western-shooter/src/systems/bullet-system.ts

import { INTERNAL_WIDTH, INTERNAL_HEIGHT } from '../constants';
import type { Color, Sprite } from '../types';

export type BulletDirection = 'left' | 'forward' | 'right';

export interface Bullet {
  x: number;
  y: number;
  prevX: number;
  prevY: number;
  vx: number;
  vy: number;
  width: number;
  height: number;
  color: Color;
  active: boolean;
  isPlayerBullet: boolean;
  damage: number;
  poolIndex: number; // Track position in pool for O(1) deactivation
}

// Bullet pool for performance
const MAX_BULLETS = 2000;

// Bullet constants
const PLAYER_BULLET_SPEED = 250; // pixels per second
const PLAYER_BULLET_WIDTH = 3;
const PLAYER_BULLET_HEIGHT = 8;
const PLAYER_BULLET_COLOR: Color = { r: 1.0, g: 0.9, b: 0.3, a: 1.0 }; // Yellow

// Direction angles (in radians from vertical)
const DIRECTION_ANGLES: Record<BulletDirection, number> = {
  left: -Math.PI / 6,    // -30 degrees
  forward: 0,            // straight up
  right: Math.PI / 6,    // +30 degrees
};

export class BulletSystem {
  private bullets: Bullet[] = [];
  private activeCount = 0;

  // Free list for O(1) bullet allocation
  private freeList: number[] = [];
  private freeListHead = 0;

  // Cached arrays to avoid allocations
  private playerBulletCache: Bullet[] = [];
  private enemyBulletCache: Bullet[] = [];
  private spriteCache: Sprite[] = [];

  constructor() {
    // Pre-allocate bullet pool
    for (let i = 0; i < MAX_BULLETS; i++) {
      this.bullets.push({
        x: 0,
        y: 0,
        prevX: 0,
        prevY: 0,
        vx: 0,
        vy: 0,
        width: PLAYER_BULLET_WIDTH,
        height: PLAYER_BULLET_HEIGHT,
        color: PLAYER_BULLET_COLOR,
        active: false,
        isPlayerBullet: true,
        damage: 1,
        poolIndex: i,
      });
      // Initialize free list
      this.freeList.push(i);
    }
    this.freeListHead = MAX_BULLETS;

    // Pre-allocate cache arrays
    this.playerBulletCache = new Array(MAX_BULLETS);
    this.enemyBulletCache = new Array(MAX_BULLETS);
    this.spriteCache = new Array(MAX_BULLETS);
    for (let i = 0; i < MAX_BULLETS; i++) {
      this.spriteCache[i] = { x: 0, y: 0, width: 0, height: 0, color: { r: 0, g: 0, b: 0, a: 0 } };
    }
  }

  spawnPlayerBullet(
    x: number,
    y: number,
    direction: BulletDirection,
    speedMultiplier: number = 1,
    damageMultiplier: number = 1
  ): void {
    const bullet = this.getInactiveBullet();
    if (!bullet) return;

    const angle = DIRECTION_ANGLES[direction];
    const speed = PLAYER_BULLET_SPEED * speedMultiplier;

    bullet.x = x - PLAYER_BULLET_WIDTH / 2;
    bullet.y = y;
    bullet.prevX = bullet.x;
    bullet.prevY = bullet.y;
    bullet.vx = Math.sin(angle) * speed;
    bullet.vy = -Math.cos(angle) * speed; // Negative because Y goes down
    bullet.width = PLAYER_BULLET_WIDTH;
    bullet.height = PLAYER_BULLET_HEIGHT;
    bullet.color = PLAYER_BULLET_COLOR;
    bullet.active = true;
    bullet.isPlayerBullet = true;
    bullet.damage = damageMultiplier;
  }

  spawnEnemyBullet(
    x: number,
    y: number,
    vx: number,
    vy: number,
    color: Color = { r: 1.0, g: 0.3, b: 0.3, a: 1.0 }
  ): void {
    const bullet = this.getInactiveBullet();
    if (!bullet) return;

    bullet.x = x;
    bullet.y = y;
    bullet.prevX = x;
    bullet.prevY = y;
    bullet.vx = vx;
    bullet.vy = vy;
    bullet.width = 4;
    bullet.height = 4;
    bullet.color = color;
    bullet.active = true;
    bullet.isPlayerBullet = false;
    bullet.damage = 1;
  }

  private getInactiveBullet(): Bullet | null {
    // O(1) allocation from free list
    if (this.freeListHead <= 0) {
      return null;
    }
    this.freeListHead--;
    const index = this.freeList[this.freeListHead];
    return this.bullets[index];
  }

  private returnToFreeList(bullet: Bullet): void {
    // O(1) return to free list
    if (this.freeListHead < MAX_BULLETS) {
      this.freeList[this.freeListHead] = bullet.poolIndex;
      this.freeListHead++;
    }
  }

  update(dt: number): void {
    this.activeCount = 0;
    let playerCount = 0;
    let enemyCount = 0;

    for (let i = 0; i < MAX_BULLETS; i++) {
      const bullet = this.bullets[i];
      if (!bullet.active) continue;

