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."

---

## Tech stack

| Concern | Choice | Reason |
|---------|--------|--------|
| Engine core | `kiln-core` crate | UI-agnostic game types + map I/O; the one crate every front-end depends on |
| Terminal front-end | `kiln-tui` — ratatui 0.30 / crossterm | Pure Rust TUI **player**; renders boards as colored text |
| Desktop front-end | `kiln-egui` — eframe / egui 0.33 | Pure Rust retained-mode GUI player + editor. **Currently not a workspace member** (see Module structure) |
| 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.

---

## Module structure

Cargo **workspace**; root `members = ["kiln-core", "kiln-tui"]`. `kiln-egui` stays in the tree but was removed from the workspace and is not built at the root.

```
kiln-core/src/                — the engine (UI-agnostic)
  game.rs          — core data types and game logic
  log.rs           — styled, UI-agnostic log messages (LogLine/LogSpan)
  script.rs        — Rhai scripting runtime (ScriptHost); object init/tick hooks
  map_file.rs      — TOML deserialization, Board load/save
kiln-tui/src/                 — terminal player (no editor)
  main.rs          — CLI arg + play loop; TerminalCaps detection; Kitty flags
  render.rs        — BoardWidget: draws the board into a ratatui buffer as text
  cp437.rs         — CP437 → Unicode table; renders tile indices as characters
  term.rs          — TerminalCaps detection + Kitty keyboard-protocol setup
kiln-egui/src/                — eframe/egui player + editor (NOT a workspace member)
  main.rs, render.rs, font.rs, font_dialog.rs, editor.rs, glyph_picker.rs
maps/
  start.toml       — example map; play via `cargo run -p kiln-tui -- maps/start.toml`
assets/
  vga-font-8x16.png — default CP437 bitmap font (256×128 px, 8×16 tiles); used by kiln-egui
```

The per-module deep-dives below are labeled by file. `game.rs`/`map_file.rs` are **kiln-core**; the kiln-tui player has its own section; `font.rs`, `font_dialog.rs`, and the egui `main.rs`/`render.rs`/`editor.rs`/`glyph_picker.rs` are **kiln-egui** (retained as design notes for when the GUI is revived).

### `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`, `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; their `init()`/`tick(dt)` hooks run via the scripting runtime, and `passable`/`opaque` affect collision
- `portals: Vec<PortalDef>` — exits to other boards (parsed, not yet runtime-wired)
- `scripts: HashMap<String, String>` — named Rhai source, referenced by objects via `script_name`
- `font: Option<FontSpec>` — per-board font override; `None` means use the app default

**`ObjectDef`**  
A scripted object placed on the board. Holds `x`, `y`, `glyph: Glyph` (owned by the object so scripts can change its appearance), `passable: bool`, `opaque: bool`, and `script_name: Option<String>`. `passable` defaults `false` and `opaque` defaults `true` in map files (objects block by default). `Board::is_passable` checks objects before consulting the grid cell's `Archetype::behavior()`, so an impassable object blocks movement even when placed over a passable floor tile. The `passable` and `opaque` flags are editable via checkboxes in the editor's Objects tab.

**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`**  
Owns the board behind an `Rc<RefCell<Board>>`, alongside the message `log` and a `ScriptHost`. Keeping the board a **sibling** of the script host (rather than owned by it) is the key ownership move: host functions capture a clone of that `Rc` — a `'static` handle to a *different* `RefCell` than the running engine — so they can read/queue writes without aliasing the borrow that's executing scripts. Front-ends and internal logic reach the board only through `board() -> Ref<Board>` / `board_mut() -> RefMut<Board>` (no `pub board`). `try_move` moves the player; `run_init()` / `tick(dt)` run object hooks; `apply_commands()` drains the script command queue *after* each batch — the deferred apply is when `&mut` is finally free of any outstanding borrow.

---

### `log.rs` — styled messages

`LogLine { spans: Vec<LogSpan> }`, where `LogSpan { text, fg: Option<Rgba8>, bg: Option<Rgba8> }`, is a **UI-agnostic** styled message: colors are core `Rgba8`, not a front-end type, so the engine stays UI-free. `LogLine::raw()`, a chainable `push()`, and `append()` build messages; each front-end converts a `LogLine` to its own styled text at render time (kiln-tui's `render::logline_to_line`). The game log lives on `GameState`; both the engine (scripts) and front-ends append to it.

