/*
 * 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 org.apache.calcite.adapter.csv;

import org.apache.calcite.plan.RelOptRule;
import org.apache.calcite.plan.RelOptRuleCall;
import org.apache.calcite.rel.core.RelFactories;
import org.apache.calcite.rel.logical.LogicalProject;
import org.apache.calcite.rex.RexInputRef;
import org.apache.calcite.rex.RexNode;
import org.apache.calcite.tools.RelBuilderFactory;

import java.util.List;

/**
 * Planner rule that projects from a {@link CsvTableScan} scan just the columns
 * needed to satisfy a projection. If the projection's expressions are trivial,
 * the projection is removed.
 */
public class CsvProjectTableScanRule extends RelOptRule {
  public static final CsvProjectTableScanRule INSTANCE =
      new CsvProjectTableScanRule(RelFactories.LOGICAL_BUILDER);

  /**
   * Creates a CsvProjectTableScanRule.
   *
   * @param relBuilderFactory Builder for relational expressions
   */
  public CsvProjectTableScanRule(RelBuilderFactory relBuilderFactory) {
    super(
        operand(LogicalProject.class,
            operand(CsvTableScan.class, none())),
        relBuilderFactory,
        "CsvProjectTableScanRule");
  }

  @Override public void onMatch(RelOptRuleCall call) {
    final LogicalProject project = call.rel(0);
    final CsvTableScan scan = call.rel(1);
    int[] fields = getProjectFields(project.getProjects());
    if (fields == null) {
      // Project contains expressions more complex than just field references.
      return;
    }
    call.transformTo(
        new CsvTableScan(
            scan.getCluster(),
            scan.getTable(),
            scan.csvTable,
            fields));
  }

  private int[] getProjectFields(List<RexNode> exps) {
    final int[] fields = new int[exps.size()];
    for (int i = 0; i < exps.size(); i++) {
      final RexNode exp = exps.get(i);
      if (exp instanceof RexInputRef) {
        fields[i] = ((RexInputRef) exp).getIndex();
      } else {
        return null; // not a simple projection
      }
    }
    return fields;
  }
}

// End CsvProjectTableScanRule.java
