Optimize BsonArray Index encoding

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

@@ -24,6 +24,7 @@
 import java.util.List;
 import java.util.Stack;
 
+import static java.lang.Math.max;
 import static java.lang.String.format;
 import static org.bson.assertions.Assertions.notNull;
 
@@ -37,13 +38,34 @@ 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 static final int ARRAY_INDEXES_CACHE_SIZE = 1000;
+    private static final byte[] ARRAY_INDEXES_BUFFER;
+    private static final int[] ARRAY_INDEXES_OFFSETS;
+    private static final int[] ARRAY_INDEXES_LENGTHS;
     private Mark mark;
 
     static {
+        ARRAY_INDEXES_LENGTHS = new int[ARRAY_INDEXES_CACHE_SIZE];
+        ARRAY_INDEXES_OFFSETS = new int[ARRAY_INDEXES_CACHE_SIZE];
+        int totalSize = 0;
         for (int i = 0; i < ARRAY_INDEXES_CACHE_SIZE; i++) {
-            ARRAY_INDEXES_CACHE[i] = Integer.toString(i);
+            totalSize += (int) (Math.log10(max(i, 1))
+                    + 1 // number of digits
+                    + 1); // +1 for null terminator
+        }
+        ARRAY_INDEXES_BUFFER = new byte[totalSize];
+
+        // Fill buffer
+        int offset = 0;
+        for (int i = 0; i < ARRAY_INDEXES_CACHE_SIZE; i++) {
+            String string = Integer.toString(i);
+            int length = string.length();
+            for (int j = 0; j < length; j++) {
+                ARRAY_INDEXES_BUFFER[offset++] = (byte) string.charAt(j);
+            }
+            ARRAY_INDEXES_BUFFER[offset++] = 0;
+            ARRAY_INDEXES_OFFSETS[i] = offset - (length + 1);
+            ARRAY_INDEXES_LENGTHS[i] = length + 1;  // +1 for null terminator
         }
     }
 
@@ -409,7 +431,9 @@ private void writeCurrentName() {
             if (index >= ARRAY_INDEXES_CACHE_SIZE) {
                 bsonOutput.writeCString(Integer.toString(index));
             } else {
-                bsonOutput.writeCString(ARRAY_INDEXES_CACHE[index]);
+                bsonOutput.writeBytes(ARRAY_INDEXES_BUFFER,
+                        ARRAY_INDEXES_OFFSETS[index],
+                        ARRAY_INDEXES_LENGTHS[index]);
             }
         } else {
             bsonOutput.writeCString(getName());

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

@@ -66,7 +66,7 @@ public void shouldThrowWhenMaxDocumentSizeIsExceeded() {
             writer.writeEndDocument();
             fail();
         } catch (BsonMaximumSizeExceededException e) {
-            assertEquals("Document size of 1037 is larger than maximum of 1024.", e.getMessage());
+            assertEquals("Document size of 12917 is larger than maximum of 12904.", e.getMessage());
         }
     }
 
@@ -201,16 +201,16 @@ public void testWriteArray() {
     public void testWriteArrayElements() throws IOException {
         ByteArrayOutputStream expectedOutput = new ByteArrayOutputStream();
         expectedOutput.write(new byte[]{
-                88, 11, 0, 0, //document length
+                -52, 25, 0, 0, //document length
                 4, // array type
                 97, 49, 0, // "a1" name + null terminator
-                79, 11, 0, 0}); // array length
+                -61, 25, 0, 0}); // array length
 
 
         writer.writeStartDocument();
         writer.writeStartArray("a1");
         int arrayIndex = 0;
-        while (arrayIndex < 500) {
+        while (arrayIndex < 1100) {
             writer.writeBoolean(true);
 
             expectedOutput.write(BsonType.BOOLEAN.getValue());