kiln/ARCHITECTURE.md

Kiln — Architecture & Design Notes

What this is

Kiln 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, AppMode, frame loop (input → menu → panels → board → dialogs)
  game.rs          — core data types and game logic
  map_file.rs      — TOML deserialization, Board loader
  render.rs        — drawing primitives; cell size comes from BitmapFont
  font.rs          — BitmapFont, tile UV mapping, placeholder font
  font_dialog.rs   — floating font-picker dialog (path, tile dims, preview, apply)
  editor.rs        — EditorTab, EditorState, show_editor_panel
  glyph_picker.rs  — floating Glyph picker dialog; decoupled (open, glyph, font) interface
maps/
  start.toml       — the starting map (loaded at launch)
assets/
  vga-font-8x16.png — default CP437 bitmap font (256×128 px, 32 cols × 8 rows, 8×16 tiles)

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, Eq, Hash)
The visual representation of one cell: a tile: u32 index into the active bitmap font, 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. Derives Hash so the glyph picker can deduplicate the board palette into a HashSet<Glyph>.

FontSpec
Optional per-board font override: path: String, tile_w: u32, tile_h: u32. Stored on Board.font. When None, App's default_font is used. The editor's Board tab exposes a "Font…" button to change this at runtime.

Behavior
A plain data struct of runtime behavioral properties — currently passable: bool and opaque: bool. Returned by Archetype::behavior(). Adding a new property (e.g. shootable) requires only a new field here; no match arms needed at call sites. This is a deliberate improvement over storing behavior as a bag of booleans on a per-cell or per-archetype basis.

Archetype
An enum of named element types: Empty, Wall, Object, ErrorBlock. Each variant knows:

• Its canonical map-file name (e.g. "wall") via name()
• Its default Behavior via behavior()
• Its default Glyph via default_glyph() — used by the editor when stamping a cell

ErrorBlock is a sentinel for map files that reference an unknown archetype name. It renders as a yellow ? on red so malformed maps are immediately visible in-game. It is excluded from ALL_ARCHETYPES (the editor's list) because it is not a valid authoring choice.

Why Behavior and Archetype are separate types:
Archetype is the named class of a thing — it lets you say "this is a wall" in a map file and in the editor. Behavior is the runtime properties that drive simulation. Keeping them separate means adding a new property (e.g. pushable) only requires a new field on Behavior; no match arms need to be updated across the codebase.

Why Glyph and Archetype 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 type. We replicate this: Glyph is the visual (per-cell), Archetype is the class (shared across cells of the same type). This lets you have a wall that's gray in one room and blue in another without creating two archetype variants.

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

• cells: Vec<(Glyph, Archetype)> — row-major grid; each cell directly owns its visual and behavioral class
• 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)
• font: Option<FontSpec> — per-board font override; None means use the app default

Why cells is Vec<(Glyph, Archetype)> with no palette:
An earlier design had cells: Vec<(Glyph, usize)> where the usize indexed a per-board elements: Vec<Element> palette. This was eliminated because the palette indirection added complexity without benefit: Archetype is a Copy enum (4 bytes), so storing it per-cell is as efficient as storing an index, and it removes the invariant that the index must stay in sync with a specific board's palette.

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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font.rs — bitmap font

BitmapFont wraps an egui TextureHandle plus tile geometry (tile_w, tile_h, img_w, img_h). The texture is preprocessed at load time: the top-left pixel of the source PNG becomes the "background" sentinel — those pixels are stored as TRANSPARENT, all others as opaque WHITE. This lets egui's image tinting do all the colorization work at render time:

1. painter.rect_filled(cell_rect, 0.0, glyph.bg) — paint the background
2. painter.image(texture_id, cell_rect, tile_uv, glyph.fg) — draw the tile; egui multiplies each pixel by the tint color, so white → fg and transparent → nothing (revealing bg)

No custom shaders required.

tile_uv(tile) delegates to the private compute_tile_uv function which is unit-tested directly (no egui Context needed). tile_cols(), tile_count(), img_w(), img_h() are helpers used by the picker and dialog to lay out grids.

create_placeholder(ctx) generates a 16×16 grid of 8×8 tiles where each tile shows the 8 bits of its index as a column bar. Used as a fallback when assets/vga-font-8x16.png is missing.

