Updated.

twentyfortyeight/src/twentyfortyeight/Main.java

@@ -2,8 +2,36 @@ package twentyfortyeight;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Random;
 import tester.Tester;
 
+// A game of 2048.
+class Game {
+    int score;
+    Grid grid;
+    Random rand;
+
+    Game(int score, Grid grid, Random rand) {
+        this.score = score;
+        this.grid = grid;
+        this.rand = rand;
+    }
+
+    // Convenience constructor for init with default settings.
+    Game() {
+        this.grid = new Grid(4, 4);
+        this.score = 0;
+        this.rand = new Random();
+    }
+
+    // Convenience constructor for init with custom settings.
+    Game(int w, int h, int seed) {
+        this.grid = new Grid(w, h);
+        this.score = 0;
+        this.rand = new Random(seed);
+    }
+}
+
 // The grid in which the game is played.
 class Grid {
     int w;                  // The width of the grid.
@@ -15,32 +43,43 @@ class Grid {
         if (w < 1 || h < 1)
             throw new IllegalArgumentException("Can't make grid that small.");
 
-        this.sz = w * h;
+        this.w = w;
+        this.h = h;
+        this.sz = this.w * this.h;
         this.buf = new ArrayList<>(this.sz);
 
-        for (int i = 0; i < this.sz; i++) this.buf.add(null);
+        // Fill with zeros.
+        for (int i = 0; i < this.sz; this.buf.add(i++ * 0));
     }
 
-    // Get the indexes of all "free" cells -- those whose value in buf is null,
-    // and are a blank tile.
+    // Get the indexes of all "free" cells -- those whose value in buf is 0,
+    // and are represented as blank tile.
     List<Integer> freeCellIdxs() {
-        List<Integer> nulls = new ArrayList<>();
+        List<Integer> free = new ArrayList<>();
         for (int i = 0; i < this.sz; i++)
-            if (this.buf.get(i) == null && nulls.add(i)) continue;
+            if (this.buf.get(i) == 0 && free.add(i)) continue;
 
-        return nulls;
+        return free;
     }
+
+    // Set the tile at the coords. Just for testing.
+    Grid set(int x, int y, int v) {
+        this.buf.set(this.w * y + x, v);
+        return this;
+    }
+
+    // Get the value at the coords. Just for testing.
+    int get(int x, int y) { return this.buf.get(this.w * y + x); }
 }
 
 class Examples {
-    Grid verySmall;
-    Grid small;
-    Grid oblong;
+    Grid verySmall, small, oblong, regular;
 
     void init() {
         verySmall = new Grid(1, 1);
         small = new Grid(2, 2);
         oblong = new Grid(1, 3);
+        regular = new Grid(4, 4);
     }
 
     void testGridFreeCellIdxs(Tester t) {
@@ -55,4 +94,26 @@ class Examples {
             oblong.freeCellIdxs(), new ArrayList<Integer>(List.of(0, 1, 2))
         );
     }
+
+    void testGridSet(Tester t) {
+        init();
+        small.set(0, 0, 1);
+        t.checkExpect(small.freeCellIdxs(), new ArrayList<>(List.of(1, 2, 3)));
+        small.set(1, 1, 1);
+        t.checkExpect(small.freeCellIdxs(), new ArrayList<>(List.of(1, 2)));
+    }
+
+    void testRegular(Tester t) {
+        init();
+        regular.set(1, 0, 16)
+            .set(2, 0, 16)
+            .set(3, 0, 32)
+            .set(0, 1, 4)
+            .set(1, 1, 4)
+            .set(4, 1, 2);
+
+        t.checkExpect(regular.get(1, 0), 16);
+        t.checkExpect(regular.get(0, 0), 0);
+        t.checkExpect(regular.get(3, 3), 0);
+    }
 }