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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF 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 opennlp.tools.formats.muc;

import java.io.IOException;
import java.io.Reader;
import java.util.HashMap;
import java.util.Map;

import opennlp.tools.util.InvalidFormatException;
import opennlp.tools.util.StringUtil;

/**
 * SAX style SGML parser.
 * 
 * Note:

 * The implementation is very limited, but good enough to
 * parse the MUC corpora. Its must very likely be extended/improved/fixed to parse
 * a different SGML corpora.
 */
public class SgmlParser {

  public static abstract class ContentHandler {

    public void startElement(String name, Map attributes) throws InvalidFormatException {
    }

    public void characters(CharSequence chars) throws InvalidFormatException{
    }

    public void endElement(String name) throws InvalidFormatException {
    }
  }

  private static String extractTagName(CharSequence tagChars) throws InvalidFormatException {

    int fromOffset = 1;

    if (tagChars.length() > 1 && tagChars.charAt(1) == '/') {
      fromOffset = 2;
    }

    for (int ci = 1; ci < tagChars.length(); ci++) {

      if (tagChars.charAt(ci) == '>' || StringUtil.isWhitespace(tagChars.charAt(ci))) {
        return tagChars.subSequence(fromOffset, ci).toString();
      }
    }

    throw new InvalidFormatException("Failed to extract tag name!");
  }

  private static Map getAttributes(CharSequence tagChars) {

    // format:
    // space
    // key
    // =
    // " <- begin
    // value chars
    // " <- end

    Map attributes = new HashMap<>();

    StringBuilder key = new StringBuilder();
    StringBuilder value = new StringBuilder();

    boolean extractKey = false;
    boolean extractValue = false;

    for (int i = 0; i < tagChars.length(); i++) {

      // White space indicates begin of new key name
      if (StringUtil.isWhitespace(tagChars.charAt(i)) && !extractValue) {
        extractKey = true;
      }
      // Equals sign indicated end of key name
      else if (extractKey && ('=' == tagChars.charAt(i) || StringUtil.isWhitespace(tagChars.charAt(i)))) {
        extractKey = false;
      }
      // Inside key name, extract all chars
      else if (extractKey) {
        key.append(tagChars.charAt(i));
      }
      // " Indicates begin or end of value chars
      else if ('"' == tagChars.charAt(i)) {

        if (extractValue) {
          attributes.put(key.toString(), value.toString());

          // clear key and value buffers
          key.setLength(0);
          value.setLength(0);
        }

        extractValue = !extractValue;
      }
      // Inside value, extract all chars
      else if (extractValue) {
        value.append(tagChars.charAt(i));
      }
    }

    return attributes;
  }

  public void parse(Reader in, ContentHandler handler) throws IOException {

    StringBuilder buffer = new StringBuilder();

    boolean isInsideTag = false;
    boolean isStartTag = true;

    int lastChar = -1;
    int c;
    while ((c = in.read()) != -1) {

      if ('<' == c) {
        if (isInsideTag) {
          throw new InvalidFormatException("Did not expect < char!");
        }

        if (buffer.toString().trim().length() > 0) {
          handler.characters(buffer.toString().trim());
        }

        buffer.setLength(0);

        isInsideTag = true;
        isStartTag = true;
      }

      buffer.appendCodePoint(c);

      if ('/' == c && lastChar == '<') {
        isStartTag = false;
      }

      if ('>' == c) {

        if (!isInsideTag) {
          throw new InvalidFormatException("Did not expect > char!");
        }

        if (isStartTag) {
          handler.startElement(extractTagName(buffer), getAttributes(buffer));
        }
        else {
          handler.endElement(extractTagName(buffer));
        }

        buffer.setLength(0);

        isInsideTag = false;
      }

      lastChar = c;
    }

    if (isInsideTag) {
      throw new InvalidFormatException("Did not find matching > char!");
    }
  }
}