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/*
 * 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.linq4j;

import org.apache.calcite.linq4j.function.BigDecimalFunction1;
import org.apache.calcite.linq4j.function.DoubleFunction1;
import org.apache.calcite.linq4j.function.EqualityComparer;
import org.apache.calcite.linq4j.function.FloatFunction1;
import org.apache.calcite.linq4j.function.Function1;
import org.apache.calcite.linq4j.function.Function2;
import org.apache.calcite.linq4j.function.Functions;
import org.apache.calcite.linq4j.function.IntegerFunction1;
import org.apache.calcite.linq4j.function.LongFunction1;
import org.apache.calcite.linq4j.function.NullableBigDecimalFunction1;
import org.apache.calcite.linq4j.function.NullableDoubleFunction1;
import org.apache.calcite.linq4j.function.NullableFloatFunction1;
import org.apache.calcite.linq4j.function.NullableIntegerFunction1;
import org.apache.calcite.linq4j.function.NullableLongFunction1;
import org.apache.calcite.linq4j.function.Predicate1;
import org.apache.calcite.linq4j.function.Predicate2;
import org.apache.calcite.linq4j.tree.Expression;
import org.apache.calcite.linq4j.tree.Expressions;
import org.apache.calcite.linq4j.tree.FunctionExpression;

import org.checkerframework.checker.nullness.qual.Nullable;

import java.lang.reflect.Type;
import java.math.BigDecimal;
import java.util.Comparator;
import java.util.Iterator;

import static java.util.Objects.requireNonNull;

/**
 * Default implementations for methods in the {@link Queryable} interface.
 */
public abstract class QueryableDefaults {

  /**
   * Applies an accumulator function over a
   * sequence.
   */
  public static  T aggregate(Queryable queryable,
      FunctionExpression> func) {
    throw Extensions.todo();
  }

  /**
   * Applies an accumulator function over a
   * sequence. The specified seed value is used as the initial
   * accumulator value.
   */
  public static  TAccumulate aggregate(Queryable queryable,
      TAccumulate seed,
      FunctionExpression> func) {
    throw Extensions.todo();
  }

  /**
   * Applies an accumulator function over a
   * sequence. The specified seed value is used as the initial
   * accumulator value, and the specified function is used to select
   * the result value.
   */
  public static  TResult aggregate(
      Queryable queryable, TAccumulate seed,
      FunctionExpression> func,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Determines whether all the elements of a sequence
   * satisfy a condition.
   */
  public static  boolean all(Queryable queryable,
      FunctionExpression> predicate) {
    throw Extensions.todo();
  }

  /**
   * Determines whether a sequence contains any
   * elements.
   */
  public static  void any(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Determines whether any element of a sequence
   * satisfies a condition.
   */
  public static  boolean any(Queryable queryable,
      FunctionExpression> predicate) {
    throw Extensions.todo();
  }

  /**
   * Converts a generic {@code Enumerable} to a generic
   * {@code IQueryable}.
   */
  public static  Queryable asQueryable(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Computes the average of a sequence of Decimal
   * values that is obtained by invoking a projection function on
   * each element of the input sequence.
   */
  public static  BigDecimal averageBigDecimal(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the average of a sequence of nullable
   * Decimal values that is obtained by invoking a projection
   * function on each element of the input sequence.
   */
  public static  BigDecimal averageNullableBigDecimal(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the average of a sequence of Double
   * values that is obtained by invoking a projection function on
   * each element of the input sequence.
   */
  public static  double averageDouble(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the average of a sequence of nullable
   * Double values that is obtained by invoking a projection
   * function on each element of the input sequence.
   */
  public static  Double averageNullableDouble(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the average of a sequence of int values
   * that is obtained by invoking a projection function on each
   * element of the input sequence.
   */
  public static  int averageInteger(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the average of a sequence of nullable
   * int values that is obtained by invoking a projection function
   * on each element of the input sequence.
   */
  public static  Integer averageNullableInteger(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the average of a sequence of Float
   * values that is obtained by invoking a projection function on
   * each element of the input sequence.
   */
  public static  float averageFloat(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the average of a sequence of nullable
   * Float values that is obtained by invoking a projection
   * function on each element of the input sequence.
   */
  public static  Float averageNullableFloat(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the average of a sequence of long values
   * that is obtained by invoking a projection function on each
   * element of the input sequence.
   */
  public static  long averageLong(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the average of a sequence of nullable
   * long values that is obtained by invoking a projection function
   * on each element of the input sequence.
   */
  public static  Long averageNullableLong(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Analogous to LINQ's Enumerable.Cast extension method.
   *
   * @param clazz Target type
   * @param  Target type
   *
   * @return Collection of T2
   */
  public static  Queryable cast(final Queryable source,
      final Class clazz) {
    return new BaseQueryable(source.getProvider(), clazz,
        source.getExpression()) {
      @Override public Enumerator enumerator() {
        return new EnumerableDefaults.CastingEnumerator<>(source.enumerator(),
            clazz);
      }
    };
  }

