Extract headless sim crate from server

Move chunk, entity, world, map, sim and load_world into a new sim library
crate together with the game rules from shared (player_action, action_delta,
delta_action, step_allowed, tile_collidable, tick constants). shared keeps
only the wire types and re-exports the rules, so server and client compile
unchanged. First step toward a local single-player build.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

sim/src/chunk.rs

@@ -0,0 +1,102 @@
+#[derive(Clone, Copy, Default, Debug, PartialEq, Eq)]
+pub struct TileFlags(pub u8);
+
+impl TileFlags {
+    pub const COLLIDABLE: u8 = 1 << 0;
+    pub const OPAQUE:     u8 = 1 << 1;
+    pub const LIQUID:     u8 = 1 << 2;
+
+    pub fn collidable(self) -> bool { self.0 & Self::COLLIDABLE != 0 }
+    pub fn opaque(self)     -> bool { self.0 & Self::OPAQUE != 0 }
+    pub fn liquid(self)     -> bool { self.0 & Self::LIQUID != 0 }
+}
+
+#[derive(Clone, Copy, Default, Debug, PartialEq, Eq)]
+pub struct TileDef { pub tile_id: u16, pub flags: TileFlags }
+
+#[derive(Debug)]
+pub struct PaletteFullError;
+
+pub struct Chunk {
+    pub palette:   [TileDef; 64],
+    pub pal_count: u8,
+    pub tiles:     Box<[u8; 1024]>,
+    pub version:   u16,
+}
+
+impl Chunk {
+    /// Runtime use: all tiles empty, pal_count=1, version=1.
+    /// Clients with version=0 will always request this chunk.
+    pub fn new() -> Self {
+        Self {
+            palette:   [TileDef::default(); 64],
+            pal_count: 1,
+            tiles:     Box::new([0u8; 1024]),
+            version:   1,
+        }
+    }
+
+    /// Find existing palette entry or insert a new one.
+    /// Index 0 is always the null sentinel; this short-circuits for default TileDef.
+    /// Returns (palette_index, was_new). Does NOT bump version.
+    fn find_or_insert_raw(&mut self, def: TileDef) -> Result<(u8, bool), PaletteFullError> {
+        if def == TileDef::default() {
+            return Ok((0, false));
+        }
+        for i in 1..self.pal_count as usize {
+            if self.palette[i] == def {
+                return Ok((i as u8, false));
+            }
+        }
+        if self.pal_count >= 64 {
+            return Err(PaletteFullError);
+        }
+        let idx = self.pal_count;
+        self.palette[idx as usize] = def;
+        self.pal_count += 1;
+        Ok((idx, true))
+    }
+
+    /// World gen path. Calls the closure for every tile coordinate.
+    /// version=0 so the first StatePacket causes all clients to request it.
+    pub fn generate(f: impl Fn(u8, u8) -> TileDef) -> Result<Self, PaletteFullError> {
+        let mut chunk = Self {
+            palette:   [TileDef::default(); 64],
+            pal_count: 1,
+            tiles:     Box::new([0u8; 1024]),
+            version:   0,
+        };
+        for ly in 0u8..32 {
+            for lx in 0u8..32 {
+                let def = f(lx, ly);
+                let (idx, _) = chunk.find_or_insert_raw(def)?;
+                chunk.tiles[ly as usize * 32 + lx as usize] = idx;
+            }
+        }
+        chunk.version = 1;
+        Ok(chunk)
+    }
+
+    /// Set a tile at local coords. Bumps version if tile index or palette changed.
+    pub fn set_tile(&mut self, lx: u8, ly: u8, def: TileDef) -> Result<(), PaletteFullError> {
+        let (idx, was_new) = self.find_or_insert_raw(def)?;
+        let slot = &mut self.tiles[ly as usize * 32 + lx as usize];
+        let changed = *slot != idx;
+        *slot = idx;
+        if changed || was_new {
+            self.version = self.version.wrapping_add(1);
+            if self.version == 0 {
+                self.version = 1;
+            }
+        }
+        Ok(())
+    }
+
+    pub fn get_tile(&self, lx: u8, ly: u8) -> TileDef {
+        self.palette[self.tiles[ly as usize * 32 + lx as usize] as usize]
+    }
+
+    pub fn tile_flags(&self, lx: u8, ly: u8) -> TileFlags {
+        self.get_tile(lx, ly).flags
+    }
+}