      // Store previous position
      bullet.prevX = bullet.x;
      bullet.prevY = bullet.y;

      // Update position
      bullet.x += bullet.vx * dt;
      bullet.y += bullet.vy * dt;

      // Check bounds (with margin)
      const margin = 20;
      if (
        bullet.x < -margin ||
        bullet.x > INTERNAL_WIDTH + margin ||
        bullet.y < -margin ||
        bullet.y > INTERNAL_HEIGHT + margin
      ) {
        bullet.active = false;
        this.returnToFreeList(bullet);
        continue;
      }

      // Cache bullets by type for collision detection
      if (bullet.isPlayerBullet) {
        this.playerBulletCache[playerCount++] = bullet;
      } else {
        this.enemyBulletCache[enemyCount++] = bullet;
      }

      this.activeCount++;
    }

    // Store counts for getters
    (this as any)._playerBulletCount = playerCount;
    (this as any)._enemyBulletCount = enemyCount;
  }

  // Iterator for sprites - avoids array allocation
  *iterateSprites(alpha: number): Generator<Sprite> {
    for (let i = 0; i < MAX_BULLETS; i++) {
      const bullet = this.bullets[i];
      if (!bullet.active) continue;

      // Interpolate position
      const x = bullet.prevX + (bullet.x - bullet.prevX) * alpha;
      const y = bullet.prevY + (bullet.y - bullet.prevY) * alpha;

      // Reuse cached sprite object
      const sprite = this.spriteCache[i];
      sprite.x = x;
      sprite.y = y;
      sprite.width = bullet.width;
      sprite.height = bullet.height;
      sprite.color = bullet.color;

      yield sprite;
    }
  }

  // Batch render method - more efficient than getSprites
  forEachSprite(alpha: number, callback: (sprite: Sprite) => void): void {
    for (let i = 0; i < MAX_BULLETS; i++) {
      const bullet = this.bullets[i];
      if (!bullet.active) continue;

      // Interpolate position
      const x = bullet.prevX + (bullet.x - bullet.prevX) * alpha;
      const y = bullet.prevY + (bullet.y - bullet.prevY) * alpha;

      // Reuse cached sprite object
      const sprite = this.spriteCache[i];
      sprite.x = x;
      sprite.y = y;
      sprite.width = bullet.width;
      sprite.height = bullet.height;
      sprite.color = bullet.color;

      callback(sprite);
    }
  }

  getSprites(alpha: number): Sprite[] {
    // Backward compatible version - still allocates but reuses sprite data
    const sprites: Sprite[] = [];

    for (let i = 0; i < MAX_BULLETS; i++) {
      const bullet = this.bullets[i];
      if (!bullet.active) continue;

      // Interpolate position
      const x = bullet.prevX + (bullet.x - bullet.prevX) * alpha;
      const y = bullet.prevY + (bullet.y - bullet.prevY) * alpha;

      sprites.push({
        x,
        y,
        width: bullet.width,
        height: bullet.height,
        color: bullet.color,
      });
    }

    return sprites;
  }

  // Get active bullets for collision detection - returns cached view
  getPlayerBullets(): Bullet[] {
    const count = (this as any)._playerBulletCount || 0;
    // Return a slice of the cached array (still allocates, but much smaller)
    return this.playerBulletCache.slice(0, count);
  }

  getEnemyBullets(): Bullet[] {
    const count = (this as any)._enemyBulletCount || 0;
    return this.enemyBulletCache.slice(0, count);
  }

  // Zero-allocation collision iteration
  forEachPlayerBullet(callback: (bullet: Bullet) => boolean | void): void {
    const count = (this as any)._playerBulletCount || 0;
    for (let i = 0; i < count; i++) {
      if (callback(this.playerBulletCache[i]) === false) break;
    }
  }

  forEachEnemyBullet(callback: (bullet: Bullet) => boolean | void): void {
    const count = (this as any)._enemyBulletCount || 0;
    for (let i = 0; i < count; i++) {
      if (callback(this.enemyBulletCache[i]) === false) break;
    }
  }

  getPlayerBulletCount(): number {
    return (this as any)._playerBulletCount || 0;
  }

  getEnemyBulletCount(): number {
    return (this as any)._enemyBulletCount || 0;
  }

  getBulletDamage(bullet: Bullet): number {
    return bullet.damage;
  }

  deactivateBullet(bullet: Bullet): void {
    if (bullet.active) {
      bullet.active = false;
      this.returnToFreeList(bullet);
    }
  }

  getActiveCount(): number {
    return this.activeCount;
  }

  clear(): void {
    // Reset free list
    this.freeListHead = MAX_BULLETS;
    for (let i = 0; i < MAX_BULLETS; i++) {
      this.bullets[i].active = false;
      this.freeList[i] = i;
    }
    this.activeCount = 0;
    (this as any)._playerBulletCount = 0;
    (this as any)._enemyBulletCount = 0;
  }
}