`kiln` is a ZZT-inspired game-making system written in Rust (edition 2024) — a runtime **and** authoring environment where game worlds are plain-text files with embedded scripts. The model is ZZT (1991, Epic MegaGames): a DOS game that shipped with a built-in editor and a scripting language (ZZT-OOP), letting players create and share worlds. ZZT's playfield was 60×25 characters; each board was a self-contained screen with scripted objects, passages to adjacent boards, and ~50 built-in element types.
**Tech-stack rationale (the "why"):**
- **WASM compatibility is a first-class requirement** — everything in the stack must compile to WASM. This is why scripting uses **Rhai** (pure Rust, sandboxed, no C deps, explicit WASM support) rather than Lua (which needs C FFI via mlua/rlua).
- **Maps are TOML + serde** — human-readable, hand-editable, good Rust tooling; grid multi-line strings make the room shape visible, and embedded Rhai scripts fit TOML multi-line strings without escaping.
- **The engine (`kiln-core`) is UI-agnostic** — no rendering/UI types leak in (e.g. log colors are core `Rgba8`, not a front-end type), so the same game data drives multiple front-ends. kiln-tui being added with zero engine changes is the proof.
- Add `///` rustdoc comments to every `pub` type, field, and function. The user reads rustdoc in their IDE to understand the code while making changes.
- Add inline `//` comments inside non-trivial function bodies to explain the *why* of each logical step — especially in `update` loops, rendering math, and conversion logic.
- Keep comments accurate: update them when the code they describe changes.
- **`kiln-core`** — the engine: all core game types (`Board`, `Glyph`, `Archetype`, `GameState`, …) plus `.toml` map-file load/save. No rendering or UI; every front-end depends on it.
- **`kiln-tui`** — a terminal **player** (no editor yet) built on **ratatui 0.30 / crossterm**. Takes a map-file path on the command line and lets you walk the player around the board with the arrow keys.
- **`kiln-egui`** — the original eframe/egui desktop app (player + editor). **Still in the tree but no longer a workspace member**: it was removed from the root `Cargo.toml``members` and its files were left untouched, so it is not built at the root and is not guaranteed to compile against the current engine. Its notes below are retained for when it is revived.
-`Glyph` (`Copy`, `Eq`, `Hash`) — per-cell visual: `tile: u32` (tilesheet index), `fg/bg: Color32`. `Glyph::player()` is a `const fn`; all other glyphs come from the map file. Derives `Hash` so boards can deduplicate glyphs into a palette.
-`FontSpec` — optional per-board bitmap font override: `path: String`, `tile_w: u32`, `tile_h: u32`. When `None`, the app default font is used.
-`Behavior` — plain data struct of runtime behavioral properties: `solid: bool` (blocks/participates in movement — inverse of the old `passable`), `opaque: bool`, `pushable: Pushable` (an enum `No`/`Any`/`Horizontal`/`Vertical` with `allows(dir)`; only meaningful for `solid` things). Returned by `Archetype::behavior()`; new properties added here require no match arms elsewhere. (`ObjectDef.pushable` is still a plain `bool` = any direction.)
-`Solid<'a>` — the single solid occupant of a cell, returned by `Board::solid_at`: `Player` (the player; pushable any direction), `Cell(Archetype)` (a solid grid archetype like a wall) or `Object(&ObjectDef)` (a solid object). At most one solid may occupy a cell — enforced at load time. The player is checked first, so its cell reports `Solid::Player` (and is impassable to movers).
-`Archetype` (`Copy`, `PartialEq`) — enum of named element types: `Empty`, `Wall`, `Crate`, `HCrate`, `VCrate`, `ErrorBlock`. Each variant provides `behavior()`, `name()` (used in map files), and `default_glyph()` (used by the editor when stamping a cell). `Crate` is solid and pushable any direction (CP437 ■, char 254, light gray on black); `HCrate`/`VCrate` are crates pushable only east/west (↔, char 29) / north/south (↕, char 18) respectively, same colors. `ErrorBlock` is a sentinel for unknown archetype names — renders as yellow `?` on red.