sim/src/entity.rs

@@ -0,0 +1,19 @@
+pub struct Entity {
+    pub id:      u32,
+    pub type_id: u16,   // 0 = player
+    pub pos:     (i16, i16),
+    pub hp:      u16,
+    pub hp_max:  u16,
+}
+
+impl Entity {
+    pub fn new(id: u32, type_id: u16, pos: (i16, i16), hp: u16) -> Self {
+        Self {
+            id,
+            type_id,
+            pos,
+            hp,
+            hp_max: hp,
+        }
+    }
+}

sim/src/lib.rs

@@ -0,0 +1,126 @@
+//! Headless world simulation: tiles, entities, movement rules and the tick loop.
+//! No platform or rendering dependencies — the game binary drives `Sim::tick`.
+
+pub mod chunk;
+pub mod entity;
+pub mod map;
+pub mod world;
+mod sim;
+
+pub use sim::Sim;
+
+use chunk::{Chunk, TileDef, TileFlags};
+use map::TileMap;
+use world::World;
+
+pub mod player_action {
+    pub const NOOP:       u16 = 0;
+    pub const NORTH:      u16 = 1;
+    pub const EAST:       u16 = 2;
+    pub const SOUTH:      u16 = 3;
+    pub const WEST:       u16 = 4;
+    pub const NORTH_EAST: u16 = 5;
+    pub const SOUTH_EAST: u16 = 6;
+    pub const SOUTH_WEST: u16 = 7;
+    pub const NORTH_WEST: u16 = 8;
+}
+
+/// The movement delta of an action, or `None` for `NOOP` and unknown values.
+pub fn action_delta(action: u16) -> Option<(i32, i32)> {
+    match action {
+        player_action::NORTH      => Some((0, -1)),
+        player_action::EAST       => Some((1,  0)),
+        player_action::SOUTH      => Some((0,  1)),
+        player_action::WEST       => Some((-1, 0)),
+        player_action::NORTH_EAST => Some((1, -1)),
+        player_action::SOUTH_EAST => Some((1,  1)),
+        player_action::SOUTH_WEST => Some((-1, 1)),
+        player_action::NORTH_WEST => Some((-1, -1)),
+        _                         => None,
+    }
+}
+
+/// The action for a single-step delta. Inverse of [`action_delta`]; panics on anything
+/// that is not a king move.
+pub fn delta_action(dx: i32, dy: i32) -> u16 {
+    match (dx, dy) {
+        (0, -1)  => player_action::NORTH,
+        (1,  0)  => player_action::EAST,
+        (0,  1)  => player_action::SOUTH,
+        (-1, 0)  => player_action::WEST,
+        (1, -1)  => player_action::NORTH_EAST,
+        (1,  1)  => player_action::SOUTH_EAST,
+        (-1, 1)  => player_action::SOUTH_WEST,
+        (-1, -1) => player_action::NORTH_WEST,
+        d        => panic!("non-step delta {d:?}"),
+    }
+}
+
+/// The single-step movement rule, shared by the sim and the pathfinder so both agree
+/// on the same physics.
+///
+/// World geometry is chessboard (Chebyshev): diagonal and cardinal steps are the same
+/// distance, so a step is any king move onto a free tile. A diagonal step additionally
+/// requires *both* orthogonal neighbor tiles to be free — no squeezing between two
+/// diagonally touching blockers (corner cutting).
+pub fn step_allowed(
+    from: (i32, i32),
+    to: (i32, i32),
+    blocked: impl Fn(i32, i32) -> bool,
+) -> bool {
+    let (dx, dy) = (to.0 - from.0, to.1 - from.1);
+    if dx.abs() > 1 || dy.abs() > 1 || (dx == 0 && dy == 0) {
+        return false;
+    }
+    if blocked(to.0, to.1) {
+        return false;
+    }
+    dx == 0 || dy == 0 || (!blocked(from.0 + dx, from.1) && !blocked(from.0, from.1 + dy))
+}
+
+/// Base tick rate. The timeline all action scheduling is expressed in.
+pub const TICK_HZ: u32 = 24;
+
+/// Movement resolves on every `TICKS_PER_MOVE`-th tick — one "movement window" per
+/// `TICKS_PER_MOVE` ticks (6 Hz). Window `w` executes at tick `w * TICKS_PER_MOVE`.
+pub const TICKS_PER_MOVE: u32 = 4;
+
+/// How many *future* movement windows an actor may address. Actions targeted beyond
+/// this horizon are dropped.
+pub const ACTION_WINDOW_HORIZON: usize = 3;
+
+/// Movement vocabulary: whether a tile id blocks movement. Keep this in lockstep with
+/// the tileset in `overworld.tga`.
+///
+/// Test values for now — a proper tile-data file format replaces this table later.
+pub fn tile_collidable(tile_id: u16) -> bool {
+    match tile_id {
+        0   => true, // empty / world border (chunks past the map rim are padded with id 0)
+        146 => true, // trees / rocks
+        _   => false,
+    }
+}
+
+/// Build a world from a loaded tile map. The map's top-left tile sits at world (0, 0); the
+/// map is sliced into 32×32 chunks. Tiles inside a loaded chunk but outside the authored map
+/// become an invisible solid border (id 0, collidable), so the walkable world edge sits flush
+/// with the visible map rim instead of with the larger chunk boundary.
+pub fn load_world(map: &TileMap) -> World {
+    let mut w = World::new();
+    let max_cx = (map.width.saturating_sub(1) >> 5) as i16;
+    let max_cy = (map.height.saturating_sub(1) >> 5) as i16;
+    for cy in 0..=max_cy {
+        for cx in 0..=max_cx {
+            let chunk = Chunk::generate(|lx, ly| {
+                let (wx, wy) = world::local_to_tile(cx, cy, lx, ly);
+                if wx < 0 || wy < 0 || wx >= map.width as i16 || wy >= map.height as i16 {
+                    return TileDef { tile_id: 0, flags: TileFlags(TileFlags::COLLIDABLE) };
+                }
+                let id = map.tile(wx, wy);
+                TileDef { tile_id: id, flags: map::tile_flags(id) }
+            }).expect("palette overflow during world gen");
+            w.set_chunk(cx, cy, chunk);
+        }
+    }
+    w
+}

