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Author: vbabanin

bson/src/main/org/bson/io/ByteBufferBsonInput.java

@@ -35,7 +35,11 @@ public class ByteBufferBsonInput implements BsonInput {
 
     private static final String[] ONE_BYTE_ASCII_STRINGS = new String[Byte.MAX_VALUE + 1];
     private static final boolean UNALIGNED_ACCESS_SUPPORTED = isUnalignedAccessAllowed();
-    private int scratchBufferSize = 0;
+    /* A dynamically sized scratch buffer, that is reused across BSON String reads:
+     * 1. Reduces garbage collection by avoiding new byte array creation.
+     * 2. Improves cache utilization through temporal locality.
+     * 3. Avoids JVM allocation and zeroing cost for new memory allocations.
+     */
     private byte[] scratchBuffer;
 
 
@@ -136,8 +140,8 @@ public String readCString() {
         return readString(size);
     }
 
-    private String readString(final int stringSize) {
-        if (stringSize == 2) {
+    private String readString(final int bsonStringSize) {
+        if (bsonStringSize == 2) {
             byte asciiByte = buffer.get();               // if only one byte in the string, it must be ascii.
             byte nullByte = buffer.get();                // read null terminator
             if (nullByte != 0) {
@@ -151,24 +155,24 @@ private String readString(final int stringSize) {
             if (buffer.isBackedByArray()) {
                 int position = buffer.position();
                 int arrayOffset = buffer.arrayOffset();
-                int newPosition = position + stringSize;
+                int newPosition = position + bsonStringSize;
                 buffer.position(newPosition);
 
                 byte[] array = buffer.array();
                 if (array[arrayOffset + newPosition - 1] != 0) {
                     throw new BsonSerializationException("Found a BSON string that is not null-terminated");
                 }
-                return new String(array, arrayOffset + position, stringSize - 1, StandardCharsets.UTF_8);
-            } else if (stringSize > scratchBufferSize) {
-                scratchBufferSize = stringSize + (stringSize >> 1); //1.5 times the size
+                return new String(array, arrayOffset + position, bsonStringSize - 1, StandardCharsets.UTF_8);
+            } else if (scratchBuffer == null || bsonStringSize > scratchBuffer.length) {
+                int scratchBufferSize = bsonStringSize + (bsonStringSize >>> 1); //1.5 times the size
                 scratchBuffer = new byte[scratchBufferSize];
             }
 
-            buffer.get(scratchBuffer, 0, stringSize);
-            if (scratchBuffer[stringSize - 1] != 0) {
+            buffer.get(scratchBuffer, 0, bsonStringSize);
+            if (scratchBuffer[bsonStringSize - 1] != 0) {
                 throw new BsonSerializationException("BSON string not null-terminated");
             }
-            return new String(scratchBuffer, 0, stringSize - 1, StandardCharsets.UTF_8);
+            return new String(scratchBuffer, 0, bsonStringSize - 1, StandardCharsets.UTF_8);
         }
     }
 
@@ -180,6 +184,10 @@ public void skipCString() {
         buffer.position(pos + length);
     }
 
+    /*
+         This method uses the SWAR (SIMD Within A Register) technique when aligned access is supported.
+         SWAR finds a null terminator by processing 8 bytes at once.
+     */
     public int computeCStringLength(final int prevPos) {
         ensureOpen();
         int pos = buffer.position();

driver-core/src/test/unit/com/mongodb/internal/connection/ByteBufferBsonInputTest.java

@@ -73,12 +73,12 @@ public boolean isBackedByArray() {
 
                     @Override
                     public byte[] array() {
-                        return Assertions.fail("array() is called, when hasArray() returns false");
+                        return Assertions.fail("array() is called, when isBackedByArray() returns false");
                     }
 
                     @Override
                     public int arrayOffset() {
-                        return Assertions.fail("arrayOffset() is called, when hasArray() returns false");
+                        return Assertions.fail("arrayOffset() is called, when isBackedByArray() returns false");
                     }
                 }
         );
@@ -558,10 +558,10 @@ public static Stream<Arguments> nonNullTerminatedStringsWithBuffers() {
     void shouldThrowIfStringIsNotNullTerminated(final byte[] nonNullTerminatedString, final BufferProvider bufferProvider) {
         // given
         ByteBuf buffer = allocateAndWriteToBuffer(bufferProvider, nonNullTerminatedString);
-        try (ByteBufferBsonInput expectedStringeam = new ByteBufferBsonInput(buffer)) {
+        try (ByteBufferBsonInput expectedString = new ByteBufferBsonInput(buffer)) {
 
             // when & then
-            assertThrows(BsonSerializationException.class, expectedStringeam::readString);
+            assertThrows(BsonSerializationException.class, expectedString::readString);
         }
     }