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
   *
   *   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.asterix.lang.sqlpp.rewrites.visitor;
  
  import java.util.ArrayList;
  import java.util.List;
  
  import org.apache.asterix.common.exceptions.CompilationException;
  import org.apache.asterix.common.functions.FunctionSignature;
  import org.apache.asterix.lang.common.base.Expression;
  import org.apache.asterix.lang.common.base.ILangExpression;
  import org.apache.asterix.lang.common.expression.CallExpr;
  import org.apache.asterix.lang.common.expression.LiteralExpr;
  import org.apache.asterix.lang.common.expression.OperatorExpr;
  import org.apache.asterix.lang.common.literal.TrueLiteral;
  import org.apache.asterix.lang.common.struct.OperatorType;
  import org.apache.asterix.lang.sqlpp.expression.CaseExpression;
  import org.apache.asterix.lang.sqlpp.util.SqlppRewriteUtil;
  import org.apache.asterix.lang.sqlpp.visitor.base.AbstractSqlppSimpleExpressionVisitor;
  import org.apache.asterix.om.functions.BuiltinFunctions;
  
  /**
   * CASE expression rewritings:
   * <ul>
   * <li>
   * Normalize WHEN expressions so that it can have correct NULL/MISSING semantics as well
   * as type promotion semantics.
   * </li>
   * <li>
   * If the CASE expression does not contain a subquery then rewrite it to a switch-case function call.
   * </li>
   * </ul>
   */
  public final class SqlppCaseExpressionVisitor extends AbstractSqlppSimpleExpressionVisitor {
  
      @Override
      public Expression visit(CaseExpression caseExpr, ILangExpression arg) throws CompilationException {
          Expression expr = super.visit(caseExpr, arg); // Visits it as usual first.
          if (expr != caseExpr) {
              return expr.accept(this, arg);
          }
          CaseExpression newCaseExpr = normalizeCaseExpr(caseExpr);
          if (SqlppRewriteUtil.constainsSubquery(newCaseExpr)) {
              // If the CASE expression contains a subquery, we do not rewrite it to a switch-case function call.
              return newCaseExpr;
          }
          // If the CASE expression does not contain a subquery, we rewrite it to a switch-case function call.
          FunctionSignature functionSignature = new FunctionSignature(BuiltinFunctions.SWITCH_CASE);
          List<Expression> whenExprList = newCaseExpr.getWhenExprs();
          List<Expression> thenExprList = newCaseExpr.getThenExprs();
          List<Expression> newExprList = new ArrayList<>();
          newExprList.add(newCaseExpr.getConditionExpr());
          for (int index = 0; index < whenExprList.size(); ++index) {
              newExprList.add(whenExprList.get(index));
              newExprList.add(thenExprList.get(index));
          }
          newExprList.add(newCaseExpr.getElseExpr());
          CallExpr callExpr = new CallExpr(functionSignature, newExprList);
          callExpr.setSourceLocation(caseExpr.getSourceLocation());
          return callExpr;
      }
  
      // Normalizes WHEN expressions so that it can have correct NULL/MISSING semantics as well
      // as type promotion semantics.
      private CaseExpression normalizeCaseExpr(CaseExpression caseExpr) throws CompilationException {
          Expression conditionExpr = caseExpr.getConditionExpr();
          if (conditionExpr.getKind() == Expression.Kind.LITERAL_EXPRESSION
                  && ((LiteralExpr) conditionExpr).getValue().equals(TrueLiteral.INSTANCE)) {
              return caseExpr;
          }
          List<Expression> normalizedWhenExprs = new ArrayList<>();
          for (Expression expr : caseExpr.getWhenExprs()) {
              OperatorExpr operatorExpr = new OperatorExpr();
              operatorExpr.addOperand((Expression) SqlppRewriteUtil.deepCopy(expr));
              operatorExpr.addOperand(caseExpr.getConditionExpr());
              operatorExpr.addOperator(OperatorType.EQ);
              operatorExpr.setSourceLocation(expr.getSourceLocation());
              normalizedWhenExprs.add(operatorExpr);
          }
          LiteralExpr trueLiteral = new LiteralExpr(TrueLiteral.INSTANCE);
          trueLiteral.setSourceLocation(caseExpr.getSourceLocation());
          CaseExpression newCaseExpr =
                  new CaseExpression(trueLiteral, normalizedWhenExprs, caseExpr.getThenExprs(), caseExpr.getElseExpr());
          newCaseExpr.setSourceLocation(caseExpr.getSourceLocation());
          return newCaseExpr;
      }
  }
  