### `script.rs` — Rhai scripting runtime

`ScriptHost` owns the Rhai `Engine`, the scripts referenced by a board's objects (compiled once per name via `Engine::compile`), a persistent per-object `Scope`, and a shared **command queue**. Built with `ScriptHost::new(&Rc<RefCell<Board>>)`, which registers the API, compiles scripts, and records each object's available hooks (detected with `AST::iter_functions()` by name/arity) but **runs nothing** — compile/unknown-script failures are queued as `GameCommand::Error`.

Two optional lifecycle hooks per object: `init()` (zero-arg, run once) and `tick(dt)` (elapsed seconds as `f64`, run per frame), driven by `run_init()` / `run_tick(dt)`. Runtime errors become `Error` commands, not fatal.

**Reads vs writes.** Scripts **read** the world directly: read getters are registered on `Rc<RefCell<Board>>` (the `BoardRef` alias, exposed to Rhai as type `Board`) — getters only, so read-only by construction — and a clone of the handle is pushed into each scope as the constant `Board`, so a script writes `Board.player_x`; the getter briefly borrows the shared `Board`. Scripts **write** by enqueuing `Command { source, kind }` where `kind: GameCommand` is `Move(Direction)` / `SetTile(u32)` / `Log(LogLine)` / `Error(String)`. The host fns `move`/`set_tile`/`log` push into a shared `Rc<RefCell<Vec<Command>>>` cloned into each closure (the same `'static`-closure trick the old log sink used, generalized); `GameState::apply_commands` drains and applies them after the batch. Reads-are-live, writes-are-deferred gives frame-coherent semantics and keeps script execution free of any `&mut GameState` borrow.

**Sender identity** (which object issued a command) rides Rhai's per-call **tag**: `run` calls `call_fn_with_options(...with_tag(object_index)...)` and the host fns read it via `NativeCallContext::tag()`, so scripts write `move(North)` without a `this` argument (`North`/`South`/`East`/`West` are `Direction` constants in scope; `impl From<Direction> for (i32,i32)` yields the delta). The object's identity is currently its index into `Board::objects` — a stopgap (a `// TODO` flags replacing it with stable unique ids that survive spawn/destroy). Default `call_fn` rewinds the scope per call, so script-local persistence across ticks is still future work; the per-object `Scope` is the seam for that. `Engine`/`Scope` are not `Send` — fine for the single-threaded kiln-tui.

---

### kiln-tui — terminal player

`kiln-tui` is a **play-only** front-end (no editor yet): it takes a map-file path on the command line, loads a `Board` via `kiln_core::map_file::load`, and lets the player walk around with the arrow keys (or `hjkl`). It depends only on `kiln-core`, `ratatui`/`crossterm`, and the `color` crate — none of the egui stack. This is the proof that `kiln-core` is genuinely UI-agnostic: the engine needed **zero changes** to add a second front-end.

**Text rendering instead of bitmap tiles.**  
A terminal can't blit a bitmap font, so kiln-tui reinterprets each `Glyph.tile` index as a *character*. The default kiln font is CP437, where the tile index equals the code point, so `cp437::tile_to_char` maps indices through a 256-entry CP437→Unicode table (graphic glyphs for 0x00–0x1F, box-drawing for 0xB0–0xDF, ASCII in between). The `[font]` map section is still parsed and stored on `Board`, but kiln-tui ignores the image path and uses the tile indices directly. `Glyph.fg`/`bg` (`Rgba8`) map to `ratatui::style::Color::Rgb` — full 24-bit truecolor, alpha dropped (256-color terminals approximate it).

**`render.rs` — `BoardWidget`.**  
A `ratatui::widgets::Widget` that draws the board into the frame buffer. The `axis(board_len, view, player) -> (offset, pad, count)` helper resolves each axis independently: a board smaller than the viewport is **centered** with screen padding; a larger one **scrolls** to keep the player visible, clamped so no empty margin shows past the board edge. Per cell, glyph priority is player > object > grid floor.