-`Board` — the complete game unit (ZZT-style "board"): `width`, `height`, `cells: Vec<(Glyph, Archetype)>` (row-major; each cell owns its visual and behavioral class directly), `player: Player`, `objects: Vec<ObjectDef>`, `portals: Vec<PortalDef>`, `font: Option<FontSpec>`. `cells` is `pub(crate)`. `solid_at(x, y) -> Option<Solid>` returns the cell's single solid occupant (object checked before grid archetype); `is_passable(x, y)` is the convenience inverse (`solid_at(...).is_none()`). `in_bounds((i32, i32))` bounds-checks a (possibly negative) coordinate. Push support is split into the read-only `can_push(x, y, dir)` (does the chain of pushable solids starting here end at open space?) and the mutating `push(x, y, dir)` (shoves that chain one cell, leaving `Empty` behind); the read-only half lets a mover answer "can I move here?" via `is_passable || can_push` before committing. `is_valid()` / `load_errors()` expose nonfatal problems collected while loading (a non-serialized `Vec<LogLine>`); `report_error(msg)` appends a red-on-black line and `is_valid()` is just "no load errors".
-`ObjectDef` — scripted object placed on the board: `x`, `y`, `glyph: Glyph`, `solid: bool`, `opaque: bool`, `pushable: bool`, `script_name: Option<String>`. `solid` and `opaque` default `true`, `pushable` defaults `false` in map files. Its `init()`/`tick(dt)`/`bump(id)` hooks are run by the scripting runtime (see `script.rs`).
-`GameState` — holds `board: Rc<RefCell<Board>>`, the message `log: Vec<LogLine>`, and a `ScriptHost`. The board sits behind `Rc<RefCell<…>>` as a sibling of the `ScriptHost` so script host functions can hold a shared handle to it without aliasing the borrow that's running the engine. Front-ends/logic reach the board through `board() -> Ref<Board>` / `board_mut() -> RefMut<Board>` (there is no `pub board` field). `try_move(dir: Direction)` moves the player (pushing a crate/solid out of the way if possible) and fires `bump(-1)` on a solid object it walks into; `run_init()` runs object `init()` hooks once at startup; `tick(dt)` advances each object's cooldown then runs `tick(dt)` hooks every frame. Both end by calling `resolve()`, which drains the board queue (`take_board_queue()`) and applies each `Action`: `Log` → `log`, `SetTile` → the source object's glyph, `Move(dir)` → the free fn `step_object` (gates on `in_bounds` then `is_passable || can_push`, pushing before it relocates, and returns the index of any solid object it bumped). Resolution is two-phase — mutate the board collecting `(bumped, bumper)` pairs, then drop the board borrow and fire `run_bump` for each (a `bump` script reads `Board.*`, so no `board_mut` borrow may be held while it runs). A bumped object's own emitted actions wait for the next tick's pump (no same-tick cascade). `drain_errors()` moves the script error sink into `log`. This deferred apply (writes after the batch) is what keeps script execution borrow-safe.
-`LogSpan { text, fg: Option<Rgba8>, bg: Option<Rgba8> }` and `LogLine { spans: Vec<LogSpan> }` — a UI-agnostic styled message (colors are core `Rgba8`, not a front-end type). `LogLine::raw()`, a chainable `push()`, `append()`, and `LogLine::error()` (a red-on-black single-span constructor used for nonfatal load errors) build messages; each front-end converts a `LogLine` to its own styled text at render time.
-`ScriptHost` — owns the Rhai `Engine`, the compiled scripts referenced by a board's objects (compiled once per name), a per-object persistent `Scope` + output queue + ready timer, and the shared board queue. Built with `ScriptHost::new(&Rc<RefCell<Board>>)` (registers the API, compiles scripts, reports compile/unknown-script failures onto the error sink; runs nothing).
- Lifecycle hooks per object: `init()` (zero-arg), `tick(dt)` (elapsed seconds as `f64`), and `bump(id)` (the bumper's object index, or `-1` for the player), all optional — detected via `AST::iter_functions()` by name and arity. Driven by `run_init()` / `run_tick(dt)` / `run_bump(idx, id)`; runtime errors go to the error sink (drained to the log), not fatal.