  /**
   * Concatenates two sequences.
   */
  public static  Queryable concat(Queryable queryable0,
      Enumerable source2) {
    throw Extensions.todo();
  }

  /**
   * Determines whether a sequence contains a specified
   * element by using the default equality comparer.
   */
  public static  boolean contains(Queryable queryable, T element) {
    throw Extensions.todo();
  }

  /**
   * Determines whether a sequence contains a specified
   * element by using a specified {@code EqualityComparer}.
   */
  public static  boolean contains(Queryable queryable, T element,
      EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Returns the number of elements in a
   * sequence.
   */
  public static  int count(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Returns the number of elements in the specified
   * sequence that satisfies a condition.
   */
  public static  int count(Queryable queryable,
      FunctionExpression> func) {
    throw Extensions.todo();
  }

  /**
   * Returns the elements of the specified sequence or
   * the type parameter's default value in a singleton collection if
   * the sequence is empty.
   */
  public static  Queryable defaultIfEmpty(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Returns the elements of the specified sequence or
   * the specified value in a singleton collection if the sequence
   * is empty.
   */
  public static  T defaultIfEmpty(Queryable queryable, T value) {
    throw Extensions.todo();
  }

  /**
   * Returns distinct elements from a sequence by using
   * the default equality comparer to compare values.
   */
  public static  Queryable distinct(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Returns distinct elements from a sequence by using
   * a specified {@code EqualityComparer} to compare values.
   */
  public static  Queryable distinct(Queryable queryable,
      EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Returns the element at a specified index in a
   * sequence.
   */
  public static  T elementAt(Queryable queryable, int index) {
    throw Extensions.todo();
  }

  /**
   * Returns the element at a specified index in a
   * sequence or a default value if the index is out of
   * range.
   */
  public static  T elementAtOrDefault(Queryable queryable, int index) {
    throw Extensions.todo();
  }

  /**
   * Produces the set difference of two sequences by
   * using the default equality comparer to compare values. (Defined
   * by Queryable.)
   */
  public static  Queryable except(Queryable queryable,
      Enumerable enumerable) {
    throw Extensions.todo();
  }

  /**
   * Produces the set difference of two sequences by
   * using the specified {@code EqualityComparer} to compare
   * values.
   */
  public static  Queryable except(Queryable queryable,
      Enumerable enumerable, EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Returns the first element of a sequence. (Defined
   * by Queryable.)
   */
  public static  T first(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Returns the first element of a sequence that
   * satisfies a specified condition.
   */
  public static  T first(Queryable queryable,
      FunctionExpression> predicate) {
    throw Extensions.todo();
  }

  /**
   * Returns the first element of a sequence, or a
   * default value if the sequence contains no elements.
   */
  public static  T firstOrDefault(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Returns the first element of a sequence that
   * satisfies a specified condition or a default value if no such
   * element is found.
   */
  public static  T firstOrDefault(Queryable queryable,
      FunctionExpression> predicate) {
    throw Extensions.todo();
  }

  /**
   * Groups the elements of a sequence according to a
   * specified key selector function.
   */
  public static  Queryable> groupBy(
      Queryable queryable,
      FunctionExpression> keySelector) {
    throw Extensions.todo();
  }

