Add string cache to pool most commonly used Java strings

src/core/org/luaj/vm2/LuaString.java

@@ -25,8 +25,8 @@ import java.io.ByteArrayInputStream;
 import java.io.DataOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.lang.ref.WeakReference;
 
-import org.luaj.vm2.lib.MathLib;
 import org.luaj.vm2.lib.StringLib;
 
 public class LuaString extends LuaValue {
@@ -37,11 +37,36 @@ public class LuaString extends LuaValue {
 	public final int    m_offset;
 	public final int    m_length;
 
+	private static final int STRINGCACHE_POW2 = 10;
+	private static final WeakReference[] stringcache 
+					= new WeakReference[1<<STRINGCACHE_POW2];
+
+	private static class LuaJavaString extends LuaString {
+		private final String string;
+		private LuaJavaString(String value, byte[] bytes) {
+			super(bytes);
+			string = value.intern();
+		}
+		public String toString() {
+			return string;
+		}
+	}
+	
 	public static LuaString valueOf(String string) {
+		int h = string.hashCode();
+		int i = h & ((1<<STRINGCACHE_POW2)-1);
+		if ( stringcache[i] != null ) {
+			LuaJavaString s = (LuaJavaString) stringcache[i].get();
+			if ( s != null && s.string == string ) {
+				return s;
+			}
+		}
 		char[] c = string.toCharArray();
 		byte[] b = new byte[lengthAsUtf8(c)];
 		encodeToUtf8(c, b, 0);
-		return new LuaString(b);
+		LuaJavaString s = new LuaJavaString(string,b);
+		stringcache[i] = new WeakReference(s);
+		return s;
 	}
 
 	public LuaString(byte[] bytes, int offset, int length) {

test/junit/org/luaj/vm2/UnaryBinaryOperatorsTest.java

@@ -127,7 +127,7 @@ public class UnaryBinaryOperatorsTest extends TestCase {
 		
 		// objects should be different
 		assertNotSame(ia, ib);
-		assertNotSame(sa, sb);
+		assertSame(sa, sb);
 		assertNotSame(ia, ic);
 		assertNotSame(sa, sc);
 		
@@ -148,7 +148,7 @@ public class UnaryBinaryOperatorsTest extends TestCase {
 		
 		// objects should be different
 		assertNotSame(da, db);
-		assertNotSame(sa, sb);
+		assertSame(sa, sb);
 		assertNotSame(da, dc);
 		assertNotSame(sa, sc);