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/**
* Copyright (C) 2021-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <memory>
#include <type_traits>
#include <utility>
#include "mongo/stdx/type_traits.h"
namespace mongo::expression_walker {
/**
* A template type which resolves to 'const T*' if 'IsConst' argument is 'true', and to 'T*'
* otherwise.
*/
template <bool IsConst, typename T>
using MaybeConstPtr = typename std::conditional<IsConst, const T*, T*>::type;
// The following types and constexpr values are used to determine if a Walker has a given member
// function at compile-time.
/**
* PreVisit provides the compiler with a type for a preVisit member function.
*/
template <typename Walker, typename Arg>
using PreVisit = decltype(std::declval<Walker>().preVisit(std::declval<Arg>()));
/**
* hasVoidPreVisit is a template variable indicating whether such a void-returning member function
* exists for a given Walker type when called on a pointer to our Node type.
*/
template <typename Node, typename Walker>
inline constexpr auto hasVoidPreVisit =
stdx::is_detected_exact_v<void, PreVisit, Walker, MaybeConstPtr<std::is_const_v<Node>, Node>>;
/**
* hasVoidPreVisit is a template variable indicating whether such a pointer-returning member
* function exists for a given Walker type when called on a pointer to our Node type.
*/
template <typename Node, typename Walker>
inline constexpr auto hasPtrPreVisit =
stdx::is_detected_convertible_v<std::unique_ptr<Node>, PreVisit, Walker, Node*>;
/**
* InVisit provides the compiler with a type for an inVisit member function.
*/
template <typename Walker, typename... Args>
using InVisit = decltype(std::declval<Walker>().inVisit(std::declval<Args>()...));
/**
* hasBasicInVisit is a template variable indicating whether such a member function exists for a
* given Walker type when called on a pointer to our Node type.
*/
template <typename Node, typename Walker>
inline constexpr auto hasBasicInVisit =
stdx::is_detected_v<InVisit, Walker, MaybeConstPtr<std::is_const_v<Node>, Node>>;
/**
* hasCountingInVisit is a template variable indicating whether such a member function exists for a
* given Walker type when called on a pointer to our Node type.
*/
template <typename Node, typename Walker>
inline constexpr auto hasCountingInVisit = stdx::
is_detected_v<InVisit, Walker, unsigned long long, MaybeConstPtr<std::is_const_v<Node>, Node>>;
/**
* PostVisit provides the compiler with a type for a postVisit member function.
*/
template <typename Walker, typename Arg>
using PostVisit = decltype(std::declval<Walker>().postVisit(std::declval<Arg>()));
/**
* hasVoidPostVisit is a template variable indicating whether such a void-returning member function
* exists for a given Walker type when called on a pointer to our Node type.
*/
template <typename Node, typename Walker>
inline constexpr auto hasVoidPostVisit =
stdx::is_detected_exact_v<void, PostVisit, Walker, MaybeConstPtr<std::is_const_v<Node>, Node>>;
/**
* hasVoidPostVisit is a template variable indicating whether such a pointer-returning member
* function exists for a given Walker type when called on a pointer to our Node type.
*/
template <typename Node, typename Walker>
inline constexpr auto hasPtrPostVisit =
stdx::is_detected_convertible_v<std::unique_ptr<Node>, PostVisit, Walker, Node*>;
/**
* hasReturningVisit is a template variable indicating whether there is a pointer-returning member
* function (pre or post) that exists for a given Walker type when called on a pointer to our
* Node type.
*/
template <typename Node, typename Walker>
inline constexpr auto hasReturningVisit =
hasPtrPreVisit<Node, Walker> || hasPtrPostVisit<Node, Walker>;
/**
* Provided with a Walker and a Node, walk() calls each of the following if they exist:
* * walker.preVisit() once before walking to each child.
* * walker.inVisit() between walking to each child. It is called multiple times, once between each
* pair of children. walker.inVisit() is skipped if the Node has fewer than two children.
* * walker.postVisit() once after walking to each child.
