001/*
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 *    http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing,
013 * software distributed under the License is distributed on an
014 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
015 * KIND, either express or implied.  See the License for the
016 * specific language governing permissions and limitations
017 * under the License.
018 */
019package org.apache.commons.io.input;
020
021import static org.apache.commons.io.IOUtils.EOF;
022
023import java.io.IOException;
024import java.io.Reader;
025
026/**
027 * A reader that imposes a limit to the number of characters that can be read from an underlying reader, returning EOF
028 * when this limit is reached, regardless of state of underlying reader.
029 *
030 * <p>
031 * One use case is to avoid overrunning the readAheadLimit supplied to {@link java.io.Reader#mark(int)}, since reading
032 * too many characters removes the ability to do a successful reset.
033 * </p>
034 *
035 * @since 2.5
036 */
037public class BoundedReader extends Reader {
038
039    private static final int INVALID = -1;
040
041    private final Reader target;
042
043    private int charsRead;
044
045    private int markedAt = INVALID;
046
047    private int readAheadLimit; // Internally, this value will never exceed the allowed size
048
049    private final int maxCharsFromTargetReader;
050
051    /**
052     * Constructs a bounded reader
053     *
054     * @param target                   The target stream that will be used
055     * @param maxCharsFromTargetReader The maximum number of characters that can be read from target
056     */
057    public BoundedReader(final Reader target, final int maxCharsFromTargetReader) {
058        this.target = target;
059        this.maxCharsFromTargetReader = maxCharsFromTargetReader;
060    }
061
062    /**
063     * Closes the target
064     *
065     * @throws IOException If an I/O error occurs while calling the underlying reader's close method
066     */
067    @Override
068    public void close() throws IOException {
069        target.close();
070    }
071
072    /**
073     * marks the target stream
074     *
075     * @param readAheadLimit The number of characters that can be read while still retaining the ability to do #reset().
076     *                       Note that this parameter is not validated with respect to maxCharsFromTargetReader. There
077     *                       is no way to pass past maxCharsFromTargetReader, even if this value is greater.
078     *
079     * @throws IOException If an I/O error occurs while calling the underlying reader's mark method
080     * @see java.io.Reader#mark(int)
081     */
082    @Override
083    public void mark(final int readAheadLimit) throws IOException {
084        this.readAheadLimit = readAheadLimit - charsRead;
085
086        markedAt = charsRead;
087
088        target.mark(readAheadLimit);
089    }
090
091    /**
092     * Reads a single character
093     *
094     * @return -1 on EOF or the character read
095     * @throws IOException If an I/O error occurs while calling the underlying reader's read method
096     * @see java.io.Reader#read()
097     */
098    @Override
099    public int read() throws IOException {
100
101        if (charsRead >= maxCharsFromTargetReader) {
102            return EOF;
103        }
104
105        if (markedAt >= 0 && charsRead - markedAt >= readAheadLimit) {
106            return EOF;
107        }
108        charsRead++;
109        return target.read();
110    }
111
112    /**
113     * Reads into an array
114     *
115     * @param cbuf The buffer to fill
116     * @param off  The offset
117     * @param len  The number of chars to read
118     * @return the number of chars read
119     * @throws IOException If an I/O error occurs while calling the underlying reader's read method
120     * @see java.io.Reader#read(char[], int, int)
121     */
122    @Override
123    public int read(final char[] cbuf, final int off, final int len) throws IOException {
124        int c;
125        for (int i = 0; i < len; i++) {
126            c = read();
127            if (c == EOF) {
128                return i == 0 ? EOF : i;
129            }
130            cbuf[off + i] = (char) c;
131        }
132        return len;
133    }
134
135    /**
136     * Resets the target to the latest mark,
137     *
138     * @throws IOException If an I/O error occurs while calling the underlying reader's reset method
139     * @see java.io.Reader#reset()
140     */
141    @Override
142    public void reset() throws IOException {
143        charsRead = markedAt;
144        target.reset();
145    }
146}