feat: projectile system for throwing elements (Phase 4.5)

F key launches first inventory item toward mouse cursor as projectile.
Projectiles travel at 350px/s, expire after 2s, destroyed on hitting
non-walkable tiles. Color matches element's periodic table color.
13 new tests (198 total).

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/projectile.test.ts

@@ -0,0 +1,188 @@
+/**
+ * Projectile System Tests — Phase 4.5
+ *
+ * Tests: projectile creation, direction calculation,
+ * lifetime expiry, tile collision removal.
+ */
+
+import { describe, it, expect } from 'vitest';
+import { createWorld, addEntity, addComponent, query } from 'bitecs';
+import { Position, Velocity, Projectile, SpriteRef } from '../src/ecs/components';
+import {
+  launchProjectile,
+  projectileSystem,
+  calculateDirection,
+  type ProjectileData,
+  PROJECTILE_SPEED,
+  PROJECTILE_LIFETIME,
+} from '../src/player/projectile';
+
+// === Direction Calculation ===
+
+describe('calculateDirection', () => {
+  it('normalizes direction vector', () => {
+    const d = calculateDirection(0, 0, 100, 0);
+    expect(d.dx).toBeCloseTo(1);
+    expect(d.dy).toBeCloseTo(0);
+  });
+
+  it('handles diagonal direction', () => {
+    const d = calculateDirection(0, 0, 100, 100);
+    const mag = Math.sqrt(d.dx * d.dx + d.dy * d.dy);
+    expect(mag).toBeCloseTo(1);
+  });
+
+  it('handles negative direction', () => {
+    const d = calculateDirection(100, 100, 0, 0);
+    expect(d.dx).toBeCloseTo(-1 / Math.SQRT2);
+    expect(d.dy).toBeCloseTo(-1 / Math.SQRT2);
+  });
+
+  it('returns zero for same position', () => {
+    const d = calculateDirection(50, 50, 50, 50);
+    expect(d.dx).toBe(1); // default to right
+    expect(d.dy).toBe(0);
+  });
+});
+
+// === Launch Projectile ===
+
+describe('launchProjectile', () => {
+  it('creates entity at source position', () => {
+    const world = createWorld();
+    const projData = new Map<number, ProjectileData>();
+
+    const eid = launchProjectile(world, projData, 100, 200, 200, 200, 'Na');
+
+    expect(Position.x[eid]).toBe(100);
+    expect(Position.y[eid]).toBe(200);
+  });
+
+  it('sets velocity toward target', () => {
+    const world = createWorld();
+    const projData = new Map<number, ProjectileData>();
+
+    const eid = launchProjectile(world, projData, 0, 0, 100, 0, 'Na');
+
+    expect(Velocity.vx[eid]).toBeCloseTo(PROJECTILE_SPEED);
+    expect(Velocity.vy[eid]).toBeCloseTo(0);
+  });
+
+  it('stores chemical data', () => {
+    const world = createWorld();
+    const projData = new Map<number, ProjectileData>();
+
+    const eid = launchProjectile(world, projData, 0, 0, 100, 0, 'Na');
+
+    expect(projData.get(eid)).toBeDefined();
+    expect(projData.get(eid)!.itemId).toBe('Na');
+  });
+
+  it('sets lifetime', () => {
+    const world = createWorld();
+    const projData = new Map<number, ProjectileData>();
+
+    const eid = launchProjectile(world, projData, 0, 0, 100, 0, 'Na');
+
+    expect(Projectile.lifetime[eid]).toBe(PROJECTILE_LIFETIME);
+  });
+
+  it('sets sprite with element color', () => {
+    const world = createWorld();
+    const projData = new Map<number, ProjectileData>();
+
+    const eid = launchProjectile(world, projData, 0, 0, 100, 0, 'Na');
+
+    expect(SpriteRef.radius[eid]).toBeGreaterThan(0);
+    expect(SpriteRef.color[eid]).toBeDefined();
+  });
+});
+
+// === Projectile System ===
+
+describe('projectileSystem', () => {
+  it('decrements lifetime', () => {
+    const world = createWorld();
+    const eid = addEntity(world);
+    addComponent(world, eid, Position);
+    addComponent(world, eid, Velocity);
+    addComponent(world, eid, Projectile);
+    Position.x[eid] = 48;
+    Position.y[eid] = 48;
+    Velocity.vx[eid] = 100;
+    Velocity.vy[eid] = 0;
+    Projectile.lifetime[eid] = 1000;
+
+    const grid = [[0, 0, 0], [0, 0, 0], [0, 0, 0]];
+    const walkable = new Set([0]);
+    const projData = new Map<number, ProjectileData>();
+    projData.set(eid, { itemId: 'Na' });
+
+    projectileSystem(world, 100, grid, 32, walkable, projData);
+
+    expect(Projectile.lifetime[eid]).toBe(900);
+  });
+
+  it('removes expired projectiles', () => {
+    const world = createWorld();
+    const eid = addEntity(world);
+    addComponent(world, eid, Position);
+    addComponent(world, eid, Velocity);
+    addComponent(world, eid, Projectile);
+    Position.x[eid] = 48;
+    Position.y[eid] = 48;
+    Projectile.lifetime[eid] = 50;
+
+    const grid = [[0, 0, 0], [0, 0, 0], [0, 0, 0]];
+    const walkable = new Set([0]);
+    const projData = new Map<number, ProjectileData>();
+    projData.set(eid, { itemId: 'Na' });
+
+    projectileSystem(world, 100, grid, 32, walkable, projData);
+
+    expect(query(world, [Projectile]).length).toBe(0);
+    expect(projData.size).toBe(0);
+  });
+
+  it('removes projectiles on non-walkable tile', () => {
+    const world = createWorld();
+    const eid = addEntity(world);
+    addComponent(world, eid, Position);
+    addComponent(world, eid, Velocity);
+    addComponent(world, eid, Projectile);
+    // Position on non-walkable tile (1,1) = tile 3
+    Position.x[eid] = 48;
+    Position.y[eid] = 48;
+    Projectile.lifetime[eid] = 5000;
+
+    const grid = [[0, 0, 0], [0, 3, 0], [0, 0, 0]];
+    const walkable = new Set([0]);
+    const projData = new Map<number, ProjectileData>();
+    projData.set(eid, { itemId: 'Na' });
+
+    projectileSystem(world, 16, grid, 32, walkable, projData);
+
+    expect(query(world, [Projectile]).length).toBe(0);
+  });
+
+  it('keeps valid projectiles alive', () => {
+    const world = createWorld();
+    const eid = addEntity(world);
+    addComponent(world, eid, Position);
+    addComponent(world, eid, Velocity);
+    addComponent(world, eid, Projectile);
+    Position.x[eid] = 48;
+    Position.y[eid] = 48;
+    Projectile.lifetime[eid] = 5000;
+
+    const grid = [[0, 0, 0], [0, 0, 0], [0, 0, 0]];
+    const walkable = new Set([0]);
+    const projData = new Map<number, ProjectileData>();
+    projData.set(eid, { itemId: 'Na' });
+
+    projectileSystem(world, 16, grid, 32, walkable, projData);
+
+    expect(query(world, [Projectile]).length).toBe(1);
+    expect(projData.size).toBe(1);
+  });
+});