sim/src/map.rs

@@ -0,0 +1,76 @@
+//! Minimal Tiled-CSV map loader.
+//!
+//! Reads a bare comma-separated tile grid (Tiled's "CSV" layer export, stripped of the
+//! surrounding TMX/XML). Dimensions are derived from the content: columns from the first
+//! row, height from the row count. Tiled encodes per-tile flip/rotation in the top bits of
+//! each 32-bit GID, which makes flipped tiles show up as large negative decimals — those
+//! bits are masked off here, leaving the bare tile_id. Flip orientation is discarded for now.
+
+use crate::chunk::TileFlags;
+
+/// Top three bits hold horizontal / vertical / diagonal flip flags; the rest is the tile id.
+const GID_FLIP_MASK: u32 = 0x1FFF_FFFF;
+
+pub struct TileMap {
+    pub width:  u16,
+    pub height: u16,
+    pub tiles:  Vec<u16>, // row-major, len == width * height
+}
+
+impl TileMap {
+    pub fn from_csv(path: &str) -> Self {
+        let text = std::fs::read_to_string(path)
+            .unwrap_or_else(|e| panic!("failed to read map {path}: {e}"));
+
+        let mut tiles = Vec::new();
+        let mut width = 0usize;
+        let mut height = 0usize;
+
+        for line in text.lines() {
+            if line.trim().is_empty() {
+                continue;
+            }
+            let mut cols = 0usize;
+            for field in line.split(',') {
+                let field = field.trim();
+                if field.is_empty() {
+                    continue; // tolerate a trailing comma
+                }
+                // Parse as i64 so the "negative" flipped GIDs fit, then reinterpret the low
+                // 32 bits as u32 and strip the flip flags.
+                let raw = field.parse::<i64>()
+                    .unwrap_or_else(|_| panic!("invalid tile value {field:?} in {path}"));
+                let gid = (raw as u32) & GID_FLIP_MASK;
+                tiles.push(gid as u16);
+                cols += 1;
+            }
+            if width == 0 {
+                width = cols;
+            } else if cols != width {
+                panic!("ragged map row in {path}: expected {width} cols, got {cols}");
+            }
+            height += 1;
+        }
+
+        TileMap { width: width as u16, height: height as u16, tiles }
+    }
+
+    /// Tile id at map coordinates, or 0 (empty) when outside the map bounds.
+    pub fn tile(&self, x: i16, y: i16) -> u16 {
+        if x < 0 || y < 0 || x >= self.width as i16 || y >= self.height as i16 {
+            return 0;
+        }
+        self.tiles[y as usize * self.width as usize + x as usize]
+    }
+}
+
+/// Gameplay flags for a global tile id. Collision comes from the shared movement
+/// vocabulary (`crate::tile_collidable`) so the client can predict it identically;
+/// other flags (opaque, …) stay sim-side. Tile ids index into `overworld.tga`.
+pub fn tile_flags(tile_id: u16) -> TileFlags {
+    let mut bits = 0u8;
+    if crate::tile_collidable(tile_id) {
+        bits |= TileFlags::COLLIDABLE;
+    }
+    TileFlags(bits)
+}

