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.rex;
  
  import org.apache.calcite.rel.type.RelDataType;
  import org.apache.calcite.sql.SqlKind;
  
  import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.util.Collection;
  
  import static java.util.Objects.requireNonNull;
  
  /**
   * Row expression.
   *
   * <p>Every row-expression has a type.
   * (Compare with {@link org.apache.calcite.sql.SqlNode}, which is created before
   * validation, and therefore types may not be available.)
   *
   * <p>Some common row-expressions are: {@link RexLiteral} (constant value),
   * {@link RexVariable} (variable), {@link RexCall} (call to operator with
   * operands). Expressions are generally created using a {@link RexBuilder}
   * factory.
   *
   * <p>All sub-classes of RexNode are immutable.
   */
  public abstract class RexNode {
  
    //~ Instance fields --------------------------------------------------------
  
    // Effectively final. Set in each sub-class constructor, and never re-set.
    protected @MonotonicNonNull String digest;
  
    //~ Methods ----------------------------------------------------------------
  
    public abstract RelDataType getType();
  
    /**
     * Returns whether this expression always returns true. (Such as if this
     * expression is equal to the literal <code>TRUE</code>.)
     */
    public boolean isAlwaysTrue() {
      return false;
    }
  
    /**
     * Returns whether this expression always returns false. (Such as if this
     * expression is equal to the literal <code>FALSE</code>.)
     */
    public boolean isAlwaysFalse() {
      return false;
    }
  
    public boolean isA(SqlKind kind) {
      return getKind() == kind;
    }
  
    public boolean isA(Collection<SqlKind> kinds) {
      return getKind().belongsTo(kinds);
    }
  
    /**
     * Returns the kind of node this is.
     *
     * @return Node kind, never null
     */
    public SqlKind getKind() {
      return SqlKind.OTHER;
    }
  
    @Override public String toString() {
      return requireNonNull(digest, "digest");
    }
  
    /** Returns the number of nodes in this expression.
     *
     * <p>Leaf nodes, such as {@link RexInputRef} or {@link RexLiteral}, have
     * a count of 1. Calls have a count of 1 plus the sum of their operands.
     *
     * <p>Node count is a measure of expression complexity that is used by some
     * planner rules to prevent deeply nested expressions.
     */
    public int nodeCount() {
      return 1;
    }
  
    /**
     * Accepts a visitor, dispatching to the right overloaded
     * {@link RexVisitor#visitInputRef visitXxx} method.
     *
     * <p>Also see {@link RexUtil#apply(RexVisitor, java.util.List, RexNode)},
     * which applies a visitor to several expressions simultaneously.
     */
    public abstract <R> R accept(RexVisitor<R> visitor);
  
    /**
     * Accepts a visitor with a payload, dispatching to the right overloaded
     * {@link RexBiVisitor#visitInputRef(RexInputRef, Object)} visitXxx} method.
     */
    public abstract <R, P> R accept(RexBiVisitor<R, P> visitor, P arg);
  
    /** {@inheritDoc}
     *
     * <p>Every node must implement {@link #equals} based on its content
     */
    @Override public abstract boolean equals(@Nullable Object obj);
  
    /** {@inheritDoc}
     *
     * <p>Every node must implement {@link #hashCode} consistent with
     * {@link #equals}
     */
    @Override public abstract int hashCode();
  }
  