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Guava IO contains a variety of types and utilities to ease IO handling in Java.

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/*
 * Copyright (C) 2007 Google Inc.
 *
 * Licensed 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 com.google.common.io;

import com.google.common.annotations.Beta;

import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;

/**
 * An {@link InputStream} that counts the number of bytes read.
 *
 * @author Chris Nokleberg
 * @since 1
 */
@Beta
public final class CountingInputStream extends FilterInputStream {

  private long count;
  private long mark = -1;

  /**
   * Wraps another input stream, counting the number of bytes read.
   *
   * @param in the input stream to be wrapped
   */
  public CountingInputStream(InputStream in) {
    super(in);
  }

  /** Returns the number of bytes read. */
  public long getCount() {
    return count;
  }

  @Override public int read() throws IOException {
    int result = in.read();
    if (result != -1) {
      count++;
    }
    return result;
  }

  @Override public int read(byte[] b, int off, int len) throws IOException {
    int result = in.read(b, off, len);
    if (result != -1) {
      count += result;
    }
    return result;
  }

  @Override public long skip(long n) throws IOException {
    long result = in.skip(n);
    count += result;
    return result;
  }

  @Override public void mark(int readlimit) {
    in.mark(readlimit);
    mark = count;
    // it's okay to mark even if mark isn't supported, as reset won't work
  }

  @Override public void reset() throws IOException {
    if (!in.markSupported()) {
      throw new IOException("Mark not supported");
    }
    if (mark == -1) {
      throw new IOException("Mark not set");
    }

    in.reset();
    count = mark;
  }
}