forgotten_caves/notes/roadmap.md

# Roadmap

Local single-player roguelike with a continuously simulated, tick-based world.

Workspace layout: `sim/` (headless world simulation — tiles, entities, movement rules,
tick loop; no platform dependencies, unit-testable) and `game/` (the executable — window,
rendering, input, pathfinding, camera; owns a `Sim` and drives it). The platform layer —
window, GPU, input, RGB565 framebuffer — lives in the external `pbio` crate (git dependency,
branch `truecolor16`),
not in this repo.

History: this fork started from a client/server multiplayer architecture (authoritative UDP
server, thin client with prediction). The scope was cut to single player in September 2026;
the sim kept its authoritative shape, everything network-related was removed. The `sim`
crate's `tick`/`set_action` interface is the seam a server could be wrapped around again.

---

## Achieved milestones

- [x] PNG loader: `Image::from_png` (via the `png` crate, quantized to RGB565 on load, alpha
      mask), `to_tileset` → `Vec<Tile>` in `game/src/assets.rs`. Framebuffer is 16-bit
      truecolor (`pbio` branch `truecolor16`)
- [x] Pixel helpers: `set_pixel`, `extract`, `blit`, `blit_tile` in `game/src/game/pixelhelper.rs`
      — signed `i32` offsets, clip against all four framebuffer edges, per-tile opacity mask
- [x] Input system: `GameAction`, `InputMap`, `InputState` in `game/src/input.rs` (over `pbio::Key`)
- [x] World model: `World` of 32×32 `Chunk`s (plain `[u16; 1024]` tile ids, flags derived
      on lookup — the network-era 64-entry palette is gone), entities indexed per chunk,
      `entities_in_viewport`. Map path is the first CLI argument; `assets/map_large` is a
      generated 256×256 stress map
- [x] Map loader: Tiled-CSV parser in `sim/src/map.rs` (flip-flag masking, content-derived
      dimensions), `load_world` slices it into chunks, invisible solid border at the map rim.
      See `notes/07-map-loader.md`
- [x] Tile rendering: chunk-based world rendering straight from `World` (`render_viewport`)
- [x] Sprite-based entity rendering: entity `type_id` → tile in `entities.png`, blitted with transparency
- [x] Hybrid tick loop: 24 Hz base tick, movement in `entity_tick` every 4th tick (6 Hz);
      the game loop drives the sim with a fixed-timestep accumulator. See `notes/06-hybrid-tick-loop.md`
- [x] Intent model: one scheduled action per entity per movement window (`Sim::set_action`),
      replaced on re-set, consumed on execution. Headless tests in `sim/src/sim.rs`
- [x] 8-directional movement in chessboard geometry: world physics use the Chebyshev
      metric — diagonal and cardinal steps are the same distance, a "circle" is a square
      of tiles, matching the square viewport. The single-step rule lives in
      `sim::step_allowed` (king move onto a free tile, diagonals additionally need both
      orthogonal neighbors free — no corner cutting) and is the one function used by the
      sim and the A*. Two held keys walk diagonally.
- [x] Collision vocabulary seeded with test values (`sim::tile_collidable`): id 146
      (trees/rocks) and id 0 (empty / the invisible world border). A proper tile-data file
      format replaces this table later.
- [x] Click-to-move: framebuffer click → world tile, A* over the world
      (`sim/src/pathfind.rs`, 8-connected, Chebyshev heuristic), route fed into the
      sim one step per movement window. Keyboard input cancels the route; each step is
      re-validated at scheduling time and a blocked step replans toward the persistent
      goal. Click-and-hold steers continuously: while the button is held the route keeps
      replanning toward the tile under the cursor (only when that tile changes — cursor
      or camera movement), sweeping across blocked tiles keeps the current route.
- [x] Smooth camera follow: the camera is a float pixel position (`cam` in `game.rs`)
      that eases toward the player's *interpolated* render position (exponential
      approach, τ = 120 ms), snapping only to whole pixels at render time (and outright
      on teleport-sized corrections). The viewport renders with sub-tile offsets
      (31×31 tile pass + right-strip clip).
- [x] Entity blocking: `World::solid` (collidable tile ∨ out of world) and
      `World::blocked` (solid ∨ occupied). The sim checks the geometry rule against
      `solid` and resolves the target tile itself; the pathfinder plans with `blocked`.
      Intents execute in id order, so the player (id 1) wins a contested tile
      deterministically.
- [x] First NPC: `EntityKind { Player, Npc }` on `Entity`; `Sim::think` runs before
      every movement window and schedules a random king move for each NPC one window
      in four (seeded xorshift in `Sim`). Sprite = tile 1 in `entities.png` (a hen from
      the Mini-Medieval pack). One is spawned at (6, 6) in `Game::start`.
- [x] Bump-to-attack and death: the geometry rule is checked against terrain only
      (`World::solid`); an entity on the target tile turns the step into an attack
      (`Entity::attack` damage, `hp` saturating). At 0 hp the entity is removed. The
      sim reports both through its event queue — `sim::Event { Attacked, Died }`,
      drained by the game with `Sim::take_events` after every window — which is the
      seam for sound, message log, camera shake, spawns. Player death quits for now.
      Click-to-move treats an occupied goal as a target (route ends in a bump) but
      walks around entities in the way; keyboard steps always bump. A bump into the
      same `type_id` (species) is a refused move, not a fight.
- [x] NPC brains: `Brain { Idle, Wander, Hunt(Prey) }` as a field on `Entity` — strategy
      is data, independent of species and swappable at runtime. `Sim::think` matches on
      it. `Hunt` takes the first step of an A* route (the pathfinder moved into `sim/`
      for this) to the nearest prey within `SIGHT_RANGE` (8), replanned every window;
      out of sight or unreachable it wanders. A greedy step-toward-target was tried
      first and oscillated at any wall between hunter and prey. Six foxes
      (`Hunt(Kind(hen))`, 8 hp / 2 dmg) clear 150 wandering hens off `map_test` in
      about a minute; `cargo test -p game population -- --ignored --nocapture` watches
      it headlessly.
- [x] Entity interpolation: per-id lerp table (`EntityLerp` in `game.rs`) — previous/current
      tile plus a clock, rendered as a pixel lerp over one movement interval. Jumps of
      more than one tile (Chebyshev) snap. Purely cosmetic; game logic keeps using the
      sim's tile positions.

