3 files changed, 118 insertions, 1 deletions
diff --git a/life2/game/index.js b/life2/game/index.js
index f6c0ff5..b8e8f9c 100644
--- a/life2/game/index.js
+++ b/life2/game/index.js
@@ -25,3 +25,4 @@ export {Board, Grid} from './board';
 export {Cell} from './cell';
 export {Rule, RuleFunction} from './rule';
 export {RulesManager} from './rules_manager';
+export {World} from './world';
diff --git a/life2/game/rule.js b/life2/game/rule.js
index e4ed3ef..a7ae574 100644
--- a/life2/game/rule.js
+++ b/life2/game/rule.js
@@ -89,6 +89,14 @@ export class Rule {
    */
   execute(cell, neighbors) {
     if (cell === Cell.BARRIER) return null;
-    return this.expression_(cell, neighbors);
+    const state = this.expression_(cell, neighbors);
+
+    // Check if the rule returned a valid state.
+    if (state !== Cell.TEAM_A && state !== Cell.TEAM_B &&
+        state !== Cell.EMPTY) {
+      throw new Error(`Rule '${this.name}' returned an invalid state.`);
+    }
+
+    return state;
   }
 }
diff --git a/life2/game/world.js b/life2/game/world.js
new file mode 100644
index 0000000..b9c52e0
--- /dev/null
+++ b/life2/game/world.js
@@ -0,0 +1,108 @@
+/**
+ * Copyright 2024 The Life2 Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * @license
+ */
+
+/**
+ * @fileoverview The world is the interface to interact with the simulation
+ * game.  It is responsible for managing the game loop, emitting events, and
+ * controlling the game state (rules and cells) in real-time, without coupling
+ * downstream users to the game's internal state.
+ *
+ * @package life2
+ */
+
+import {Board} from './board';
+import {Cell} from './cell';
+import {Rule} from './rule';
+import {RulesManager} from './rules_manager';
+
+/**
+ * The World class is the main interface to interact with the game state in
+ * real-time without coupling the game's internal state to external
+ * applications.  The World is also the main manager of the game loop and
+ * applies the rules to the cells.
+ */
+export class World {
+  /**
+   * Initializes the game world with a board and rules.
+   * @param {!Board} board The game board.
+   * @param {?Array<!Rule>} rules A list of rules to add and enable by default.
+   */
+  constructor(board, rules) {
+    /**
+     * The game board.
+     * @type {!Board}
+     */
+    this.board = board;
+    /**
+     * The game rules.
+     * @type {!RulesManager}
+     */
+    this.rules = new RulesManager(rules);
+  }
+
+  /**
+   * Given a list of cell states, reduces the list to the most occurring one.
+   * @param {!Array<!Cell>} states The list of states.
+   * @return {!Cell}
+   * @private
+   */
+  reduceStates_(states) {
+    let countA = 0;
+    let countB = 0;
+    let countEmpty = 0;
+
+    // One loop is better than three ".filter"s btw.
+    for (const state of states) {
+      if (state === Cell.ALIVE) {
+        countA++;
+      } else if (state === Cell.DEAD) {
+        countB++;
+      } else {
+        countEmpty++;
+      }
+    }
+
+    return Math.max(countA, countB, countEmpty);
+  }
+
+  /**
+   * Calculates the next state of the game and applies it to the board.
+   */
+  nextState() {
+    const rules = this.rules.getAll();
+    const width = this.board.getWidth();
+    const height = this.board.getHeight();
+    const newGrid = this.board.getGrid();
+
+    for (let x = 0; x < width; x++) {
+      for (let y = 0; y < height; y++) {
+        if (this.board.isOutOfBounds(x, y)) continue;
+
+        const cell = this.board.getCell(x, y);
+        if (cell === Cell.EMPTY) continue;
+
+        const neighbors = this.board.getNeighbors(x, y);
+        const states = rules.map((rule) => rule.execute(cell, neighbors))
+                           .filter((state) => state != null);
+
+        newGrid[x][y] = this.reduceStates_(states);
+      }
+    }
+
+    this.board.setGrid(newGrid);
+  }
+}