Remove the repeated logic of the method that overwrites the lower nib…

…ble of container type descriptor. (#548)

src/main/java/com/amazon/ion/impl/bin/IonRawBinaryWriter.java

@@ -614,22 +614,18 @@ private ContainerInfo popContainer()
                 // The number of bytes we need to shift by is determined by the writer's preallocation mode.
                 final int numberOfBytesToShiftBy = preallocationMode.numberOfLengthBytes();
 
-                // `length` is the encoded length of the container/wrapper we're stepping out of. It does not
-                // include any header bytes. In this `if` branch, we've confirmed that `length` is <= 0xD,
-                // so this downcast from `long` to `int` is safe.
-                final int lengthOfSliceToShift = (int) length;
-
                 // Shift the container/wrapper body backwards in the buffer. Because this only happens when
                 // `lengthOfSliceToShift` is 13 or fewer bytes, this will usually be a very fast memcpy.
                 // It's slightly more work if the slice we're shifting happens to straddle two memory blocks
                 // inside the buffer.
-                buffer.shiftBytesLeft(lengthOfSliceToShift, numberOfBytesToShiftBy);
+                // `length` is the encoded length of the container/wrapper we're stepping out of. It does not
+                // include any header bytes. In this `if` branch, we've confirmed that `length` is <= 0xD,
+                // so this downcast from `long` to `int` is safe.
+                buffer.shiftBytesLeft((int)length, numberOfBytesToShiftBy);
 
                 // Overwrite the lower nibble of the original type descriptor byte with the body's encoded length.
                 final long typeDescriptorPosition = positionOfFirstLengthByte - 1;
-                final long type = (buffer.getUInt8At(typeDescriptorPosition) & 0xF0) | length;
-                buffer.writeUInt8At(typeDescriptorPosition, type);
-
+                buffer.writeLowerNibbleAt(typeDescriptorPosition, length);
                 // We've reclaimed some number of bytes; adjust the container length as appropriate.
                 length -= numberOfBytesToShiftBy;
             }

src/main/java/com/amazon/ion/impl/bin/WriteBuffer.java

@@ -104,17 +104,6 @@ public long position()
         return (((long) index) * allocator.getBlockSize()) + current.limit;
     }
 
-    private static final int OCTET_MASK = 0xFF;
-
-    /** Returns the octet at the logical position given. */
-    public int getUInt8At(final long position)
-    {
-        final int index = index(position);
-        final int offset = offset(position);
-        final Block block = blocks.get(index);
-        return block.data[offset] & OCTET_MASK;
-    }
-
     /** Writes a single octet to the buffer, expanding if necessary. */
     public void writeByte(final byte octet)
     {
@@ -1266,12 +1255,24 @@ public void writeUInt8At(final long position, final long value)
     {
         final int index = index(position);
         final int offset = offset(position);
-
         // XXX we'll never overrun a block unless we're given a position past our block array
         final Block block = blocks.get(index);
         block.data[offset] = (byte) value;
     }
 
+    /**
+     * Overwrite the lower nibble of the specified type descriptor byte with the length information.
+     * @param position represents the position of the byte that will be overwritten.
+     * @param value represents the length value of the container.
+     */
+    public void writeLowerNibbleAt(final long position, final long value) {
+        final int index = index(position);
+        final int offset = offset(position);
+        final Block block = blocks.get(index);
+        long bitValue = block.data[offset];
+        block.data[offset] = (byte) (bitValue & 0xF0 | value) ;
+    }
+
     /** Get the length of FlexInt for the provided value. */
     public static int flexIntLength(final long value) {
         int numMagnitudeBitsRequired;