Simplest possible streaming (filter) expansion of property references like ${property}

PropertyFilter.java

/*
 * Copyright (C) 2011, Mark Renouf
 *
 * This code is licensed 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 com.bitgrind.util;

import java.io.BufferedReader;
import java.io.FilterReader;
import java.io.IOException;
import java.io.Reader;
import java.io.StringReader;
import java.nio.CharBuffer;
import java.util.HashMap;
import java.util.Map;

public class PropertyFilter extends FilterReader {

  static int maxVariableNameLength = 255;
  CharBuffer buffer = CharBuffer.allocate(65535);
  private Map<String, String> map;

  protected PropertyFilter(Map<String, String> variables, Reader in) {
    super(new BufferedReader(in, maxVariableNameLength + 3));
    this.map = variables;
    buffer.limit(0);
  }

  int rewindAndRead() throws IOException {
    in.reset();
    return in.read();
  }


  @Override
  public int read() throws IOException {
    // read from previously expanded buffer
    if (buffer.remaining() > 0) {
      return buffer.get();
    }

    in.mark(maxVariableNameLength + 3);
    char ch = (char) in.read();
    if (ch != '$') {
      return rewindAndRead();
    }

    ch = (char) in.read();
    if (ch != '{') {
      return rewindAndRead();
    }

    int charsRead = 0;
    ch = (char) in.read();

    buffer.clear();
    while (ch != '}' && ch != -1 && !Character.isWhitespace(ch) && charsRead < maxVariableNameLength) {
      charsRead++;
      buffer.append(ch);
      ch = (char) in.read();
    }

    // abnormal termination or empty variable name
    if (ch != '}' || buffer.position() == 0) {
      buffer.clear();
      buffer.limit(0);
      return rewindAndRead();
    }

    buffer.flip();
    String variable = buffer.toString();
    if (map.containsKey(variable)) {
      // add expanded value to buffer
      buffer.clear();
      buffer.put(map.get(variable));
      buffer.flip();

      // clear mark and skip over variable
      in.reset();
      in.skip(charsRead + 3);
      return buffer.get();
    } else {
      // no matching key so output unchanged
      return rewindAndRead();
    }
  }

  @Override
  public int read(char[] buffer, int offset, int count) throws IOException {
    int i;
    for (i = 0; i < count; i++) {
      int ch = this.read();
      if (ch == -1) {
        if (i == 0) return -1;
        break;
      }
      buffer[offset++] = (char) ch;
    }
    return i;
  }
}