text files with embedded scripts, and the engine interprets them.

This document records the architectural decisions made so far and the reasoning behind them, so future development sessions don't have to rediscover the "why."

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Tech stack

Concern	Choice	Reason
GUI/windowing	eframe 0.33 / egui 0.33	Pure Rust, retained-mode, works on desktop and (eventually) WASM
Scripting	Rhai 1.x	Pure Rust, sandboxed, WASM-compatible; designed for embedding
Map format	TOML + serde	Human-readable, good Rust tooling, standard in the ecosystem

WASM compatibility is a first-class requirement. Everything in the stack must compile to WASM. This ruled out Lua (C FFI via mlua/rlua) for scripting. Rhai was chosen specifically because it is pure Rust with no C dependencies and explicit no_std/WASM support.

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Module structure

src/
  main.rs       — app entry point, rendering, input
  game.rs       — core data types and game logic
  map_file.rs   — TOML deserialization, Board loader
maps/
  start.toml    — the starting map (loaded at launch)

game.rs — core types

The types here are the runtime representation of a game world. They are deliberately free of any file-loading or rendering concerns.

Glyph (Copy)
The visual representation of one cell: a character, a foreground color, and a background color. Stored per cell (not per element type) so individual cells can animate or vary their appearance independently — e.g. a "fire" element where each flame tile has a slightly different color — without changing their behavior.

Element
The behavioral definition of a tile type. Currently just passable: bool; future fields will include opaque: bool (for line-of-sight), shootable, etc. Elements are stored once in a palette on Board; cells reference them by index. Many cells can share the same Element.

Why Glyph and Element are separate:
In ZZT, each board tile had both a visual (character + color pair) and an element type. The visual could vary per-tile even for the same element. We replicate this: Glyph is the visual (per-cell), Element is the behavior (shared via palette). This lets you have a wall that's gray in one room and blue in another without creating two "wall" element types.

Board
The complete unit of a game world — one "room" or "screen" in ZZT terminology. Holds:

• elements: Vec<Element> — the behavior palette for this board
• cells: Vec<(Glyph, usize)> — row-major grid; each cell is a visual + an index into elements
• player: Player — current player position on this board
• objects: Vec<ObjectDef> — scripted objects (parsed, not yet runtime-wired)
• portals: Vec<PortalDef> — exits to other boards (parsed, not yet runtime-wired)

Why cells is Vec<(Glyph, usize)> not Vec<Cell>:
An anonymous tuple prevents the element index from being detached from its board and misused. The usize has no meaning on its own — it only makes sense relative to a specific Board's elements palette. Wrapping it in a named Cell struct would give it false independence.

Why Board is the complete unit (no wrapper struct):
An earlier design had GameMap { board: Board, player: Player, ... }. This was eliminated because the split was artificial: there's no meaningful use of a Board without a player position, and no meaningful use of a player without a Board. ZZT itself treats a board as containing everything — the grid, the objects, and the player entry point. Collapsing to a single struct matches the domain model.

GameState
Currently a thin wrapper around Board that provides game-logic methods (try_move). It exists to keep mutation logic (collision checking, movement) separate from the data. As the game grows, event processing and scripting dispatch will live here.

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map_file.rs — file loading

MapFile and supporting structs are serde deserialization types only — they exist solely to parse TOML and are never used at runtime.

impl From<MapFile> for Board — the single conversion point. Reads the palette, builds the elements vec, then walks the grid string character-by-character to build cells. This is the only place that knows about both the file format and the runtime representation.

pub fn load(path: &str) -> Result<Board, ...> — reads a file, deserializes, converts. Called from main() before the window is created.

Why loading happens before window creation:
The window minimum size is derived from board.width and board.height. eframe requires NativeOptions (including window size) to be set before calling run_native. So the board must be loaded first.

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main.rs — app + rendering

App holds a GameState. The update method:

1. Reads arrow key input and calls GameState::try_move
2. Draws the menu bar
3. Draws the board: for each cell, a filled background rect then a centered monospace character
4. Draws the player on top using Glyph::player() (hardcoded @ in cyan — the player is not a board cell)

Cell rendering constants: CELL_W = 14.0, CELL_H = 20.0 pixels. The board is centered in the CentralPanel when the window is larger than the minimum size.

