/*
    * Copyright 2006-2012 the original author or authors.
    *
    * 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.springframework.batch.item.database.support;
  
  import org.springframework.util.StringUtils;
  
  /**
   * Postgres implementation of a  {@link org.springframework.batch.item.database.PagingQueryProvider} using database specific features.
   * 
   * When using the groupClause, this implementation expects all select fields not used in aggregate functions to be included in the 
   * groupClause (the provider does not add them for you).
   *
   * @author Thomas Risberg
   * @author Michael Minella
   * @since 2.0
   */
  public class PostgresPagingQueryProvider extends AbstractSqlPagingQueryProvider {
  
  	@Override
  	public String generateFirstPageQuery(int pageSize) {
  		return SqlPagingQueryUtils.generateLimitSqlQuery(this, false, buildLimitClause(pageSize));
  	}
  
  	@Override
  	public String generateRemainingPagesQuery(int pageSize) {
  		if(StringUtils.hasText(getGroupClause())) {
  			return SqlPagingQueryUtils.generateLimitGroupedSqlQuery(this, true, buildLimitClause(pageSize));
  		}
  		else {
  			return SqlPagingQueryUtils.generateLimitSqlQuery(this, true, buildLimitClause(pageSize));
  		}
  	}
  
  	private String buildLimitClause(int pageSize) {
  		return new StringBuilder().append("LIMIT ").append(pageSize).toString();
  	}
  	
  	@Override
  	public String generateJumpToItemQuery(int itemIndex, int pageSize) {
  		int page = itemIndex / pageSize;
  		int offset = (page * pageSize) - 1;
  		offset = offset<0 ? 0 : offset;
  		String limitClause = new StringBuilder().append("LIMIT 1 OFFSET ").append(offset).toString();
  		return SqlPagingQueryUtils.generateLimitJumpToQuery(this, limitClause);
  	}
  
  }