**`term.rs` — capability detection + Kitty protocol.**  
`TerminalCaps { keyboard_enhancement, truecolor }` is detected once at startup: `keyboard_enhancement` via crossterm's `supports_keyboard_enhancement()` (a real query/response with the terminal), `truecolor` from `$COLORTERM`. When keyboard enhancement is supported, kiln-tui pushes the Kitty keyboard-protocol flags (`DISAMBIGUATE_ESCAPE_CODES | REPORT_EVENT_TYPES | REPORT_ALTERNATE_KEYS | REPORT_ALL_KEYS_AS_ESCAPE_CODES`) after `ratatui::init()` and pops them before `restore()`. This unlocks richer input — modifier-only keypresses, key release/repeat events, unambiguous Ctrl combos — which the engine intends to hand to scripts so they can choose bindings the terminal actually supports.

**Real-time loop.**  
The loop runs at ~30 FPS: 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` (which drives object `tick(dt)` hooks). Because `REPORT_EVENT_TYPES` makes the terminal emit Release (and Repeat) events, it acts on `Press` **and** `Repeat` — so holding a key moves continuously at the terminal's own repeat cadence — and skips `Release`, which would otherwise fire a second, spurious move. Mouse capture is enabled so the log panel can be scrolled (wheel) and resized (drag the divider). `game.run_init()` runs object `init()` hooks after the board loads and before the loop.

**Log panel + capability line.**  
The bottom-left border previews the latest log line after a `[l]og:` label; pressing `l` opens a bottom panel listing the log newest-first (`render::logline_to_line` converts each `LogLine`). `TerminalCaps::status_logline()` returns a `LogLine` (not a border badge) seeded into the log at startup: when truecolor is available it draws "truecolor" with each letter a different rainbow color (`hue_to_rgb`, a full-saturation HSV→RGB helper), else a plain "256-color", followed by the input descriptor.

**Why a capability struct, not a terminal name.**  
Scripts should branch on *what works* (truecolor, keyboard enhancement), not on a brittle terminal brand string that is often unset over SSH or inside tmux. `TerminalCaps` deliberately carries no program-identity field.

---

### `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.

---

### `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.

---

### `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).

---

### `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`. `App::active_font()` resolves `board_font.as_ref().unwrap_or(&default_font)`; at the editor-panel and glyph-picker sites the same expression is used inline instead so the font borrow stays disjoint from the `&mut editor`/`&mut board` borrows (a method call would borrow all of `self`).

`update` is a thin sequence of phase methods, each owning one concern:

1. `handle_input` — table-driven arrow keys → `GameState::try_move` (**Play mode only**)
2. `menu_bar` — File menu (Save/Save As via `save_to`/`save_as`, Exit) and the Play/Edit toggle
3. `try_show_script_editor` — if a script is open, fills the `CentralPanel` with the code editor and returns `true`, ending the frame early
4. Editor side panel (Edit mode) — `editor::show_editor_panel`, declared before `CentralPanel`; then the font-reload check
5. `show_board` — draws the viewport and returns the clicked cell; the click is dispatched to `handle_board_click` *after* it returns
6. `show_glyph_pickers` — the Palette and Objects glyph pickers; then the selected object's glyph is flushed back to `board.objects[i]`

**Click dispatch borrow split:** `show_board(&self)` only draws and returns `Option<(usize, usize)>` (validated cell coords). The mutating `handle_board_click(&mut self, cx, cy)` runs afterward, so the immutable font borrow held during drawing never conflicts with the `&mut self` handler.

**Viewport — Play mode:**  
`render::board_origin(available, board_w, board_h, player, font, zoom)` 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.palette.glyph, self.editor.palette.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.

---

### `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. Per-cell pixel size comes from `BitmapFont::cell_size(zoom)`.

**`rgba8_to_color32(c)` / `color32_to_rgba8(c)`** — inverse conversions between the core `color::Rgba8` and egui's `Color32`, used at the glyph-picker boundary where colors cross between the two type systems.

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

**`glyph_preview_button(ui, glyph, font) -> Response`** — allocates a one-cell swatch (unzoomed), paints `glyph`, and returns the click `Response`. Shared by the Palette and Objects editor tabs for their editable glyph swatches.