  /**
   * Groups the elements of a sequence according to a
   * specified key selector function and compares the keys by using
   * a specified comparer.
   */
  public static  Queryable> groupBy(
      Queryable queryable,
      FunctionExpression> keySelector,
      EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Groups the elements of a sequence according to a
   * specified key selector function and projects the elements for
   * each group by using a specified function.
   */
  public static  Queryable> groupBy(
      Queryable queryable,
      FunctionExpression> keySelector,
      FunctionExpression> elementSelector) {
    throw Extensions.todo();
  }

  /**
   * Groups the elements of a sequence according to a
   * specified key selector function and creates a result value from
   * each group and its key.
   *
   * NOTE: Renamed from {@code groupBy} to distinguish from
   * {@link #groupBy(org.apache.calcite.linq4j.Queryable, org.apache.calcite.linq4j.tree.FunctionExpression, org.apache.calcite.linq4j.tree.FunctionExpression)},
   * which has the same erasure.
   */
  public static  Queryable> groupByK(
      Queryable queryable,
      FunctionExpression> keySelector,
      FunctionExpression, TResult>>
        elementSelector) {
    throw Extensions.todo();
  }

  /**
   * Groups the elements of a sequence and projects the
   * elements for each group by using a specified function. Key
   * values are compared by using a specified comparer.
   */
  public static  Queryable> groupBy(
      Queryable queryable,
      FunctionExpression> keySelector,
      FunctionExpression> elementSelector,
      EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Groups the elements of a sequence according to a
   * specified key selector function and creates a result value from
   * each group and its key. Keys are compared by using a specified
   * comparer.
   *
   * 
NOTE: Renamed from {@code groupBy} to distinguish from
   * {@link #groupBy(org.apache.calcite.linq4j.Queryable, org.apache.calcite.linq4j.tree.FunctionExpression, org.apache.calcite.linq4j.tree.FunctionExpression, org.apache.calcite.linq4j.function.EqualityComparer)},
   * which has the same erasure.
   */
  public static  Queryable groupByK(
      Queryable queryable,
      FunctionExpression> keySelector,
      FunctionExpression, TResult>>
        elementSelector,
      EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Groups the elements of a sequence according to a
   * specified key selector function and creates a result value from
   * each group and its key. The elements of each group are
   * projected by using a specified function.
   */
  public static  Queryable groupBy(
      Queryable queryable,
      FunctionExpression> keySelector,
      FunctionExpression> elementSelector,
      FunctionExpression, TResult>>
        resultSelector) {
    throw Extensions.todo();
  }

  /**
   * Groups the elements of a sequence according to a
   * specified key selector function and creates a result value from
   * each group and its key. Keys are compared by using a specified
   * comparer and the elements of each group are projected by using
   * a specified function.
   */
  public static  Queryable groupBy(
      Queryable queryable,
      FunctionExpression> keySelector,
      FunctionExpression> elementSelector,
      FunctionExpression, TResult>>
        resultSelector,
      EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Correlates the elements of two sequences based on
   * key equality and groups the results. The default equality
   * comparer is used to compare keys.
   */
  public static  Queryable groupJoin(
      Queryable outer, Enumerable inner,
      FunctionExpression> outerKeySelector,
      FunctionExpression> innerKeySelector,
      FunctionExpression, TResult>>
        resultSelector) {
    throw Extensions.todo();
  }

  /**
   * Correlates the elements of two sequences based on
   * key equality and groups the results. A specified
   * {@code EqualityComparer} is used to compare keys.
   */
  public static  Enumerable groupJoin(
      Queryable outer, Enumerable inner,
      FunctionExpression> outerKeySelector,
      FunctionExpression> innerKeySelector,
      FunctionExpression, TResult>>
        resultSelector,
      EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Produces the set intersection of two sequences by
   * using the default equality comparer to compare values. (Defined
   * by Queryable.)
   */
  public static  Queryable intersect(Queryable queryable,
      Enumerable enumerable) {
    throw Extensions.todo();
  }

