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.calcite.sql.fun;
  
  import org.apache.calcite.rel.core.AggregateCall;
  import org.apache.calcite.rel.type.RelDataType;
  import org.apache.calcite.rex.RexBuilder;
  import org.apache.calcite.rex.RexNode;
  import org.apache.calcite.sql.SqlKind;
  import org.apache.calcite.sql.SqlOperatorBinding;
  import org.apache.calcite.sql.SqlStaticAggFunction;
  import org.apache.calcite.sql.type.OperandTypes;
  import org.apache.calcite.util.ImmutableBitSet;
  
  import com.google.common.collect.ImmutableList;
  import com.google.common.collect.Iterables;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * The {@code LITERAL_AGG} aggregate function.
   *
   * <p>It accepts zero regular aggregate arguments (the kind that reference
   * columns of the input group) but one argument of type
   * {@link org.apache.calcite.rex.RexLiteral}, and its return is that literal.
   */
  public class SqlLiteralAggFunction {
    /** This is utility class. It is never instantiated. */
    private SqlLiteralAggFunction() {
    }
  
    public static final SqlBasicAggFunction INSTANCE =
        SqlBasicAggFunction.create(SqlKind.LITERAL_AGG,
                SqlLiteralAggFunction::inferReturnType, OperandTypes.NILADIC)
            .withStatic(SqlLiteralAggFunction::constant);
  
    /** Implements {@link org.apache.calcite.sql.type.SqlReturnTypeInference}. */
    private static RelDataType inferReturnType(SqlOperatorBinding opBinding) {
      // LITERAL_AGG takes one pre-operand and zero (post-)operands.
      if (opBinding.getPreOperandCount() != 1
          || opBinding.getOperandCount() != 1) {
        throw new AssertionError();
      }
      return opBinding.getOperandType(0);
    }
  
    /** Implements {@link SqlStaticAggFunction}. */
    private static @Nullable RexNode constant(RexBuilder rexBuilder,
        ImmutableBitSet groupSet, ImmutableList<ImmutableBitSet> groupSets,
        AggregateCall aggregateCall) {
      // LITERAL_AGG[literal]() evaluates to "literal".
      return Iterables.getOnlyElement(aggregateCall.rexList);
    }
  }
  