Add stats calculation within the game loop

This patch changes the behavior of the calculation loop of a board
(nextState) to keep track of the changes that happened during the round
(number of cells that died or were born in a given team).

These stats may be accessed by called the getStats() method on a World
instance, which returns a !Stats object:

    const world = Life2.World(...);
    // ...
    const stats = world.getStats();
    displayStats(stats);

Close: https://github.com/nc0fr/life2/issues/8
Signed-off-by: Nicolas Paul <n@nc0.fr>

2 files changed, 60 insertions, 2 deletions

diff --git a/life2/game/index.js b/life2/game/index.js
index b8e8f9c..5d9277e 100644
--- a/life2/game/index.js
+++ b/life2/game/index.js
@@ -25,4 +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';
+export {Stats, World} from './world';
diff --git a/life2/game/world.js b/life2/game/world.js
index b9c52e0..d6981dc 100644
--- a/life2/game/world.js
+++ b/life2/game/world.js
@@ -30,6 +30,17 @@ import {Rule} from './rule';
 import {RulesManager} from './rules_manager';
 
 /**
+ * Stats about a board in general, and a state in particular.
+ * @typedef {Object} Stats
+ * @property {number} birthACount The number of cells that became TEAM_A.
+ * @property {number} deathACount The number of cells from TEAM_A that died.
+ * @property {number} birthBCount The number of cells that became TEAM_B.
+ * @property {number} deathBCount The number of cells from TEAM_B that died.
+ * @const
+ */
+export let Stats;
+
+/**
  * 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
@@ -52,6 +63,28 @@ export class World {
      * @type {!RulesManager}
      */
     this.rules = new RulesManager(rules);
+
+    // Stats
+    /**
+     * The amount of cells from TEAM_A that died in the last game iteration.
+     * @type {number}
+     */
+    this.deathACount_ = 0;
+    /**
+     * The amount of cells from TEAM_B that died in the last game iteration.
+     * @type {number}
+     */
+    this.deathBCount_ = 0;
+    /**
+     * The amount of cells that became from TEAM_A in the last game iteration.
+     * @type {number}
+     */
+    this.birthACount_ = 0;
+    /**
+     * The amount of cells that became TEAM_A in the last game iteration.
+     * @type {number}
+     */
+    this.birthBCount_ = 0;
   }
 
   /**
@@ -99,10 +132,35 @@ export class World {
         const states = rules.map((rule) => rule.execute(cell, neighbors))
                            .filter((state) => state != null);
 
-        newGrid[x][y] = this.reduceStates_(states);
+        const newCell = this.reduceStates_(states);
+
+        if (newCell === Cell.EMPTY && cell === Cell.TEAM_A) {
+          this.deathACount_ += 1;
+        } else if (newCell === Cell.EMPTY && cell === Cell.TEAM_B) {
+          this.deathBCount_ += 1;
+        } else if (newCell === Cell.TEAM_A && cell !== Cell.TEAM_A) {
+          this.birthACount_ += 1;
+        } else if (newCell === Cell.TEAM_B && cell !== Cell.TEAM_B) {
+          this.birthBCount_ += 1;
+        }
+
+        newGrid[x][y] = newCell;
       }
     }
 
     this.board.setGrid(newGrid);
   }
+
+  /**
+   * Returns the stats of the world and of the last nextState().
+   * @return {!Stats}
+   */
+  getStats() {
+    return {
+      birthACount: this.birthACount_,
+      deathACount: this.deathACount_,
+      birthBCount: this.birthBCount_,
+      deathBCount: this.deathBCount_,
+    };
+  }
 }