  /**
   * Produces the set intersection of two sequences by
   * using the specified {@code EqualityComparer} to compare
   * values.
   */
  public static  Queryable intersect(Queryable queryable,
      Enumerable enumerable, EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Correlates the elements of two sequences based on
   * matching keys. The default equality comparer is used to compare
   * keys.
   */
  public static  Queryable join(
      Queryable outer, Enumerable inner,
      FunctionExpression> outerKeySelector,
      FunctionExpression> innerKeySelector,
      FunctionExpression> resultSelector) {
    throw Extensions.todo();
  }

  /**
   * Correlates the elements of two sequences based on
   * matching keys. A specified {@code EqualityComparer} is used to
   * compare keys.
   */
  public static  Queryable join(
      Queryable outer, Enumerable inner,
      FunctionExpression> outerKeySelector,
      FunctionExpression> innerKeySelector,
      FunctionExpression> resultSelector,
      EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Returns the last element in a sequence. (Defined
   * by Queryable.)
   */
  public static  T last(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Returns the last element of a sequence that
   * satisfies a specified condition.
   */
  public static  T last(Queryable queryable,
      FunctionExpression> predicate) {
    throw Extensions.todo();
  }

  /**
   * Returns the last element in a sequence, or a
   * default value if the sequence contains no elements.
   */
  public static  T lastOrDefault(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Returns the last element of a sequence that
   * satisfies a condition or a default value if no such element is
   * found.
   */
  public static  T lastOrDefault(Queryable queryable,
      FunctionExpression> predicate) {
    throw Extensions.todo();
  }

  /**
   * Returns an long that represents the total number
   * of elements in a sequence.
   */
  public static  long longCount(Queryable xable) {
    throw Extensions.todo();
  }

  /**
   * Returns an long that represents the number of
   * elements in a sequence that satisfy a condition.
   */
  public static  long longCount(Queryable queryable,
      FunctionExpression> predicate) {
    throw Extensions.todo();
  }

  /**
   * Returns the maximum value in a generic
   * {@code IQueryable}.
   */
  public static  T max(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Invokes a projection function on each element of a
   * generic {@code IQueryable} and returns the maximum resulting
   * value.
   */
  public static  TResult max(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Returns the minimum value in a generic
   * {@code IQueryable}.
   */
  public static  T min(Queryable queryable) {
    throw Extensions.todo();
  }

  /**
   * Invokes a projection function on each element of a
   * generic {@code IQueryable} and returns the minimum resulting
   * value.
   */
  public static  TResult min(Queryable queryable,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Filters the elements of an IQueryable based on a
   * specified type.
   *
   * 
This method generates a
   * {@link org.apache.calcite.linq4j.tree.MethodCallExpression} that
   * represents calling {@code ofType} itself as a constructed generic method.
   * It then passes the {@code MethodCallExpression} to the
   * {@link org.apache.calcite.linq4j.QueryProvider#createQuery createQuery}
   * method of the
   * {@link org.apache.calcite.linq4j.QueryProvider} represented by
   * the Provider property of the source parameter.
   *
   * 
The query behavior that occurs as a result of executing an expression
   * tree that represents calling OfType depends on the implementation of the
   * type of the source parameter. The expected behavior is that it filters
   * out any elements in source that are not of type TResult.
   *
   * 
NOTE: clazz parameter not present in C# LINQ; necessary because of
   * Java type erasure.
   */
  public static  Queryable ofType(Queryable queryable,
      Class clazz) {
    throw Extensions.todo();
  }

  /**
   * Sorts the elements of a sequence in ascending
   * order according to a key.
   *
   * @see #thenBy
   */
  public static  OrderedQueryable orderBy(
      Queryable source, FunctionExpression> keySelector) {
    throw Extensions.todo();
  }

  /**
   * Sorts the elements of a sequence in ascending
   * order by using a specified comparer.
   */
  public static  OrderedQueryable orderBy(Queryable source,
      FunctionExpression> keySelector,
      Comparator comparator) {
    throw Extensions.todo();
  }

  /**
   * Sorts the elements of a sequence in descending
   * order according to a key.
   */
  public static  OrderedQueryable orderByDescending(
      Queryable source,
      FunctionExpression> keySelector) {
    throw Extensions.todo();
  }

  /**
   * Sorts the elements of a sequence in descending
   * order by using a specified comparer.
   */
  public static  OrderedQueryable orderByDescending(
      Queryable source, FunctionExpression> keySelector,
      Comparator comparator) {
    throw Extensions.todo();
  }