- **Reads (direct):** read getters (`player_x`, `player_y`, `width`, `height`) are registered on `Rc<RefCell<Board>>` (the `BoardRef` alias, exposed to Rhai under the type name `Board`) — getters only, so read-only by construction. A clone of the handle is pushed into each scope as the constant `Board`. `blocked(dir) -> bool` is also a read fn: true if the caller's target cell is off-board, already solid, or the destination of a pending move on the board queue (an object never sees its *own* move, which is pumped only after its script returns). Each getter briefly borrows the shared `Board`.
- **Actions & rate limiting (two-tier queues):** host fns `move(dir)`, `set_tile(n)`, `log(s)` append an `Action` (`Move`/`SetTile`/`Log`; `Action::time_cost()` is `MOVE_COST = 0.25` s for `Move`, else 0) to the *issuing object's* output queue (routed by the per-call tag via a `HashMap<usize, ObjQueue>`). `ScriptHost::pump(i)` promotes ready actions onto the shared **board queue** (`Vec<BoardAction { source, action }>`): the leading run of zero-cost actions plus at most one timed action, which arms that object's **ready timer** and ends the pump. While a ready timer is `> 0` nothing is pulled (this caps object speed); `advance_timers(dt)` counts them down each frame. `GameState` drains the board queue with `take_board_queue()` and applies it after the batch — so scripts mutate without a `&mut GameState` borrow. Errors (compile/runtime) bypass the queues via the error sink (`take_errors()`).
- The `Queue` object (a handle to the calling object's output queue) is pushed into each scope; scripts call `Queue.length()` and `Queue.clear()`. Safe to mutate from script because the host only touches output queues between calls (during `pump`).
- **Sender identity:** `source` (which object issued an action) rides the per-call **tag** — `run` calls `call_fn_with_options(...with_tag(object_index)...)` and the host fns read it via `NativeCallContext::tag()`. Scripts write `move(North)` without naming themselves; `North`/`South`/`East`/`West` are `Direction` constants in scope, and `impl From<Direction> for (i32,i32)` gives the delta.
- **TODO in-code:** `BoardAction.source` (and `ObjectRuntime.object_index`, and the `QueueMap` keys) are array indices into `Board::objects` — a stopgap that breaks under object spawn/destroy/reorder; should become a monotonically-increasing unique object id with objects keyed by it.
-`PlayerStart` — `#[serde(untagged)]` enum (`Coord([i32;2])` | `Char(char)`); `player_start = [x, y]` or `player_start = "@"` (locate that char in the grid). Same dual-form trick as `TileIndex`
-`impl TryFrom<MapFile> for Board` — the single load conversion. **Best-effort/nonfatal**: only a grid-dimension mismatch returns `Err`; every other problem (unknown archetype/grid char → `ErrorBlock`; object/player placement-char issues; one-solid-per-cell conflicts) is recovered and recorded on `Board::load_errors` (see `Board::is_valid`). Resolves the player (coord or `@` char, first-occurrence, falling back to `(0,0)`) and lets it *win* its cell; places objects by `x`/`y` or `palette` char
-`pub fn load(path: &str) -> Result<Board, Box<dyn std::error::Error>>` — reads and converts a `.toml` map file
-`pub fn save(board: &Board, path: &Path) -> Result<(), Box<dyn std::error::Error>>` — serializes `Board` back to TOML and writes to disk
### kiln-tui modules
The terminal player. Renders the board as text: each `Glyph.tile` index is reinterpreted as a character (the default kiln font is CP437, where the tile index equals the code point) and drawn with the glyph's RGB colors. No bitmap font or UV mapping is involved.
**`kiln-tui/src/main.rs`** — entry point and play loop:
- Parses a single positional arg (the map path); prints usage and exits non-zero if missing. Loads the board via `kiln_core::map_file::load`*before* touching the terminal so load errors print to a normal screen.
-`ratatui::init()` → detect `TerminalCaps` → push Kitty flags when supported → `run` loop → pop Kitty flags → `ratatui::restore()`.
-`run` is a ~30 FPS real-time loop: it `event::poll`s only until the next frame deadline (so input stays responsive) and otherwise wakes to advance the game by the real elapsed `dt` via `GameState::tick`. It acts on `Press`**and**`Repeat` key events (so holding a key moves continuously) and ignores `Release` (which the Kitty protocol also emits, and which would otherwise double-fire each move). Arrow keys move the player via `GameState::try_move`; `l` toggles the log panel; `q`/`Esc` quit. Mouse capture is enabled so the log panel scrolls (wheel) and resizes (drag the divider). `game.run_init()` runs object `init()` hooks after the board is loaded and before the loop.