sim/src/sim.rs

@@ -0,0 +1,85 @@
+use std::collections::{BTreeMap, HashMap};
+use crate::{ACTION_WINDOW_HORIZON, TICKS_PER_MOVE};
+use crate::world::World;
+
+pub struct Sim {
+    pub world:   World,
+    /// Scheduled actions per entity, keyed by movement window (`tick / TICKS_PER_MOVE`).
+    /// The window slot is the whole ordering model: an action addressed to an occupied
+    /// window *replaces* it (retraction and rescheduling by the client, dedup of
+    /// retransmits), a late action only fills the next window if it is empty, and
+    /// anything past `ACTION_WINDOW_HORIZON` is dropped — so no client can grow server
+    /// memory or move faster than one action per window.
+    pending:     HashMap<u32, BTreeMap<u32, u16>>,
+}
+
+impl Sim {
+    pub fn new(world: World) -> Self {
+        Self { world, pending: HashMap::new() }
+    }
+
+    pub fn tick(&mut self, tick: u32, actions: &[(u32, u32, u16)]) {
+        // The earliest window still addressable at this tick. On a movement tick that is
+        // the window executing *this call* — actions arriving the same tick still count.
+        let min_window = tick.div_ceil(TICKS_PER_MOVE);
+
+        for &(entity_id, target_tick, action) in actions {
+            let slots = self.pending.entry(entity_id).or_default();
+            let window = target_tick.div_ceil(TICKS_PER_MOVE);
+            if window < min_window {
+                // Late. A movement action keeps its *order* instead of its time: it
+                // fills the first still-empty upcoming window, so bunched late arrivals
+                // don't collapse onto one slot and eat each other. A late NOOP is
+                // dropped — once its window has passed, its cancellation intent is
+                // ambiguous, and as a gap-filler it would block real steps (worst case
+                // one stale step executes; the client's reconciliation handles that).
+                if crate::action_delta(action).is_some() {
+                    let gap = (min_window..min_window + ACTION_WINDOW_HORIZON as u32)
+                        .find(|w| !slots.contains_key(w));
+                    if let Some(w) = gap {
+                        slots.insert(w, action);
+                    }
+                }
+            } else if window - min_window < ACTION_WINDOW_HORIZON as u32 {
+                slots.insert(window, action); // newest addressing wins
+            }
+        }
+
+        if tick.is_multiple_of(TICKS_PER_MOVE) {
+            self.entity_tick(tick / TICKS_PER_MOVE);
+        }
+    }
+
+    fn entity_tick(&mut self, window: u32) {
+        // Exactly one action per entity per movement window, then the window is gone.
+        let actions: Vec<(u32, u16)> = self.pending.iter_mut()
+            .filter_map(|(&id, slots)| {
+                let action = slots.remove(&window);
+                slots.retain(|&w, _| w > window); // drop anything the timeline passed by
+                action.map(|a| (id, a))
+            })
+            .collect();
+        self.pending.retain(|_, slots| !slots.is_empty());
+
+        for (entity_id, action) in actions {
+            let delta = match crate::action_delta(action) {
+                Some(d) => d,
+                None => continue, // NOOP or garbage
+            };
+            let pos = match self.world.entities.get(&entity_id) {
+                Some(e) => e.pos,
+                None => continue,
+            };
+            let from = (pos.0 as i32, pos.1 as i32);
+            let to = (from.0 + delta.0, from.1 + delta.1);
+            // `step_allowed` is the shared movement rule (chessboard geometry, no corner
+            // cutting) — the client predicts with the exact same function.
+            let allowed = crate::step_allowed(from, to, |x, y| {
+                self.world.tile_flags(x as i16, y as i16).map_or(true, |f| f.collidable())
+            });
+            if allowed {
+                self.world.move_entity(entity_id, (to.0 as i16, to.1 as i16));
+            }
+        }
+    }
+}