  /**
   * Inverts the order of the elements in a
   * sequence.
   */
  public static  Queryable reverse(Queryable source) {
    throw Extensions.todo();
  }

  /**
   * Projects each element of a sequence into a new form.
   */
  public static  Queryable select(Queryable source,
      FunctionExpression> selector) {
    return source.getProvider().createQuery(
        Expressions.call(requireNonNull(source.getExpression()), "select", selector),
        functionResultType(selector));
  }

  private static  Type functionResultType(
      FunctionExpression> selector) {
    return requireNonNull(selector.body, "selector.body").getType();
  }

  /**
   * Projects each element of a sequence into a new
   * form by incorporating the element's index.
   *
   * 
NOTE: Renamed from {@code select} because had same erasure as
   * {@link #select(org.apache.calcite.linq4j.Queryable, org.apache.calcite.linq4j.tree.FunctionExpression)}.
   */
  public static  Queryable selectN(Queryable source,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Projects each element of a sequence to an
   * {@code Enumerable} and combines the resulting sequences into one
   * sequence.
   */
  public static  Queryable selectMany(Queryable source,
      FunctionExpression>> selector) {
    throw Extensions.todo();
  }

  /**
   * Projects each element of a sequence to an
   * {@code Enumerable} and combines the resulting sequences into one
   * sequence. The index of each source element is used in the
   * projected form of that element.
   *
   * 
NOTE: Renamed from {@code selectMany} because had same erasure as
   * {@link #selectMany(org.apache.calcite.linq4j.Queryable, org.apache.calcite.linq4j.tree.FunctionExpression)}.
   */
  public static  Queryable selectManyN(Queryable source,
      FunctionExpression>> selector) {
    throw Extensions.todo();
  }

  /**
   * Projects each element of a sequence to an
   * {@code Enumerable} that incorporates the index of the source
   * element that produced it. A result selector function is invoked
   * on each element of each intermediate sequence, and the
   * resulting values are combined into a single, one-dimensional
   * sequence and returned.
   */
  public static  Queryable selectMany(
      Queryable source,
      FunctionExpression>>
        collectionSelector,
      FunctionExpression> resultSelector) {
    throw Extensions.todo();
  }

  /**
   * Projects each element of a sequence to an
   * {@code Enumerable} and invokes a result selector function on each
   * element therein. The resulting values from each intermediate
   * sequence are combined into a single, one-dimensional sequence
   * and returned.
   *
   * NOTE: Renamed from {@code selectMany} because had same erasure as
   * {@link #selectMany(org.apache.calcite.linq4j.Queryable, org.apache.calcite.linq4j.tree.FunctionExpression, org.apache.calcite.linq4j.tree.FunctionExpression)}.
   */
  public static  Queryable selectManyN(
      Queryable source,
      FunctionExpression>>
        collectionSelector,
      FunctionExpression> resultSelector) {
    throw Extensions.todo();
  }

  /**
   * Determines whether two sequences are equal by
   * using the default equality comparer to compare
   * elements.
   */
  public static  boolean sequenceEqual(Queryable queryable,
      Enumerable enumerable) {
    throw Extensions.todo();
  }

  /**
   * Determines whether two sequences are equal by
   * using a specified {@code EqualityComparer} to compare
   * elements.
   */
  public static  boolean sequenceEqual(Queryable queryable,
      Enumerable enumerable, EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Returns the only element of a sequence, and throws
   * an exception if there is not exactly one element in the
   * sequence.
   */
  public static  T single(Queryable source) {
    throw Extensions.todo();
  }

  /**
   * Returns the only element of a sequence that
   * satisfies a specified condition, and throws an exception if
   * more than one such element exists.
   */
  public static  T single(Queryable source,
      FunctionExpression> predicate) {
    throw Extensions.todo();
  }

  /**
   * Returns the only element of a sequence, or a
   * default value if the sequence is empty; this method throws an
   * exception if there is more than one element in the
   * sequence.
   */
  public static  T singleOrDefault(Queryable source) {
    throw Extensions.todo();
  }