---

## Upcoming work

### 06 — Hybrid tick loop  *(partially done)*

Design of record: **`notes/06-hybrid-tick-loop.md`**.

- [x] 24 Hz base loop driven from the game loop
- [x] Stride layout settled: `entity_tick` at `%4` (6 Hz, sets movement rate)
- [ ] Action-point model (`energy` / `speed` per entity, act when `energy >= ACTION_COST`).
      Removed from `entity.rs` for now as premature — reintroduce when NPCs (08) actually need it.
      Then `entity_tick` can move to `%2` (12 Hz) and `speed` sets the effective movement rate.

---

### 07 — Tiled map loader  *(done — see Achieved / `notes/07-map-loader.md`)*

Remaining follow-ups when the need is concrete:

- Collision vocabulary: seeded with test values (0, 146) in `sim::tile_collidable`.
  Decide on a proper file format for tile data (collision, opacity, …) instead of a
  hardcoded match.
- Tile flipping: orientation is discarded on load; revisit with the sprite pass (09).
- Multiple / object layers (spawns, triggers) — not yet parsed. The player currently
  spawns at (0, 0).

---

### 08 — Basic NPC entity + AI budget  *(wanderer done — see Achieved)*

The wandering NPC and entity blocking have landed; `Sim::think` is the hook where the
budget model goes once there are enough NPCs to need it. The action-point model from
06 is still pending and only matters once NPC speeds should differ.

- [x] `EntityKind::Npc`, `Sim::think` scheduling intents through the same intent table
- [x] Bump-to-attack, death, `Event` queue
- [x] `Brain` enum, `Hunt` via A*
- A* per hunter per window is fine for six foxes; the think budget below becomes
  necessary once hunters number in the dozens (or `hunt` caches its route and replans
  only when the target moved).
- Hunters don't defend themselves: a fox bitten by a hen does not retaliate unless
  the hen happens to be its nearest prey. `Brain::Hunt` could switch target on `Attacked`.
- Stats beyond `hp`/`attack` (defense, speed) as plain fields; a `Stats` substruct once
  they crowd `Entity`. No ECS until entities actually differ in *which* components they
  carry — see the discussion of 2026-09-19.
- NPC reactions (flee when hit, aggro on the player) — `think` would need to see the
  previous window's events; today the game drains them right after `step`
- Per sim-tick: distribute a fixed `think_budget` across entities ordered by player proximity
- Complex entities consume more budget; simple ones less. Loop breaks at zero — natural load shedding.
- No framework. No trait objects yet. A match on `EntityKind` is fine.

```rust
let mut budget: u32 = BASE_BUDGET;
for entity in entities_by_player_proximity() {
    if budget == 0 { break; }
    budget = budget.saturating_sub(entity.think(&mut world));
}
```

---

### 09 — Client camera  *(done — see Achieved)*

Sprite rendering, camera follow and entity interpolation have all landed.

---

### Later — UI / HUD

- Bitmap font renderer: 8×8 glyph sheet PNG, `draw_text(frame, font, x, y, color, text)` — see `notes/04`
- UI primitives in `game/src/ui.rs`: `fill_rect`, `draw_rect`, `draw_hbar` — see `notes/05`
- HUD layout (320×240): HP bar + player name bottom 16px, message log right panel
- A real clip-rect on the blit primitives belongs here — once the viewport and HUD panels are
  two distinct regions, the abstraction earns its keep. Not before.

---

### Later — Distribution

- Asset embedding: `include_bytes!` for single-binary distribution (assets are currently
  read relative to the working directory)
- Save / load: the world is a plain `Sim` value — serialize it. Roguelike convention is a
  single save slot deleted on load

---

### Later — World depth

Once the simulation architecture is stable and a real level exists:

- Doors, interactive objects
- Combat: melee range check, HP, death, permadeath
- Inventory
- Day/night and weather on the slow tick (`% 192`) — affect visibility, spawns
- Procedural dungeon generation (BSP or cellular automata feeding into the same tile format)