diff --git a/src/main/java/dotsandboxes/DotsAndBoxesGrid.java b/src/main/java/dotsandboxes/DotsAndBoxesGrid.java
new file mode 100644
index 0000000000000000000000000000000000000000..32667af6c63a0287bd4f91c37682acb30c08e9a4
--- /dev/null
+++ b/src/main/java/dotsandboxes/DotsAndBoxesGrid.java
@@ -0,0 +1,198 @@
+package dotsandboxes;
+
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Observer;
+import java.util.function.Consumer;
+
+/**
+ * The state of a dots and boxes grid.
+ *
+ * A (4, 3) dots and boxes grid looks like this:
+ *
+ * *-*-*-*
+ * | | | |
+ * *-*-*-*
+ * | | | |
+ * *-*-*-*
+ *
+ * Notice that:
+ *
+ * - for each row, there is one less horizontal than the number of corner dots
+ * - for each row, there are as many verticals as there are corner dots
+ * - for each row, there is one less box than the number of corner dots
+ * - for each column, there is one less vertical than the number of corner dots.
+ * - for each column, there are as many horizontals as there are corner dots.
+ * - for each column, there is one less box than the number of corner dots
+ *
+ * For example, in this (4, 3) grid, there are (3, 3) horizontal lines, and (4, 2) vertical lines, and (3, 2) boxes.
+ *
+ * We number all lines and boxes by their top-left coordinate.
+ *
+ * In Java 14+, we might use a Record class for this, but we're using 11+ as an LTS version, so we don't have that yet.
+ */
+public class DotsAndBoxesGrid {
+
+ final int width;
+ final int height;
+
+ /** The horizontal lines in the grid. True if drawn. */
+ private boolean[][] horizontals;
+
+ /** The vertical lines in the grid. True if drawn. */
+ private boolean[][] verticals;
+
+ /** Which owner (if any) claimed any given box. */
+ private int[][] boxOwners;
+
+ /** A list of functions to notify when there is an update */
+ private ArrayList<Consumer<DotsAndBoxesGrid>> watchers = new ArrayList<>();
+
+ final int players;
+ private int player = 1;
+ public int getPlayer() {
+ return player;
+ }
+
+ /** Moves to the next player */
+ private void nextPlayer() {
+ player++;
+ if (player > players) {
+ player = 1;
+ }
+ }
+
+ public DotsAndBoxesGrid(int width, int height, int players) {
+ this.width = width;
+ this.height = height;
+ this.players = players;
+
+ this.horizontals = new boolean[width - 1][height];
+ this.verticals = new boolean[width][height - 1];
+ this.boxOwners = new int[width - 1][height - 1];
+ }
+
+ private void notifyObservers() {
+ for (Consumer<DotsAndBoxesGrid> consumer : watchers) {
+ consumer.accept(this);
+ }
+ }
+
+ /** Listens to this grid for changes */
+ public void addConsumer(Consumer<DotsAndBoxesGrid> consumer) {
+ watchers.add(consumer);
+ }
+
+ /** Returns whether a horizontal line has been drawn */
+ public boolean getHorizontal(int x, int y) {
+ return horizontals[x][y];
+ }
+
+ /** Returns whether a vertical line has been drawn */
+ public boolean getVertical(int x, int y) {
+ return verticals[x][y];
+ }
+
+ /** Returns which player owns a box. By convention, 0 is unowned. */
+ public int getBoxOwner(int x, int y) {
+ return boxOwners[x][y];
+ }
+
+
+ /**
+ * Checks whether a box has been fully drawn (all four sides)
+ * @param x coordinate of the left side of the box
+ * @param y coordinate of the top of the box
+ * @return true if all four sides have been drawn.
+ */
+ public boolean boxComplete(int x, int y) {
+ if (x >= width - 1 || x < 0 || y >= height - 1 || y < 0) {
+ return false;
+ }
+
+ // A box is complete if the north and south horizontals and the east and west verticals have all been drawn.
+ // FIXME: You'll need to fix this code (after writing a test first).
+ return true;
+ }
+
+ /** Tries to claim a box for a player. If the box is complete, sets the ownership and returns true. */
+ private boolean claimBox(int x, int y, int p) {
+ if (boxComplete(x, y)) {
+ boxOwners[x][y] = player;
+ return true;
+ } else {
+ return false;
+ }
+ }
+
+ /**
+ * "Draws" a horizontal line, from grid point (x, y) to (x + 1, y). (i.e. sets that line to true)
+ * @param x
+ * @param y
+ * @return true if it completes a box
+ */
+ public boolean drawHorizontal(int x, int y, int player) {
+ if (x >= width - 1 || x < 0) {
+ throw new IndexOutOfBoundsException(String.format("x was %d, which is out of range. Range is 0 to %d", x, width - 1));
+ }
+ if (y >= height || y < 0) {
+ throw new IndexOutOfBoundsException(String.format("y was %d, which is out of range. Range is 0 to %d", y, height));
+ }
+
+ // FIXME: You need to throw an exception if the line was already drawn.
+
+ this.horizontals[x][y] = true;
+
+ // Try to claim the north or south boxes
+ boolean claimN = claimBox(x, y-1, player);
+ boolean claimS = claimBox(x, y, player);
+ if (claimN || claimS) {
+ notifyObservers();
+ return true;
+ } else {
+ nextPlayer();
+ notifyObservers();
+ return false;
+ }
+ }
+
+ /**
+ * "Draws" a vertical line, from grid point (x, y) to (x, y + 1). (i.e. sets that line to true)
+ * @param x
+ * @param y
+ * @return true if it completes a box
+ */
+ public boolean drawVertical(int x, int y, int player) {
+ if (x >= width || x < 0) {
+ throw new IndexOutOfBoundsException(String.format("x was %d, which is out of range. Range is 0 to %d", x, width));
+ }
+ if (y >= height - 1 || y < 0) {
+ throw new IndexOutOfBoundsException(String.format("y was %d, which is out of range. Range is 0 to %d", y, height - 1));
+ }
+
+ // You need to throw an exception if the line was already drawn.
+
+ this.verticals[x][y] = true;
+ // Try to claim the north or south boxes
+ boolean claimE = claimBox(x, y, player);
+ boolean claimW = claimBox(x-1, y, player);
+ if (claimE || claimW) {
+ notifyObservers();
+ return true;
+ } else {
+ nextPlayer();
+ notifyObservers();
+ return false;
+ }
+
+ }
+
+ public boolean gameComplete() {
+ // Students who took COSC250 might recognise this style of code. This is Java's version of higher order functions.
+ // The grid is complete if "for all rows, all the boxes in that row have a non-zero owner"
+ return Arrays.stream(boxOwners).allMatch((row) -> Arrays.stream(row).allMatch((owner) -> owner > 0));
+ }
+
+
+}