  /**
   * Returns the only element of a sequence that
   * satisfies a specified condition or a default value if no such
   * element exists; this method throws an exception if more than
   * one element satisfies the condition.
   */
  public static  T singleOrDefault(Queryable source,
      FunctionExpression> predicate) {
    throw Extensions.todo();
  }

  /**
   * Bypasses a specified number of elements in a
   * sequence and then returns the remaining elements.
   */
  public static  Queryable skip(Queryable source, int count) {
    return EnumerableDefaults.skip(source.asEnumerable(), count).asQueryable();
  }

  /**
   * Bypasses elements in a sequence as long as a
   * specified condition is true and then returns the remaining
   * elements.
   */
  public static  Queryable skipWhile(Queryable source,
      FunctionExpression> predicate) {
    return skipWhileN(source,
        Expressions.lambda(
            Functions.toPredicate2(predicate.getFunction())));
  }

  /**
   * Bypasses elements in a sequence as long as a
   * specified condition is true and then returns the remaining
   * elements. The element's index is used in the logic of the
   * predicate function.
   */
  public static  Queryable skipWhileN(final Queryable source,
      final FunctionExpression> predicate) {
    return new BaseQueryable(source.getProvider(), source.getElementType(),
        source.getExpression()) {
      @Override public Enumerator enumerator() {
        return new EnumerableDefaults.SkipWhileEnumerator<>(
            source.enumerator(), predicate.getFunction());
      }
    };
  }

  /**
   * Computes the sum of the sequence of Decimal values
   * that is obtained by invoking a projection function on each
   * element of the input sequence.
   */
  public static  BigDecimal sumBigDecimal(Queryable sources,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the sum of the sequence of nullable
   * Decimal values that is obtained by invoking a projection
   * function on each element of the input sequence.
   */
  public static  BigDecimal sumNullableBigDecimal(Queryable source,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the sum of the sequence of Double values
   * that is obtained by invoking a projection function on each
   * element of the input sequence.
   */
  public static  double sumDouble(Queryable source,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the sum of the sequence of nullable
   * Double values that is obtained by invoking a projection
   * function on each element of the input sequence.
   */
  public static  Double sumNullableDouble(Queryable source,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the sum of the sequence of int values
   * that is obtained by invoking a projection function on each
   * element of the input sequence.
   */
  public static  int sumInteger(Queryable source,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the sum of the sequence of nullable int
   * values that is obtained by invoking a projection function on
   * each element of the input sequence.
   */
  public static  Integer sumNullableInteger(Queryable source,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the sum of the sequence of long values
   * that is obtained by invoking a projection function on each
   * element of the input sequence.
   */
  public static  long sumLong(Queryable source,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the sum of the sequence of nullable long
   * values that is obtained by invoking a projection function on
   * each element of the input sequence.
   */
  public static  Long sumNullableLong(Queryable source,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the sum of the sequence of Float values
   * that is obtained by invoking a projection function on each
   * element of the input sequence.
   */
  public static  float sumFloat(Queryable source,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Computes the sum of the sequence of nullable
   * Float values that is obtained by invoking a projection
   * function on each element of the input sequence.
   */
  public static  Float sumNullableFloat(Queryable source,
      FunctionExpression> selector) {
    throw Extensions.todo();
  }

  /**
   * Returns a specified number of contiguous elements
   * from the start of a sequence.
   */
  public static  Queryable take(Queryable source, int count) {
    return EnumerableDefaults.take(source.asEnumerable(), count).asQueryable();
  }

  /**
   * Returns elements from a sequence as long as a
   * specified condition is true.
   */
  public static  Queryable takeWhile(Queryable source,
      FunctionExpression> predicate) {
    return takeWhileN(source,
        Expressions.lambda(
            Functions.toPredicate2(predicate.getFunction())));
  }

  /**
   * Returns elements from a sequence as long as a
   * specified condition is true. The element's index is used in the
   * logic of the predicate function.
   */
  public static  Queryable takeWhileN(final Queryable source,
      final FunctionExpression> predicate) {
    return new BaseQueryable(source.getProvider(), source.getElementType(),
        source.getExpression()) {
      @Override public Enumerator enumerator() {
        return new EnumerableDefaults.TakeWhileEnumerator<>(
            source.enumerator(), predicate.getFunction());
      }
    };
  }