-`draw` wraps the board in a bordered `Block` titled with the board name and controls. When the log panel is closed, the latest log line is previewed in the bottom-left border after a `[l]og:` label; when open, a bottom panel lists the log newest-first (`render::logline_to_line` converts each `LogLine`).
-`Ui { log_open, log_height, scroll }` — view-only panel state kept in the front-end (not on `GameState`).
-`BoardWidget<'a>` — a `ratatui::widgets::Widget` that draws the board. Per axis, `axis(board_len, view, player) -> (offset, pad, count)`: a board smaller than the viewport is **centered** with screen padding, a larger one **scrolls** to keep the player on screen with no empty margins. Each cell resolves glyph priority player > object > grid floor, then writes char + fg/bg into the buffer.
**`kiln-tui/src/term.rs`** — terminal capability detection and Kitty setup:
-`TerminalCaps { keyboard_enhancement: bool, truecolor: bool }` — detected once at startup. `keyboard_enhancement` is a real query/response (`supports_keyboard_enhancement()`); `truecolor` reads `$COLORTERM`. Intended to be handed to scripts so they can pick bindings the terminal actually supports.
-`summary_line() -> Line` — styled one-line badge for the bottom border; when `truecolor`, the word "truecolor" is drawn with each letter a different rainbow color (`hue_to_rgb`, an HSV→RGB helper), else a plain "256-color".
-`push_kitty_flags()` / `pop_kitty_flags()` — enable/disable the Kitty keyboard protocol (`DISAMBIGUATE_ESCAPE_CODES | REPORT_EVENT_TYPES | REPORT_ALTERNATE_KEYS | REPORT_ALL_KEYS_AS_ESCAPE_CODES`); the last flag makes modifier-only keypresses observable. Pushed only when supported, popped before restore. Enabling `REPORT_EVENT_TYPES` is why the input loop must filter out `Release` events.
### kiln-egui modules (no longer a workspace member)
**`kiln-egui/src/font.rs`** — bitmap font loading and UV mapping:
-`BitmapFont` — wraps an egui `TextureHandle` (preprocessed to opaque-white / transparent) plus tile dimensions. The top-left pixel of the source PNG defines the "background" color; those pixels become `TRANSPARENT`, all others become `WHITE`. At render time, `painter.image(…, tile_uv, fg_color)` tints white pixels to `fg` and transparent pixels reveal the `bg` fill behind.
-`BitmapFont::load(ctx, path, tile_w, tile_h)` — loads from disk
-`BitmapFont::from_bytes(ctx, bytes, tile_w, tile_h, label)` — loads from embedded bytes
-`BitmapFont::create_placeholder(ctx)` — 16×16 grid of 8×8 procedural tiles (bit-pattern bars), used as fallback when no font file is present
-`BitmapFont::tile_uv(tile)` — returns a UV `Rect` for the given tile index (left-to-right, top-to-bottom). Unit-tested via `compute_tile_uv`.
-`BitmapFont::cell_size(zoom)` — pixel size (`Vec2`) of one rendered cell at the given integer zoom; centralizes the `tile_w * zoom` math used across rendering and hit-testing
-`FontDialogState::from_spec(spec)` — initializes from an existing `FontSpec` or defaults (8×16, empty path)
-`show(ctx, open, state, font_spec)` — floating dialog with file browser (`rfd::FileDialog`), tile dimension `DragValue` controls, scrollable tilesheet preview with grid overlay, and Apply / Use default buttons. Uses a `should_close` flag to avoid the egui `.open()` borrow conflict.