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font_dialog.rs — font picker dialog

FontDialogState holds the in-progress edits: path, tile_w, tile_h, and a preview: Option<BitmapFont> loaded when the user clicks "Load". Separating dialog state from FontSpec means the user can explore settings without committing.

show(ctx, open, state, font_spec) renders a floating window with:

• A text field + "Browse…" button (rfd::FileDialog)
• DragValue controls for tile dimensions
• A "Load" button that decodes the PNG and stores it in state.preview
• A scrollable preview of the full tilesheet with grid overlay at tile boundaries
• "Apply" (enabled only after a successful Load) writes to font_spec and closes; "Use default" clears font_spec

Uses a should_close: bool flag outside the closure to work around egui's .open(open) borrow conflict.

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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.

TileIndex is a #[serde(untagged)] enum (Num(u32) | Chr(char)) that accepts either an integer (tile = 35) or a single-character string (tile = "#") in palette entries. This preserves backward compatibility with existing map files while allowing numeric indices in new ones.

FontHeader deserializes the optional [font] section and is converted to FontSpec in From<MapFile> for Board.

impl From<MapFile> for Board — the single conversion point. Reads the palette (each entry maps a character to a (Glyph, Archetype) pair), then walks the grid string character-by-character to build cells. Unknown archetype names produce an ErrorBlock cell and log a warning. 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:
eframe requires NativeOptions (including window size) to be set before calling run_native. Loading the board first means its dimensions are available if they're ever needed for sizing logic (currently the window uses fixed defaults, but the board must exist before App::new is called).

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main.rs — app + frame loop

App holds a GameState, an AppMode (Play | Edit), an EditorState, a default_font: BitmapFont (loaded from assets/vga-font-8x16.png at startup, falling back to create_placeholder), and a board_font: Option<BitmapFont> (loaded from board.font when a per-board font is set). App::new takes &egui::Context to upload textures before the first frame.

Font change detection: before calling show_editor_panel, font_spec_before is cloned; after the call, if board.font differs, apply_font_spec reloads board_font. The active font for the frame is board_font.as_ref().unwrap_or(&default_font), extracted as a let binding before mutable borrows to satisfy the borrow checker.

The update method phases:

1. Reads arrow key input and calls GameState::try_move — Play mode only
2. Draws the menu bar with a File menu and a Play/Edit mode toggle
3. In Edit mode: calls editor::show_editor_panel — declared before CentralPanel so egui allocates its space first
4. Draws the board and player via render::draw_board (see viewport sections below)
5. In Edit mode: calls glyph_picker::show if editor.glyph_picker_open

Viewport — Play mode:
render::board_origin(available, board_w, board_h, player) computes the pixel position of cell (0,0). If the board fits along an axis it is centered; if it overflows the viewport is centered on the player and clamped so no empty space appears at the board edges. No scroll bars.

Viewport — Edit mode:
The board is wrapped in egui::ScrollArea::new([true, true]), which shows scroll bars when the board overflows the viewport. Content is allocated at exact board size; rect.min from allocate_exact_size serves as the origin passed to render::draw_board.

Stamp action: *board.get_mut(cx, cy) = (self.editor.glyph, self.editor.selected) — the custom glyph and selected archetype are written together. The glyph can differ from the archetype's default_glyph() if the user has customized it.

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render.rs — drawing primitives

All pixel-level rendering knowledge lives here. No dependency on EditorState or App. Cell size is no longer a fixed constant — it comes from the active BitmapFont.

