/**
 * engine.js — Single-player grid world for the browser game.
 * Pure ES module, zero dependencies.
 */

// ─── Constants ───────────────────────────────────────────────────────────────

export const EMOJIS = {
  tree: '🌳', rock: '🪨', blank: '⬜', diamond: '💎', agent: '🤖',
};

export const DIRECTIONS = {
  up:    { dx: 0,  dy: -1 },
  down:  { dx: 0,  dy:  1 },
  left:  { dx: -1, dy:  0 },
  right: { dx:  1, dy:  0 },
};

export const DEFAULTS = {
  WORLD_WIDTH: 600,
  WORLD_HEIGHT: 600,
  DIAMOND_COUNT: 500,
  VIEWPORT_RADIUS: 6,
  LOOP_DELAY_MS: 500,
  TERRAIN_SCALE: 0.1,
  TERRAIN_THRESHOLDS: [
    { emoji: '🌳', threshold: 0.4 },
    { emoji: '🪨', threshold: 0.5 },
  ],
};

// ─── Seedable PRNG (mulberry32) — used when a seed is passed; otherwise Math.random ──

function mulberry32(a) {
  return function () {
    let t = (a += 0x6d2b79f5);
    t = Math.imul(t ^ (t >>> 15), t | 1);
    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
}

// ─── Perlin Noise ────────────────────────────────────────────────────────────

function makePerlin(rng = Math.random) {
  const p = Array.from({ length: 256 }, () => Math.floor(rng() * 256));
  const perm = [...p, ...p];
  const fade = t => t * t * t * (t * (t * 6 - 15) + 10);
  const lerp = (t, a, b) => a + t * (b - a);
  const grad = (h, x, y) => {
    const u = (h & 3) < 2 ? x : y, v = (h & 3) < 2 ? y : x;
    return (h & 1 ? -u : u) + (h & 2 ? -2 * v : 2 * v);
  };
  return (x, y) => {
    const X = Math.floor(x) & 255, Y = Math.floor(y) & 255;
    x -= Math.floor(x); y -= Math.floor(y);
    const u = fade(x), v = fade(y);
    const A = perm[X] + Y, B = perm[X + 1] + Y;
    return lerp(v,
      lerp(u, grad(perm[A], x, y), grad(perm[B], x - 1, y)),
      lerp(u, grad(perm[A + 1], x, y - 1), grad(perm[B + 1], x - 1, y - 1))
    );
  };
}

// ─── World Generation ────────────────────────────────────────────────────────

function generateWorld(
  width = DEFAULTS.WORLD_WIDTH,
  height = DEFAULTS.WORLD_HEIGHT,
  thresholds = DEFAULTS.TERRAIN_THRESHOLDS,
  scale = DEFAULTS.TERRAIN_SCALE,
  rng = Math.random,
) {
  const perlin = makePerlin(rng);
  const tiles = new Array(width * height);
  for (let y = 0; y < height; y++) {
    for (let x = 0; x < width; x++) {
      const n = (perlin(x * scale, y * scale) + 1) / 2;
      let tile = EMOJIS.blank;
      for (const { emoji, threshold } of thresholds) {
        if (n < threshold) { tile = emoji; break; }
      }
      tiles[y * width + x] = tile;
    }
  }
  return { width, height, tiles };
}

function getTile(world, x, y) {
  if (x < 0 || y < 0 || x >= world.width || y >= world.height) return EMOJIS.rock;
  return world.tiles[y * world.width + x];
}

function isPassable(world, x, y) {
  if (x < 0 || y < 0 || x >= world.width || y >= world.height) return false;
  return world.tiles[y * world.width + x] === EMOJIS.blank;
}

function findRandomEmpty(world, occupied = [], maxAttempts = 1000, rng = Math.random) {
  for (let i = 0; i < maxAttempts; i++) {
    const x = Math.floor(rng() * world.width);
    const y = Math.floor(rng() * world.height);
    if (isPassable(world, x, y) && !occupied.some(p => p.x === x && p.y === y)) {
      return { x, y };
    }
  }
  throw new Error('Could not find empty cell');
}

// ─── Viewport Rendering ──────────────────────────────────────────────────────

function renderViewport(world, objects, viewerAgent, radius = DEFAULTS.VIEWPORT_RADIUS) {
  let out = '';
  for (let dy = -radius; dy <= radius; dy++) {
    for (let dx = -radius; dx <= radius; dx++) {
      const wx = viewerAgent.x + dx, wy = viewerAgent.y + dy;
      if (wx === viewerAgent.x && wy === viewerAgent.y) { out += viewerAgent.emoji; continue; }
      const obj = objects.find(o => o.x === wx && o.y === wy);
      if (obj) { out += obj.emoji; continue; }
      out += getTile(world, wx, wy);
    }
    out += '\n';
  }
  return out;
}

// ─── SinglePlayerGame ────────────────────────────────────────────────────────

export class SinglePlayerGame {
  constructor(opts = {}) {
    const {
      width = DEFAULTS.WORLD_WIDTH,
      height = DEFAULTS.WORLD_HEIGHT,
      diamondCount = DEFAULTS.DIAMOND_COUNT,
      seed = null, // null → Math.random; integer → deterministic mulberry32
    } = typeof opts === 'object' ? opts : { width: opts };
    const rng = seed != null ? mulberry32(seed) : Math.random;

    this.world = generateWorld(width, height, undefined, undefined, rng);
    this.objects = [];
    this.agent = { x: 0, y: 0, emoji: EMOJIS.agent };
    this.inventory = [];
    this.stats = { moves: 0, diamonds: 0, picks: 0 };

    for (let i = 0; i < diamondCount; i++) {
      const pos = findRandomEmpty(this.world, this.objects, 1000, rng);
      this.objects.push({ x: pos.x, y: pos.y, emoji: EMOJIS.diamond });
    }

    const agentPos = findRandomEmpty(this.world, this.objects, 1000, rng);
    this.agent.x = agentPos.x;
    this.agent.y = agentPos.y;
  }

  getViewport(radius = DEFAULTS.VIEWPORT_RADIUS) {
    return renderViewport(this.world, this.objects, this.agent, radius);
  }

  move(direction) {
    const d = DIRECTIONS[direction];
    if (!d) throw new Error('Invalid direction');
    const nx = this.agent.x + d.dx, ny = this.agent.y + d.dy;
    if (!isPassable(this.world, nx, ny)) throw new Error('Cannot move into obstacle');
    // Disallow moving onto a pickable object — the agent must use `pick` instead.
    // The error string names the right tool so the model can self-correct next turn.
    const blocker = this.objects.find(o => o.x === nx && o.y === ny);
    if (blocker) {
      throw new Error(`Cannot move onto ${blocker.emoji} — use pick ${direction} instead`);
    }
    this.agent.x = nx;
    this.agent.y = ny;
    this.stats.moves++;
    return { x: nx, y: ny };
  }

  pick(direction) {
    const d = DIRECTIONS[direction];
    if (!d) throw new Error('Invalid direction');
    const tx = this.agent.x + d.dx, ty = this.agent.y + d.dy;
    const idx = this.objects.findIndex(o => o.x === tx && o.y === ty);
    if (idx === -1) throw new Error('No object in that direction');
    const obj = this.objects.splice(idx, 1)[0];
    this.inventory.push(obj.emoji);
    this.stats.picks++;
    if (obj.emoji === EMOJIS.diamond) this.stats.diamonds++;
    return { pickedUp: obj.emoji };
  }

  executeAction(action) {
    if (action.action === 'move') return this.move(action.detail);
    if (action.action === 'pick') return this.pick(action.detail);
  }
}