  /**
   * Performs a subsequent ordering of the elements in a sequence in
   * ascending order according to a key.
   */
  public static > OrderedQueryable thenBy(
      OrderedQueryable source,
      FunctionExpression> keySelector) {
    throw Extensions.todo();
  }

  /**
   * Performs a subsequent ordering of the elements in a sequence in
   * ascending order according to a key, using a specified comparator.
   */
  public static  OrderedQueryable thenBy(OrderedQueryable source,
      FunctionExpression> keySelector,
      Comparator comparator) {
    throw Extensions.todo();
  }

  /**
   * Performs a subsequent ordering of the elements in a sequence in
   * descending order according to a key.
   */
  public static > OrderedQueryable thenByDescending(
      OrderedQueryable source,
      FunctionExpression> keySelector) {
    throw Extensions.todo();
  }

  /**
   * Performs a subsequent ordering of the elements in a sequence in
   * dscending order according to a key, using a specified comparator.
   */
  public static  OrderedQueryable thenByDescending(
      OrderedQueryable source,
      FunctionExpression> keySelector,
      Comparator comparator) {
    throw Extensions.todo();
  }

  /**
   * Produces the set union of two sequences by using
   * the default equality comparer.
   */
  public static  Queryable union(Queryable source0,
      Enumerable source1) {
    throw Extensions.todo();
  }

  /**
   * Produces the set union of two sequences by using a
   * specified {@code EqualityComparer}.
   */
  public static  Queryable union(Queryable source0,
      Enumerable source1, EqualityComparer comparer) {
    throw Extensions.todo();
  }

  /**
   * Filters a sequence of values based on a
   * predicate.
   */
  public static  Queryable where(final Queryable source,
      final FunctionExpression> predicate) {
    return new NonLeafReplayableQueryable(source) {
      @Override public void replay(QueryableFactory factory) {
        factory.where(source, predicate);
      }
    };
  }

  /**
   * Filters a sequence of values based on a
   * predicate. Each element's index is used in the logic of the
   * predicate function.
   */
  public static  Queryable whereN(Queryable source,
      FunctionExpression> predicate) {
    throw Extensions.todo();
  }

  /**
   * Merges two sequences by using the specified
   * predicate function.
   */
  public static  Queryable zip(Queryable source0,
      Enumerable source1,
      FunctionExpression> resultSelector) {
    throw Extensions.todo();
  }

  /** Replayable.
   *
   * @param  element type */
  public interface Replayable extends Queryable {
    void replay(QueryableFactory factory);
  }

  /** Replayable queryable.
   *
   * @param  element type */
  public abstract static class ReplayableQueryable
      extends DefaultQueryable implements Replayable {
    @Override public void replay(QueryableFactory factory) {
    }

    @Override public Iterator iterator() {
      return Linq4j.enumeratorIterator(enumerator());
    }

    @Override public Enumerator enumerator() {
      return getProvider().executeQuery(this);
    }

    /**
     * Convenience method, for {@link QueryableRecorder} methods that
     * return a scalar value such as {@code boolean} or
     * {@link BigDecimal}.
     */
    @SuppressWarnings("unchecked")
     U castSingle() {
      return ((Queryable) (Queryable) this).single();
    }

    /**
     * Convenience method, for {@link QueryableRecorder} methods that
     * return a Queryable of a different element type than the source.
     */
    @SuppressWarnings("unchecked")
    public  Queryable castQueryable() {
      return (Queryable) (Queryable) this;
    }
  }

  /** Non-leaf replayable queryable.
   *
   * @param  element type */
  public abstract static class NonLeafReplayableQueryable
      extends ReplayableQueryable {
    private final Queryable original;

    protected NonLeafReplayableQueryable(Queryable original) {
      this.original = original;
    }

    @Override public Type getElementType() {
      return original.getElementType();
    }

    @Override public @Nullable Expression getExpression() {
      return original.getExpression();
    }

    @Override public QueryProvider getProvider() {
      return original.getProvider();
    }
  }
}