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Map file format

XPM-inspired (XPM is an old X11 image format that uses a character palette to define pixel colors). A [palette] section maps single characters to both a Glyph (visual) and an Element (behavior). The [grid] content is a TOML multi-line string where each character is a palette key.

[map]
name = "Room Name"
width = 60
height = 25
player_start = [30, 12]

[palette]
" " = { passable = true,  ch = " ", fg = "#000000", bg = "#000000" }
"#" = { passable = false, ch = "#", fg = "#808080", bg = "#606060" }

[grid]
content = """
############################################################
#                                                          #
############################################################
"""

[[objects]]
x = 10
y = 5
script = """
  on_touch(|| { send_message("open"); });
"""

[[portals]]
x = 59
y = 12
target_map = "cave"
target_entry = "west_door"

Why TOML over a custom format:
The toml crate gives us deserialization with minimal code. Multi-line strings for the grid give a visual representation of the map. Embedded Rhai scripts fit naturally in TOML multi-line strings without escaping issues.

Why the palette approach:
A direct mapping from palette character → (Glyph, Element) means the map file is both human-readable (you can see the shape of the room from the grid string) and efficient (shared element definitions, per-cell visual variation possible by using different palette chars with the same passable value but different colors).

Colors are "#RRGGBB" hex strings — universally understood, hand-editable.

player_start is a header field, not a palette character. The player is not a board cell; they are an entity that moves over the board. Using a palette character for player start (like @ in many roguelikes) would mean the tile under the player is always that character, which makes it awkward to place a player over different terrain.

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What's not yet implemented

Object scripting — ObjectDef and PortalDef are parsed from map files and stored on Board, but they have no runtime effect yet. The next step here is:

1. Wire ObjectDef scripts to Rhai: when the player moves to an object's cell, fire its on_touch handler
2. Define the Rhai API surface (what functions scripts can call: send_message, movement, board queries)

Portal navigation — PortalDef stores a target_map and target_entry but there's no multi-board loading or board switching yet.

Multi-board world — right now the engine loads a single maps/start.toml. Future: a world file or directory of boards, lazy-loaded as the player moves through portals.

Game creation tools — the long-term goal is an in-app editor. Not started.

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Future considerations

These are not current requirements but intended future directions. Where a planned change conflicts with current design, the tension is called out explicitly so it can be addressed before it becomes a problem.

The player may become an object; boards may have no player

The long-term goal is for the "player" to be an object on the board that happens to respond to arrow key events — not a hardcoded special entity. Some boards may have no player-like object at all and do something else with input events (a cutscene, a menu, a puzzle that reacts to keys differently).

ZZT hard-coded the player as a special element and many game authors had to work around this limitation (e.g. hiding the real player behind a wall and scripting a fake one). This is a deliberate improvement over that model.

Current design tension:

The following assumptions are baked in today and will need to change when this is implemented:

• Board.player: Player is a required non-optional field. A board with no player can't be represented. This should eventually become Option<Player>, or the player should be removed from Board entirely and tracked by the engine layer only when present.

• player_start in the map file is a required header field. It will need to become optional, or player spawning will move into the object/script system (an object with a special role, spawned at its x/y position).

• GameState::try_move directly mutates board.player. Once the player is an object driven by Rhai, movement will go through the scripting dispatch layer instead. try_move will likely be replaced by something like engine.dispatch_event(ArrowKey(dx, dy)).

• In main.rs, the player is rendered as a hardcoded overlay using Glyph::player(). Once the player is an object, it should be rendered as part of the normal object layer, not as a special case.

None of these are blockers for current work, but avoid making Board.player more central than it already is (e.g. don't add methods that assume player presence, don't derive window sizing from player position).

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ZZT reference

ZZT (1991) was a text-mode game for DOS. Its playfield was 60×25 characters (the right 20 columns were the stats panel). Each board was a self-contained screen with objects (tiles with embedded ZZT-OOP scripts), passageways to adjacent boards, and a fixed element type system (about 50 built-in element types). Players could create worlds with the built-in editor and share .ZZT files.

viberogue takes the core ideas — tile-based boards, embedded scripts per object, named portals between boards — and rebuilds them in a modern, WASM-capable stack with a more flexible scripting language and a human-readable file format.