* Each of the Node's child Nodes is recursively walked and the same three methods are called for
* it. Although each of the methods are individually optional, at least one of them must exist.
*
* If the caller doesn't intend to modify the tree, then the type of Node should be 'const'.
*
* If Node is not const, preVisit() and postVisit() may return a pointer to a Node. If either does,
* walk() will replace the current Node with the return value. If no change is needed during a
* particular call, preVisit() and postVisit() may return null. walk() returns a unique_ptr
* containing a new root node, if it is modified by a value returning preVisit() or postVisit(),
* nullptr if it is not modified, or void if modification is impossible for the given Walker.
*/
template <typename Node, typename Walker>
auto walk(Node* node, Walker* walker)
-> std::conditional_t<hasReturningVisit<Node, Walker>, std::unique_ptr<Node>, void> {
if constexpr (std::is_const_v<Node>) {
static_assert(
hasVoidPreVisit<Node, Walker> || hasBasicInVisit<Node, Walker> ||
hasCountingInVisit<Node, Walker> || hasVoidPostVisit<Node, Walker>,
"Walker that is const must have at least one of the following functions: 'preVisit', "
"'inVisit', 'postVisit'.");
} else {
static_assert(hasVoidPreVisit<Node, Walker> || hasPtrPreVisit<Node, Walker> ||
hasBasicInVisit<Node, Walker> || hasCountingInVisit<Node, Walker> ||
hasVoidPostVisit<Node, Walker> || hasPtrPostVisit<Node, Walker>,
"Walker that is non-const must have at least one of the following functions: "
"'preVisit', 'inVisit', 'postVisit'.");
}
static_assert(!hasBasicInVisit<Node, Walker> || !hasCountingInVisit<Node, Walker>,
"Walker must include only one signature for inVisit: inVisit(num, node) "
"or inVisit(node).");
// Calls walk on a child node. Then replaces that node if walk returns a non-null value.
auto walkChild = [&](auto&& child) {
if constexpr (hasReturningVisit<Node, Walker>) {
if (auto newChild = walk(child.get(), walker))
child = newChild.release();
} else {
walk(child.get(), walker);
}
};
auto newExpr = std::unique_ptr<Node>{};
if (node) {
if constexpr (hasVoidPreVisit<Node, Walker>) {
walker->preVisit(node);
} else if constexpr (hasPtrPreVisit<Node, Walker>) {
newExpr = walker->preVisit(node);
node = newExpr.get() != nullptr ? newExpr.get() : node;
}
// InVisit needs to be called between every two nodes which requires more complicated
// branching logic. InVisit is forbidden from replacing its node through the return value
// and must return void since it would break our iteration and be confusing to replace a
// node while only a portion of its children have been walked.
if constexpr (hasBasicInVisit<Node, Walker>) {
static_assert(
std::is_void_v<InVisit<Walker, Node*>>,
"Walker::inVisit must return void. Modification is forbidden between walking "
"children.");
auto skippingFirst = true;
for (auto&& child : node->getChildren()) {
if (skippingFirst)
skippingFirst = false;
else
walker->inVisit(node);
walkChild(child);
}
}
// If the signature of InVisit includes a count, maintaing it while walking and pass it to
// the function.
else if constexpr (hasCountingInVisit<Node, Walker>) {
static_assert(
std::is_void_v<InVisit<Walker, unsigned long long, Node*>>,
"Walker::inVisit must return void. Modification is forbidden between walking "
"children.");
auto count = 0ull;
for (auto&& child : node->getChildren()) {
if (count != 0ull)
walker->inVisit(count, node);
count++;
walkChild(child);
}
} else {
for (auto&& child : node->getChildren())
walkChild(child);
}
if constexpr (hasVoidPostVisit<Node, Walker>)
walker->postVisit(node);
else if constexpr (hasPtrPostVisit<Node, Walker>)
if (auto postResult = walker->postVisit(node))
newExpr = std::move(postResult);
}
if constexpr (hasReturningVisit<Node, Walker>)
return newExpr;
}
} // namespace mongo::expression_walker
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