Add String cache for BsonArray indexes. (#1664)

JAVA-5836
---------
Co-authored-by: Ross Lawley <ross.lawley@gmail.com>

Author: vbabanin

bson/src/main/org/bson/BsonBinaryWriter.java

@@ -37,8 +37,16 @@ public class BsonBinaryWriter extends AbstractBsonWriter {
 
     private final BsonOutput bsonOutput;
     private final Stack<Integer> maxDocumentSizeStack = new Stack<>();
+    private static final int ARRAY_INDEXES_CACHE_SIZE = 256;
+    private static final String[] ARRAY_INDEXES_CACHE = new String[ARRAY_INDEXES_CACHE_SIZE];
     private Mark mark;
 
+    static {
+        for (int i = 0; i < ARRAY_INDEXES_CACHE_SIZE; i++) {
+            ARRAY_INDEXES_CACHE[i] = Integer.toString(i);
+        }
+    }
+
     /**
      * Construct an instance.
      *
@@ -397,7 +405,12 @@ public void reset() {
 
     private void writeCurrentName() {
         if (getContext().getContextType() == BsonContextType.ARRAY) {
-            bsonOutput.writeCString(Integer.toString(getContext().index++));
+            int index = getContext().index++;
+            if (index >= ARRAY_INDEXES_CACHE_SIZE) {
+                bsonOutput.writeCString(Integer.toString(index));
+            } else {
+                bsonOutput.writeCString(ARRAY_INDEXES_CACHE[index]);
+            }
         } else {
             bsonOutput.writeCString(getName());
         }

bson/src/test/unit/org/bson/BsonBinaryWriterTest.java

@@ -26,6 +26,7 @@
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.nio.ByteBuffer;
+import java.nio.charset.StandardCharsets;
 import java.util.List;
 
 import static java.util.Arrays.asList;
@@ -49,7 +50,7 @@ public class BsonBinaryWriterTest {
     @BeforeEach
     public void setup() {
         buffer = new BasicOutputBuffer();
-        writer = new BsonBinaryWriter(new BsonWriterSettings(100), new BsonBinaryWriterSettings(1024), buffer);
+        writer = new BsonBinaryWriter(new BsonWriterSettings(100), new BsonBinaryWriterSettings(12904), buffer);
     }
 
     @AfterEach
@@ -61,7 +62,7 @@ public void tearDown() {
     public void shouldThrowWhenMaxDocumentSizeIsExceeded() {
         try {
             writer.writeStartDocument();
-            writer.writeBinaryData("b", new BsonBinary(new byte[1024]));
+            writer.writeBinaryData("b", new BsonBinary(new byte[12904]));
             writer.writeEndDocument();
             fail();
         } catch (BsonMaximumSizeExceededException e) {
@@ -197,16 +198,39 @@ public void testWriteArray() {
     }
 
     @Test
-    public void testWriteArrayElements() {
+    public void testWriteArrayElements() throws IOException {
+        ByteArrayOutputStream expectedOutput = new ByteArrayOutputStream();
+        expectedOutput.write(new byte[]{
+                88, 11, 0, 0, //document length
+                4, // array type
+                97, 49, 0, // "a1" name + null terminator
+                79, 11, 0, 0}); // array length
+
 
         writer.writeStartDocument();
         writer.writeStartArray("a1");
-        writer.writeBoolean(true);
-        writer.writeBoolean(false);
+        int arrayIndex = 0;
+        while (arrayIndex < 500) {
+            writer.writeBoolean(true);
+
+            expectedOutput.write(BsonType.BOOLEAN.getValue());
+            expectedOutput.write(Integer.toString(arrayIndex++).getBytes(StandardCharsets.UTF_8));
+            expectedOutput.write(0); // null terminator
+            expectedOutput.write(1); // boolean value
+
+            writer.writeBoolean(false);
+
+            expectedOutput.write(BsonType.BOOLEAN.getValue());
+            expectedOutput.write(Integer.toString(arrayIndex++).getBytes(StandardCharsets.UTF_8));
+            expectedOutput.write(0); // null terminator
+            expectedOutput.write(0); // boolean value
+        }
         writer.writeEndArray();
+        expectedOutput.write(0); // end of array
         writer.writeEndDocument();
-        byte[] expectedValues = {22, 0, 0, 0, 4, 97, 49, 0, 13, 0, 0, 0, 8, 48, 0, 1, 8, 49, 0, 0, 0, 0};
-        assertArrayEquals(expectedValues, buffer.toByteArray());
+        expectedOutput.write(0); // end of a document
+
+        assertArrayEquals(expectedOutput.toByteArray(), buffer.toByteArray());
     }
 
     @Test