mobile/bind: fix RefMap invariant

RefMap tracks its number of live Ref instances in the 'live' member.
However, when a reference was removed and later added, 'live' wasn't
updated accordingly. Fix and add a test.

Change-Id: I806e17ea0319d76db4d07b5f8d9107b146ee80db
Reviewed-on: https://go-review.googlesource.com/19975
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/Seq.java

@@ -154,7 +154,7 @@ public class Seq {
 
 		public Seq.Object obj;  // for Java obj: pointers to the Java obj.
 
-		private Ref(int refnum, Seq.Object o) {
+		Ref(int refnum, Seq.Object o) {
 			this.refnum = refnum;
 			this.refcnt = 0;
 			this.obj = o;
@@ -281,7 +281,7 @@ public class Seq {
 	// RefMap is a mapping of integers to Ref objects.
 	//
 	// The integers can be sparse. In Go this would be a map[int]*Ref.
-	private static final class RefMap {
+	static final class RefMap {
 		private int next = 0;
 		private int live = 0;
 		private int[] keys = new int[16];
@@ -315,6 +315,7 @@ public class Seq {
 			if (i >= 0) {
 				if (objs[i] == null) {
 					objs[i] = obj;
+					live++;
 				}
 				if (objs[i] != obj) {
 					throw new RuntimeException("replacing an existing ref (with key "+key+")");
@@ -337,7 +338,7 @@ public class Seq {
 			next++;
 		}
 
-                private void grow() {
+		private void grow() {
 			// Compact and (if necessary) grow backing store.
 			int[] newKeys;
 			Ref[] newObjs;
@@ -370,7 +371,7 @@ public class Seq {
 			if (live != next) {
 				throw new RuntimeException("bad state: live="+live+", next="+next);
 			}
-                }
+		}
 
 		private static int roundPow2(int x) {
 			int p = 1;

bind/java/SeqTest.java

@@ -33,6 +33,22 @@ public class SeqTest extends InstrumentationTestCase {
     assertEquals("const Log2E", 1/0.693147180559945309417232121458176568075500134360255254120680009, Testpkg.Log2E, 0.0001);
   }
 
+  public void testRefMap() {
+    // Ensure that the RefMap.live count is kept in sync
+    // even a particular reference number is removed and
+    // added again
+    Seq.RefMap m = new Seq.RefMap();
+    Seq.Ref r = new Seq.Ref(1, null);
+    m.put(r.refnum, r);
+    m.remove(r.refnum);
+    m.put(r.refnum, r);
+    // Force the RefMap to grow, to activate the sanity
+    // checking of the live count in RefMap.grow.
+    for (int i = 2; i < 24; i++) {
+      m.put(i, new Seq.Ref(i, null));
+    }
+  }
+
   public void testVar() {
     assertEquals("var StringVar", "a string var", Testpkg.getStringVar());