sim/src/world.rs

@@ -0,0 +1,126 @@
+use std::collections::HashMap;
+use crate::chunk::{Chunk, TileDef, TileFlags, PaletteFullError};
+use crate::entity::Entity;
+
+pub fn tile_to_chunk(tx: i16, ty: i16) -> (i16, i16) {
+    (tx >> 5, ty >> 5)
+}
+
+pub fn tile_to_local(tx: i16, ty: i16) -> (u8, u8) {
+    ((tx & 31) as u8, (ty & 31) as u8)
+}
+
+pub fn local_to_tile(cx: i16, cy: i16, lx: u8, ly: u8) -> (i16, i16) {
+    ((cx << 5) | lx as i16, (cy << 5) | ly as i16)
+}
+
+pub struct World {
+    chunks:         HashMap<(i16, i16), Chunk>,
+    pub entities:   HashMap<u32, Entity>,
+    chunk_entities: HashMap<(i16, i16), Vec<u32>>,
+    next_entity_id: u32,
+}
+
+impl World {
+    pub fn new() -> Self {
+        Self {
+            chunks:         HashMap::new(),
+            entities:       HashMap::new(),
+            chunk_entities: HashMap::new(),
+            next_entity_id: 1,
+        }
+    }
+
+    pub fn chunk(&self, cx: i16, cy: i16) -> Option<&Chunk> {
+        self.chunks.get(&(cx, cy))
+    }
+
+    pub fn set_chunk(&mut self, cx: i16, cy: i16, chunk: Chunk) {
+        self.chunks.insert((cx, cy), chunk);
+    }
+
+    pub fn tile_flags(&self, wx: i16, wy: i16) -> Option<TileFlags> {
+        let (cx, cy) = tile_to_chunk(wx, wy);
+        let (lx, ly) = tile_to_local(wx, wy);
+        self.chunks.get(&(cx, cy)).map(|c| c.tile_flags(lx, ly))
+    }
+
+    pub fn set_tile(&mut self, wx: i16, wy: i16, def: TileDef) -> Result<(), PaletteFullError> {
+        let (cx, cy) = tile_to_chunk(wx, wy);
+        let (lx, ly) = tile_to_local(wx, wy);
+        let chunk = self.chunks.entry((cx, cy)).or_insert_with(Chunk::new);
+        chunk.set_tile(lx, ly, def)
+    }
+
+    pub fn spawn_entity(&mut self, type_id: u16, pos: (i16, i16), hp: u16) -> u32 {
+        let id = self.next_entity_id;
+        self.next_entity_id += 1;
+        let entity = Entity::new(id, type_id, pos, hp);
+        self.add_entity(entity);
+        id
+    }
+
+    pub fn add_entity(&mut self, entity: Entity) {
+        let chunk_key = tile_to_chunk(entity.pos.0, entity.pos.1);
+        let id = entity.id;
+        self.entities.insert(id, entity);
+        self.chunk_entities.entry(chunk_key).or_default().push(id);
+    }
+
+    pub fn remove_entity(&mut self, id: u32) {
+        if let Some(entity) = self.entities.remove(&id) {
+            let chunk_key = tile_to_chunk(entity.pos.0, entity.pos.1);
+            if let Some(list) = self.chunk_entities.get_mut(&chunk_key) {
+                list.retain(|&eid| eid != id);
+            }
+        }
+    }
+
+    pub fn move_entity(&mut self, id: u32, new_pos: (i16, i16)) {
+        if let Some(entity) = self.entities.get_mut(&id) {
+            let old_chunk = tile_to_chunk(entity.pos.0, entity.pos.1);
+            let new_chunk = tile_to_chunk(new_pos.0, new_pos.1);
+            entity.pos = new_pos;
+            if old_chunk != new_chunk {
+                if let Some(list) = self.chunk_entities.get_mut(&old_chunk) {
+                    list.retain(|&eid| eid != id);
+                }
+                self.chunk_entities.entry(new_chunk).or_default().push(id);
+            }
+        }
+    }
+
+    pub fn entities_in_chunk(&self, cx: i16, cy: i16) -> &[u32] {
+        self.chunk_entities.get(&(cx, cy)).map_or(&[], Vec::as_slice)
+    }
+
+    /// Returns all entities within a 30×30 tile viewport centered on `center`.
+    pub fn entities_in_viewport(&self, center: (i16, i16)) -> Vec<&Entity> {
+        let half: i16 = 15;
+        let min_x = center.0 - half;
+        let max_x = center.0 + half - 1;
+        let min_y = center.1 - half;
+        let max_y = center.1 + half - 1;
+
+        let min_cx = tile_to_chunk(min_x, min_y).0;
+        let max_cx = tile_to_chunk(max_x, max_y).0;
+        let min_cy = tile_to_chunk(min_x, min_y).1;
+        let max_cy = tile_to_chunk(max_x, max_y).1;
+
+        let mut result = Vec::new();
+        for cy in min_cy..=max_cy {
+            for cx in min_cx..=max_cx {
+                for &eid in self.entities_in_chunk(cx, cy) {
+                    if let Some(e) = self.entities.get(&eid) {
+                        if e.pos.0 >= min_x && e.pos.0 <= max_x
+                            && e.pos.1 >= min_y && e.pos.1 <= max_y
+                        {
+                            result.push(e);
+                        }
+                    }
+                }
+            }
+        }
+        result
+    }
+}