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Core YAML 1.2 parser and emitter for Java
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/*
 * Copyright (c) 2018, SnakeYAML
 *
 * 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 org.snakeyaml.engine.v2.api;

import java.io.InputStream;
import java.io.Reader;
import java.util.Iterator;
import java.util.Objects;
import java.util.Optional;
import org.snakeyaml.engine.v2.composer.Composer;
import org.snakeyaml.engine.v2.constructor.BaseConstructor;
import org.snakeyaml.engine.v2.constructor.StandardConstructor;
import org.snakeyaml.engine.v2.nodes.Node;
import org.snakeyaml.engine.v2.parser.ParserImpl;
import org.snakeyaml.engine.v2.scanner.StreamReader;

/**
 * Common way to load Java instance(s). This class is not thread-safe. Which means that all the
 * methods of the same instance can be called only by one thread. It is better to create an instance
 * for every YAML stream. The instance is stateful. Only one of the 'load' methods may be called,
 * and it may be called only once.
 */
public class Load {

  private final LoadSettings settings;
  private final BaseConstructor constructor;

  /**
   * Create instance to parse the incoming YAML data and create Java instances
   *
   * @param settings - configuration
   */
  public Load(LoadSettings settings) {
    this(settings, new StandardConstructor(settings));
  }

  /**
   * Create instance to parse the incoming YAML data and create Java instances
   *
   * @param settings - configuration
   * @param constructor - custom YAML constructor
   */
  public Load(LoadSettings settings, BaseConstructor constructor) {
    Objects.requireNonNull(settings, "LoadSettings cannot be null");
    Objects.requireNonNull(constructor, "BaseConstructor cannot be null");
    this.settings = settings;
    this.constructor = constructor;
  }

  /**
   * Create Composer
   *
   * @param streamReader - the input
   * @return configured Composer
   */
  private Composer createComposer(StreamReader streamReader) {
    return new Composer(settings, new ParserImpl(settings, streamReader));
  }

  /**
   * Create Composer
   *
   * @param yamlStream - the input
   * @return configured Composer
   */
  protected Composer createComposer(InputStream yamlStream) {
    return createComposer(new StreamReader(settings, new YamlUnicodeReader(yamlStream)));
  }

  /**
   * Create Composer
   *
   * @param yaml - the input
   * @return configured Composer
   */
  protected Composer createComposer(String yaml) {
    return createComposer(new StreamReader(settings, yaml));
  }

  /**
   * Create Composer
   *
   * @param yamlReader - the input
   * @return configured Composer
   */
  protected Composer createComposer(Reader yamlReader) {
    return createComposer(new StreamReader(settings, yamlReader));
  }

  // Load a single document

  /**
   * Load with provided Composer
   *
   * @param composer - the component to create the Node
   * @return deserialised YAML document
   */
  protected Object loadOne(Composer composer) {
    Optional nodeOptional = composer.getSingleNode();
    return constructor.constructSingleDocument(nodeOptional);
  }

  /**
   * Parse the only YAML document in a stream and produce the corresponding Java object.
   *
   * @param yamlStream - data to load from (BOM is respected to detect encoding and removed from the
   *        data)
   * @return parsed Java instance
   */
  public Object loadFromInputStream(InputStream yamlStream) {
    Objects.requireNonNull(yamlStream, "InputStream cannot be null");
    return loadOne(createComposer(yamlStream));
  }

  /**
   * Parse a YAML document and create a Java instance
   *
   * @param yamlReader - data to load from (BOM must not be present)
   * @return parsed Java instance
   */
  public Object loadFromReader(Reader yamlReader) {
    Objects.requireNonNull(yamlReader, "Reader cannot be null");
    return loadOne(createComposer(yamlReader));
  }

  /**
   * Parse a YAML document and create a Java instance
   *
   * @param yaml - YAML data to load from (BOM must not be present)
   * @return parsed Java instance
   * @throws org.snakeyaml.engine.v2.exceptions.YamlEngineException if the YAML is not valid
   */
  public Object loadFromString(String yaml) {
    Objects.requireNonNull(yaml, "String cannot be null");
    return loadOne(createComposer(yaml));
  }

  // Load all the documents

  private Iterable