**`draw_glyph(painter, origin, x, y, glyph, font, zoom)`** — computes the cell rect from `font.cell_size(zoom)`, delegates to `paint_glyph`.

**`draw_board(painter, origin, board, font, zoom)`** — iterates all cells calling `draw_glyph`, then draws objects and the player as overlays on top.

**`draw_object_overlays(painter, origin, board, font, selected, zoom)`** — called from Edit mode when the Objects tab is active. Draws a dim border on every object cell and a bright border on the selected one, making objects discoverable without obscuring the underlying tile.

**`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, zoom) -> Pos2`** — computes the pixel origin for Play-mode rendering (centering or player-tracking with edge clamping).

---

### `editor.rs` — editor state and side panel

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

**`EditorState`** — transient editor state, grouped by concern into sub-structs so each tab can take only the borrow it needs (the dispatch in `show_editor_panel` passes disjoint sub-struct borrows, e.g. `&mut editor.objects` and `&mut editor.scripts`):
- `tab: EditorTab` — which side-panel tab is active
- `palette: PaletteState` — `selected: Archetype` (class to stamp), `glyph: Glyph` (visual to stamp; resets to `archetype.default_glyph()` when the archetype changes), `picker_open: bool`
- `font_dialog: FontDialog` — `open: bool`, `state: FontDialogState`
- `objects: ObjectEdit` — `selected: Option<usize>` (index into `board.objects`), `picker_open: bool`, `editing_glyph: Glyph` (local copy written back to `board.objects[i].glyph` each frame), `placing: bool` (next board click creates a new `ObjectDef`)
- `scripts: ScriptEdit` — `editing: Option<String>` (script open in the code editor), `content: String` (working copy), `new_name: String` (new-script draft)

**`show_editor_panel(ctx, editor, board, active_font)`** — renders the resizable right-side panel (default 200 px): a tab bar, then dispatch to one function per tab. `palette_tab`: scrollable archetype list and glyph preview button. `objects_tab`: "Add Object" placement toggle, click-to-select, and for the selected object a glyph preview, **Passable**/**Opaque** checkboxes (writing directly to `board.objects[i]`), and a script combobox. `board_tab`: font path label + "Font…" button + zoom slider. `scripts_tab`: create and edit named scripts. World tab: placeholder. The glyph preview swatches use `render::glyph_preview_button`.

---

### `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.

---

## 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.**

```toml
[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
tile = "#"
fg = "#aa3333"
bg = "#000000"
passable = true
script_name = "greeter"

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

[scripts]
greeter = """
fn init() {
    log(`player at ${Board.player_x}, ${Board.player_y}`);  # read
    set_tile(2);                                            # write
}
fn tick(dt) { move(North); }   # dt = elapsed seconds; dir ∈ North/South/East/West
"""
```

**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** — the runtime exists (`script.rs`): objects run optional `init()`/`tick(dt)` hooks, **read** the world via `Board.*`, and **write** via queued commands (`move`, `set_tile`, `log`). Still to come:
1. More event hooks beyond lifecycle — e.g. fire `on_touch` when the player moves onto an object's cell
2. Grow the command vocabulary and read API (`send_message`, randomness, board queries, more glyph setters)
3. **Stable object ids** — `Command.source` / `ObjectRuntime.object_index` are array indices into `Board::objects`, which break under spawn/destroy/reorder; replace with monotonically-increasing unique ids and key objects by them (a `// TODO` marks this in code)
4. Persist script-local state across ticks (needs `call_fn_raw` so the per-object `Scope` isn't rewound each call)
5. `PortalDef` is still parsed-only — no multi-board navigation yet

**Object behavior from scripts** — `ObjectDef.passable` and `ObjectDef.opaque` are live and editable in the editor, but they are static author-set values. Eventually they should be overridable by the object's Rhai script at runtime (e.g. a door script that flips `passable` when opened). `Board::is_passable` already has the right shape for this; the script runtime just needs to be wired in.

**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.

**Terminal editor** — `kiln-tui` is play-only; it has no editing UI. The editor currently lives only in `kiln-egui`, which is out of the workspace. A future TUI editor (or reviving the egui one) is an open direction.

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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.