code: // Licensed to the Software Freedom Conservancy (SFC) under one
  // or more contributor license agreements.  See the NOTICE file
  // distributed with this work for additional information
  // regarding copyright ownership.  The SFC licenses this file
  // to you under the Apache License, Version 2.0 (the
  // "License"); you may not use this file except in compliance
  // with the License.  You may obtain a copy of the License at
  //
  //   http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing,
  // software distributed under the License is distributed on an
  // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  // KIND, either express or implied.  See the License for the
  // specific language governing permissions and limitations
  // under the License.
  
  package org.openqa.selenium.io;
  
  import java.io.OutputStream;
  import java.nio.charset.Charset;
  
  /** Captures the last N bytes of output. */
  public class CircularOutputStream extends OutputStream {
    private static final int DEFAULT_SIZE = 4096;
    private int start;
    private int end;
    private boolean filled = false;
    private byte[] buffer;
  
    public CircularOutputStream(int maxSize) {
      buffer = new byte[maxSize];
    }
  
    public CircularOutputStream() {
      this(DEFAULT_SIZE);
    }
  
    @Override
    public synchronized void write(int b) {
      if (end == buffer.length) {
        filled = true;
        end = 0;
      }
  
      if (filled && end == start) {
        start = start == buffer.length - 1 ? 0 : start + 1;
      }
  
      buffer[end++] = (byte) b;
    }
  
    @Override
    public String toString() {
      int size = filled ? buffer.length : end;
      byte[] toReturn = new byte[size];
  
      // Handle the partially filled array as a special case
      if (!filled) {
        System.arraycopy(buffer, 0, toReturn, 0, end);
        return new String(toReturn, Charset.defaultCharset());
      }
  
      int copyStart = buffer.length - start;
      if (copyStart == buffer.length) {
        copyStart = 0;
      }
  
      System.arraycopy(buffer, start, toReturn, 0, copyStart);
      System.arraycopy(buffer, 0, toReturn, copyStart, end);
      return new String(toReturn, Charset.defaultCharset());
    }
  }
  