-`App` holds `GameState`, `AppMode`, `EditorState`, `default_font: BitmapFont`, and `board_font: Option<BitmapFont>`
-`App::new(board, egui_ctx)` — loads `assets/vga-font-8x16.png` as the default font (falls back to `create_placeholder`); loads per-board font from `board.font` if present
-`App::update` is a thin sequence of phase methods: `handle_input` (table-driven arrow keys, Play only) → `menu_bar` (File Save/Save As via `save_to`/`save_as`, Exit, mode toggle) → `try_show_script_editor` (returns `true` to take over the frame) → editor side panel + font reload → `show_board` → `handle_board_click` → `show_glyph_pickers` → object-glyph writeback
-`App::active_font()` — resolves the per-board font or the default; used wherever an exclusive borrow of `self` is not also needed
-`App::show_board(&self) -> Option<(usize, usize)>` — draws the viewport and returns the clicked cell (Edit mode); the click is dispatched to `handle_board_click(&mut self, cx, cy)`*after*`show_board` returns, so the `&mut` handler never conflicts with the font borrow
- Borrow split: at the editor-panel and glyph-picker call sites, `self.board_font.as_ref().unwrap_or(&self.default_font)` is used inline (not `active_font()`) so the font borrow stays disjoint from the `&mut self.editor`/`&mut board` borrows
- Window sizing constants (`DEFAULT_WINDOW_W = 840`, `DEFAULT_WINDOW_H = 524`, `MIN_WINDOW_W/H`); no fixed `CELL_W/H` — cell size comes from the active `BitmapFont`
-`glyph_preview_button(ui, glyph, font) -> Response` — allocates a one-cell swatch (unzoomed), paints `glyph`, returns the click response; used by the Palette and Objects tabs
-`draw_glyph(painter, origin, x, y, glyph, font, zoom)` — sizes cell via `font.cell_size(zoom)`, delegates to `paint_glyph`
-`draw_board(painter, origin, board, font, zoom)` — draws all cells then player overlay
-`draw_object_overlays(painter, origin, board, font, selected, zoom)` — highlights all object cells in the Objects editor tab; the selected object gets a brighter border
-`board_origin(available, board_w, board_h, player, font, zoom)` — centers board or clamps to player with no empty space
-`show_editor_panel(ctx, editor, board, active_font)` — resizable right-side panel (default 200 px); renders the tab bar then dispatches to a per-tab function (`palette_tab`, `objects_tab`, `board_tab`, `scripts_tab`; World is empty), passing the relevant sub-struct(s). Palette shows the archetype list and glyph preview button; Board shows the font path, "Font…" button, and zoom slider; Objects lists the selected object's glyph preview, **Passable/Opaque checkboxes**, and script combobox; Scripts creates/edits named scripts
-`show(ctx, open, glyph, board_cells, font)` — takes `open: &mut bool` and `glyph: &mut Glyph` directly; no dependency on `EditorState`
- Three sections: board palette (unique glyphs deduped via `HashSet<Glyph>`, click to select), FG/BG color pickers, scrollable tile grid (all tiles in the active font at `tile_w × tile_h` per cell)
XPM-inspired: a `[palette]` maps single characters to `(Glyph, Archetype)` definitions; `[grid] content` is a TOML multi-line string where each character indexes the palette.
Colors are `"#RRGGBB"` hex strings. `player_start` accepts either `[x, y]` coordinates **or** a single grid char to locate (convention `"@"`; that cell loads as `Empty`, like an object placeholder). The `tile` field accepts either a single-character string (`tile = " "`) or an integer (`tile = 35`); both are valid and existing char-style map files continue to work. The grid multi-line string's leading newline is trimmed by TOML; trailing newline is handled correctly by `str::lines()`. Unknown archetype names produce an `ErrorBlock` cell and a logged warning, as does any grid character that is neither a `[palette]` key nor an object/player `palette` char. Objects may be placed by `x`/`y` or by a single grid `palette` char (uppercase by convention). **Loading is best-effort and nonfatal** (only a grid-dimension mismatch is a hard error): a placement char that appears multiple times uses the first occurrence, a missing object char drops that object, a missing player char falls back to `(0, 0)`, and one-solid-per-cell conflicts drop the later solid — each problem is recorded on `Board` (see `is_valid()` / `load_errors()`). The **player joins one-solid-per-cell** and *wins* its cell: it is placed first, silently clearing solid terrain under it and dropping any solid object that lands there. Script source lives in the `[scripts]` table (name → Rhai source); objects reference a script by `script_name`. Scripts may define optional `init()`, `tick(dt)`, and `bump(id)` functions; they **read** the world through `Board.*` (e.g. `Board.player_x`), check `blocked(dir) -> bool`, inspect/empty their pending actions via `Queue.length()`/`Queue.clear()`, and **write** via host functions `move(dir)` (`dir` ∈ `North`/`South`/`East`/`West`), `set_tile(n)`, and `log(s)`. Writes don't take effect immediately: each is queued and **at most one `move` resolves per 250 ms** per object (a blocked move still costs the cooldown), so objects can't act infinitely fast. When two objects move into the same cell on one tick, **array order wins** (the earlier object lands; the later is pushed or blocked) and the object that was moved into receives `bump(id)`.
