content: /*
   * 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
   *
   *     https://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.apache.avro.idl;
  
  import org.apache.avro.FormattedSchemaParser;
  import org.apache.avro.ParseContext;
  import org.apache.avro.Schema;
  import org.apache.avro.SchemaParseException;
  
  import java.io.IOException;
  import java.net.URI;
  import java.util.regex.Pattern;
  
  public class IdlSchemaParser implements FormattedSchemaParser {
    /**
     * Pattern to check if the input can be IDL. It matches initial whitespace and
     * comments, plus all possible starting keywords. The match on the start of the
     * input as well as the use of possessive quantifiers is deliberate: it should
     * fail as fast as possible, as it is assumed that most schemata will not be in
     * IDL format.
     */
    private static final Pattern START_OF_IDL_PATTERN = Pattern.compile("\\A" + // Start of input
        "(?:\\s*+|/\\*(?:[^*]|\\*(?!/))*+\\*/|//(?:(?!\\R).)*+\\R)*+" + // Initial whitespace & comments
        "(?:@|(?:namespace|schema|protocol|record|enum|fixed|import)\\s)", // First keyword or @
        Pattern.UNICODE_CHARACTER_CLASS | Pattern.MULTILINE);
  
    @Override
    public Schema parse(ParseContext parseContext, URI baseUri, CharSequence formattedSchema)
        throws IOException, SchemaParseException {
      boolean inputCanBeIdl = START_OF_IDL_PATTERN.matcher(formattedSchema).find();
      if (inputCanBeIdl) {
        IdlReader idlReader = new IdlReader(parseContext);
        IdlFile idlFile = idlReader.parse(baseUri, formattedSchema);
        return idlFile.getMainSchema();
      }
      return null;
    }
  }
  