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.json;
  
  import java.io.IOException;
  import java.io.Reader;
  import java.io.UncheckedIOException;
  import org.openqa.selenium.internal.Require;
  
  /**
   * Similar to a {@link Reader} but with the ability to peek a single character ahead.
   *
   * <p>For the sake of providing a useful {@link #toString()} implementation, keeps the most recently
   * read characters in the input buffer.
   */
  class Input {
    public static final char EOF = (char) -1;
    // the number of chars to buffer
    private static final int BUFFER_SIZE = 4096;
    // the number of chars to remember, safe to set to 0
    private static final int MEMORY_SIZE = 128;
  
    private final Reader source;
    // a buffer used to minimize read calls and to keep the chars to remember
    private final char[] buffer;
    // the filled area in the buffer
    private int filled;
    // the last position read in the buffer
    private int position;
  
    public Input(Reader source) {
      this.source = Require.nonNull("Source", source);
      this.buffer = new char[BUFFER_SIZE + MEMORY_SIZE];
      this.filled = 0;
      this.position = -1;
    }
  
    public char peek() {
      return fill() ? buffer[position + 1] : EOF;
    }
  
    public char read() {
      return fill() ? buffer[++position] : EOF;
    }
  
    @Override
    public String toString() {
      int offset;
      int length;
  
      if (position < MEMORY_SIZE) {
        offset = 0;
        length = position + 1;
      } else {
        offset = position + 1 - MEMORY_SIZE;
        length = MEMORY_SIZE;
      }
  
      return "Last " + length + " characters read: " + new String(buffer, offset, length);
    }
  
    private boolean fill() {
      // do we need to fill the buffer?
      while (filled == position + 1) {
        try {
          // free the buffer, keep only the chars to remember
          int shift = filled - MEMORY_SIZE;
          if (shift > 0) {
            position -= shift;
            filled -= shift;
  
            System.arraycopy(buffer, shift, buffer, 0, filled);
          }
  
          // try to fill the buffer
          int n = source.read(buffer, filled, buffer.length - filled);
  
          if (n == -1) {
            // EOF reached
            return false;
          } else {
            // n might be 0, the outer loop will handle this
            filled += n;
          }
        } catch (IOException e) {
          throw new UncheckedIOException(e.getMessage(), e);
        }
      }
  
      return true;
    }
  }
  