Constants: Window sizing (DEFAULT_WINDOW_W = 840, DEFAULT_WINDOW_H = 524, MIN_WINDOW_W/H) defined here. No CELL_W/CELL_H — those were removed when bitmap fonts were introduced.

paint_glyph(painter, rect, glyph, font) — fills rect with glyph.bg, then draws the tile image tinted by glyph.fg. Used by both draw_glyph and glyph_picker.

draw_glyph(painter, origin, x, y, glyph, font) — computes the cell rect from font.tile_w/tile_h, delegates to paint_glyph.

draw_board(painter, origin, board, font) — iterates all cells calling draw_glyph, then draws the player as an overlay on top.

pos_to_cell(origin, pos, tile_w, tile_h) -> (i32, i32) — converts a pixel position to cell coordinates using floor division. Returns negative values for clicks above/left of the origin; callers must guard >= 0. Mirrors the rendering math so clicks land on the correct cell.

board_origin(available, board_w, board_h, player, font) -> Pos2 — computes the pixel origin for Play-mode rendering (centering or player-tracking with edge clamping).

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editor.rs — editor state and side panel

EditorTab (Palette | Board | World) — which tab is active in the side panel.

EditorState — transient editor state:

• selected: Archetype — the archetype class to stamp
• glyph: Glyph — the visual to stamp (independent of the archetype's default; resets to archetype.default_glyph() when the archetype selection changes)
• glyph_picker_open: bool — whether the glyph picker dialog is visible
• tab: EditorTab — which side-panel tab is active
• font_dialog_open: bool — whether the font picker dialog is visible
• font_dialog_state: FontDialogState — in-progress edits for the font dialog

show_editor_panel(ctx, editor, font_spec, active_font) — renders the resizable right-side panel (default 200 px). Palette tab: scrollable archetype list and glyph preview button. Board tab: current font path label + "Font…" button that initializes font_dialog_state from font_spec and opens the font dialog. World tab: placeholder.

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glyph_picker.rs — glyph picker dialog

show(ctx, open, glyph, board_cells, font) — renders a floating egui::Window. Takes open: &mut bool and glyph: &mut Glyph directly, with no dependency on EditorState, so it can be invoked from any context that needs glyph selection.

Three sections:

1. Board palette — unique glyphs found on the current board (deduplicated via HashSet<Glyph>), shown as clickable cells rendered with paint_glyph. The current glyph is highlighted with a white stroke.
2. Colors — color_edit_button_srgba pickers for glyph.fg and glyph.bg.
3. Tile grid — all tiles from the active BitmapFont, displayed at tile_w × tile_h per cell in a scrollable area. Column count matches the font image layout. Clicking a cell updates glyph.tile. The current tile is highlighted.

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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.

Keep this example in sync with map_file.rs whenever the format changes.

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

# Optional: override the default font for this board.
[font]
path = "assets/my_font.png"
tile_w = 8
tile_h = 16

[palette]
" " = { archetype = "empty", tile = " ", fg = "#000000", bg = "#000000" }
"#" = { archetype = "wall",  tile = 35,  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 tile accepts both char and integer:
TileIndex is a #[serde(untagged)] enum. Old map files using tile = " " (a single-character string) continue to work unchanged. New map files can use tile = 32 (integer) for clarity or when the tile has no printable character equivalent.

Why archetypes by name, not by property:
Palette entries used to store passable = true/false directly. Switching to archetype = "wall" means adding a new behavioral property (e.g. opaque) only requires a code change — existing map files don't need to be updated. It also makes files more readable: archetype = "wall" is self-documenting. Unknown names produce a visible ErrorBlock cell rather than silently defaulting.

Why the palette approach:
A direct mapping from palette character → (Glyph, Archetype) means the map file is both human-readable (you can see the shape of the room from the grid string) and flexible (per-cell visual variation is possible by using different palette chars with the same archetype 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)

Object behavior from scripts — Archetype::Object.behavior() currently returns a static default (passable: false, opaque: false). Eventually, Board::is_passable will need a special branch for Object cells that reads the behavior from the cell's Rhai script instead.

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.

File open dialog — the editor has no way to load a different map file from the UI. rfd is already a dependency (used by the font dialog), so a "Open map…" button is straightforward to add. WASM support for rfd file dialogs is an open question.

Map save — the editor can paint cells but has no way to write changes back to a .toml file. Saving will require serializing the Board back to MapFile format and writing it to disk.

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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.

Kiln 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.