Added twentyfortyeight.

twentyfortyeight/.project

@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<projectDescription>
+	<name>twentyfortyeight</name>
+	<comment></comment>
+	<projects>
+	</projects>
+	<buildSpec>
+		<buildCommand>
+			<name>org.eclipse.jdt.core.javabuilder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+	</buildSpec>
+	<natures>
+		<nature>org.eclipse.jdt.core.javanature</nature>
+	</natures>
+	<filteredResources>
+		<filter>
+			<id>1743009076709</id>
+			<name></name>
+			<type>30</type>
+			<matcher>
+				<id>org.eclipse.core.resources.regexFilterMatcher</id>
+				<arguments>node_modules|\.git|__CREATED_BY_JAVA_LANGUAGE_SERVER__</arguments>
+			</matcher>
+		</filter>
+	</filteredResources>
+</projectDescription>

twentyfortyeight/.settings/org.eclipse.core.resources.prefs

@@ -0,0 +1,2 @@
+eclipse.preferences.version=1
+encoding/<project>=UTF-8

twentyfortyeight/.settings/org.eclipse.jdt.core.prefs

@@ -0,0 +1,11 @@
+eclipse.preferences.version=1
+org.eclipse.jdt.core.compiler.codegen.targetPlatform=21
+org.eclipse.jdt.core.compiler.codegen.unusedLocal=preserve
+org.eclipse.jdt.core.compiler.compliance=21
+org.eclipse.jdt.core.compiler.debug.lineNumber=generate
+org.eclipse.jdt.core.compiler.debug.localVariable=generate
+org.eclipse.jdt.core.compiler.debug.sourceFile=generate
+org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures=disabled
+org.eclipse.jdt.core.compiler.problem.reportPreviewFeatures=warning
+org.eclipse.jdt.core.compiler.release=enabled
+org.eclipse.jdt.core.compiler.source=21

twentyfortyeight/src/twentyfortyeight/Main.java

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+package twentyfortyeight;
+
+import java.util.ArrayList;
+import java.util.List;
+import tester.Tester;
+
+// The grid in which the game is played.
+class Grid {
+    int w;                  // The width of the grid.
+    int h;                  // The height of the grid.
+    ArrayList<Integer> buf; // The buffer containing the tiles.
+    int sz;                 // The size of the buffer -- always w * h.
+
+    Grid(int w, int h) {
+        if (w < 1 || h < 1)
+            throw new IllegalArgumentException("Can't make grid that small.");
+
+        this.sz = w * h;
+        this.buf = new ArrayList<>(this.sz);
+
+        for (int i = 0; i < this.sz; i++) this.buf.add(null);
+    }
+
+    // Get the indexes of all "free" cells -- those whose value in buf is null,
+    // and are a blank tile.
+    List<Integer> freeCellIdxs() {
+        List<Integer> nulls = new ArrayList<>();
+        for (int i = 0; i < this.sz; i++)
+            if (this.buf.get(i) == null && nulls.add(i)) continue;
+
+        return nulls;
+    }
+}
+
+class Examples {
+    Grid verySmall;
+    Grid small;
+    Grid oblong;
+
+    void init() {
+        verySmall = new Grid(1, 1);
+        small = new Grid(2, 2);
+        oblong = new Grid(1, 3);
+    }
+
+    void testGridFreeCellIdxs(Tester t) {
+        init();
+        t.checkExpect(
+            small.freeCellIdxs(), new ArrayList<Integer>(List.of(0, 1, 2, 3))
+        );
+        t.checkExpect(
+            verySmall.freeCellIdxs(), new ArrayList<Integer>(List.of(0))
+        );
+        t.checkExpect(
+            oblong.freeCellIdxs(), new ArrayList<Integer>(List.of(0, 1, 2))
+        );
+    }
+}