- **`Behavior` and `Archetype` are separate types** — `Archetype` is the named class of a thing (`Wall`, `Empty`, `Object`); `Behavior` is its runtime properties (`solid`, `opaque`, `pushable`). Adding a new property means adding a field to `Behavior`, not a match arm at every call site.
- **One solid per cell** — at most one solid entity (the player, a solid grid archetype, *or* a solid object) may occupy a cell. The player is a first-class solid: `Board::solid_at` reports `Solid::Player` for the player's cell (checked first), it blocks movers, and it is **pushable in any direction** — a push chain that reaches the player slides the player along (and is rejected if the player has nowhere to go, e.g. against a wall). The map loader enforces the invariant: a solid object landing on an already-solid cell is dropped (recorded via `report_error`); the player *wins* its cell — its resolved position (including the `(0, 0)` fallback when `player_start` is unresolvable) silently clears any solid terrain under it and drops a conflicting object. `Board::solid_at` relies on this invariant.
- **`cells: Vec<(Glyph, Archetype)>`** — each cell owns its visual and behavioral class directly; there is no per-board element palette or integer indirection. `Archetype` is `Copy` so this is efficient.
- **Archetypes are referenced by name in map files** — so `ALL_ARCHETYPES` can be reordered or extended without breaking saved games. Adding a variant: the `match`es in `behavior()`/`name()`/`default_glyph()` are exhaustive (the compiler flags them), but **`TryFrom<&str>` and `ALL_ARCHETYPES` are not** — forget the `TryFrom` arm and the archetype silently loads as `ErrorBlock`. Also update the map-format example.
- **`Board` is the complete unit** — grid, player, objects, and portals all live on `Board`, matching how ZZT treats a "board". No separate wrapper struct.
- **Glyph (visual) and Archetype (behavior) are decoupled** — each cell has its own `Glyph` (so colors can vary per-cell, e.g. fire flickering) while sharing an `Archetype` with other cells of the same type.
### Future direction: the player may become an object
The long-term goal is for the "player" to be a board *object* that happens to respond to arrow-key events — not a hardcoded special entity — and for some boards to have **no** player at all (a cutscene/menu/puzzle that handles input differently). ZZT hardcoded the player as a special element and authors had to hack around it; avoiding that is deliberate.
Today's baked-in assumptions that will need to change (don't make `Board.player`*more* central in the meantime — e.g. don't add methods that assume player presence):
-`Board.player: Player` is required and non-optional; a player-less board can't be represented (should become `Option<Player>`, or move the player out of `Board` to the engine layer).
-`player_start` is effectively required; player spawning should eventually move into the object/script system.
-`GameState::try_move` mutates `board.player` directly; once the player is script-driven, movement should go through event dispatch (e.g. `dispatch_event(ArrowKey(dir))`) rather than a dedicated method.
- The player is rendered as a hardcoded overlay (`Glyph::player()`); it should become part of the normal object layer.
- **Scripting growth** — more event hooks beyond `init`/`tick`/`bump` (e.g. `on_touch` when the player steps onto an object), a larger action/read vocabulary (more actions could carry a `time_cost`), and persisting script-local state across ticks (needs `call_fn_raw` so the per-object `Scope` isn't rewound each call).
- **Stable object ids** — `BoardAction.source` / `ObjectRuntime.object_index` / `QueueMap` keys / `bump` ids are array indices into `Board::objects`, which break under spawn/destroy/reorder; replace with monotonic unique ids (a `// TODO` marks this in `script.rs`).
- **Portals & multi-board** — `PortalDef` is parsed but not runtime-wired; there is no multi-board world/loading yet (the engine loads a single map).
- **Load-error surfacing** — `Board::load_errors()` is collected but no front-end displays it yet (kiln-tui could append it to the in-game log at startup).
