SERVER-24508 BSONObj::ComparatorInterface

BSONObj instances should now be compared via the comparator
interface's evaluate() method. This preferred over using
BSONObj::woCompare() directly. If the comparison doesn't
require any database semantics (e.g. there is no collation),
there is a global instance of the SimpleBSONObjComparator
which should be used for BSONObj comparisons. If the
comparison requires special semantics, then callers must
instantiate their own comparator object.

1 files changed, 195 insertions, 0 deletions

diff --git a/src/mongo/bson/bsonobj_comparator_interface.h b/src/mongo/bson/bsonobj_comparator_interface.h
new file mode 100644
index 00000000000..56fa736dab0
--- /dev/null
+++ b/src/mongo/bson/bsonobj_comparator_interface.h
@@ -0,0 +1,195 @@
+/**
+ *    Copyright (C) 2016 MongoDB Inc.
+ *
+ *    This program is free software: you can redistribute it and/or  modify
+ *    it under the terms of the GNU Affero General Public License, version 3,
+ *    as published by the Free Software Foundation.
+ *
+ *    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
+ *    GNU Affero General Public License for more details.
+ *
+ *    You should have received a copy of the GNU Affero General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *    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 GNU Affero General 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 <map>
+#include <set>
+#include <unordered_map>
+#include <unordered_set>
+
+#include "mongo/base/disallow_copying.h"
+#include "mongo/bson/bsonobj.h"
+#include "mongo/util/assert_util.h"
+
+namespace mongo {
+
+/**
+ * A BSONObj::ComparatorInterface is an abstract class for comparing BSONObj objects. Usage for
+ * comparing two BSON objects, 'lhs' and 'rhs', where 'comparator' is an instance of a class
+ * implementing this interface, is as shown below:
+ *
+ *  bool lessThan = comparator.evaluate(lhs < rhs);
+ *  bool lessThanOrEqual = comparator.evaluate(lhs <= rhs);
+ *  bool equal = comparator.evaluate(lhs == rhs);
+ *  bool greaterThanOrEqual = comparator.evaluate(lhs >= rhs);
+ *  bool greaterThan = comparator.evaluate(lhs > rhs);
+ *  bool notEqual = comparator.evaluate(lhs != rhs);
+ *
+ * Can also be used to obtain function objects compatible for use with standard library algorithms
+ * such as std::sort, and to construct STL sets and maps which respect this comparator.
+ *
+ * All methods are thread-safe.
+ */
+class BSONObj::ComparatorInterface {
+    MONGO_DISALLOW_COPYING(ComparatorInterface);
+
+public:
+    /**
+     * Functor compatible for use with ordered STL containers.
+     */
+    class LessThan {
+    public:
+        explicit LessThan(const ComparatorInterface* comparator) : _comparator(comparator) {}
+
+        bool operator()(const BSONObj& lhs, const BSONObj& rhs) const {
+            return _comparator->compare(lhs, rhs) < 0;
+        }
+
+    private:
+        const ComparatorInterface* _comparator;
+    };
+
+    /**
+     * Functor compatible for use with unordered STL containers.
+     */
+    class EqualTo {
+    public:
+        explicit EqualTo(const ComparatorInterface* comparator) : _comparator(comparator) {}
+
+        bool operator()(const BSONObj& lhs, const BSONObj& rhs) const {
+            return _comparator->compare(lhs, rhs) == 0;
+        }
+
+    private:
+        const ComparatorInterface* _comparator;
+    };
+
+    using BSONObjSet = std::set<BSONObj, BSONObj::ComparatorInterface::LessThan>;
+
+    // TODO SERVER-23990: Make the BSONObj hash collation-aware.
+    using BSONObjUnorderedSet =
+        std::unordered_set<BSONObj, BSONObj::Hasher, BSONObj::ComparatorInterface::EqualTo>;
+
+    template <typename T>
+    using BSONObjMap = std::map<BSONObj, T, BSONObj::ComparatorInterface::LessThan>;
+
+    // TODO SERVER-23990: Make the BSONObj hash collation-aware.
+    template <typename T>
+    using BSONObjIndexedMap =
+        std::unordered_map<BSONObj, T, BSONObj::Hasher, BSONObj::ComparatorInterface::EqualTo>;
+
+    virtual ~ComparatorInterface() = default;
+
+    /**
+     * Compares two BSONObj objects. Returns <0, 0, >0 if 'lhs' < 'rhs', 'lhs' == 'rhs', or 'lhs' >
+     * 'rhs' respectively.
+     */
+    virtual int compare(const BSONObj& lhs, const BSONObj& rhs) const = 0;
+
+    /**
+     * Evaluates a deferred comparison object generated by invocation of one of the BSONObj operator
+     * overloads for relops.
+     */
+    bool evaluate(BSONObj::DeferredComparison deferredComparison) const {
+        int cmp = compare(deferredComparison.lhs, deferredComparison.rhs);
+        switch (deferredComparison.type) {
+            case BSONObj::DeferredComparison::Type::kLT:
+                return cmp < 0;
+            case BSONObj::DeferredComparison::Type::kLTE:
+                return cmp <= 0;
+            case BSONObj::DeferredComparison::Type::kEQ:
+                return cmp == 0;
+            case BSONObj::DeferredComparison::Type::kGT:
+                return cmp > 0;
+            case BSONObj::DeferredComparison::Type::kGTE:
+                return cmp >= 0;
+            case BSONObj::DeferredComparison::Type::kNE:
+                return cmp != 0;
+        }
+
+        MONGO_UNREACHABLE;
+    }
+
+    /**
+     * Returns a function object which computes whether one BSONObj is less than another under this
+     * comparator. This comparator must outlive the returned function object.
+     */
+    LessThan makeLessThan() const {
+        return LessThan(this);
+    }
+
+    /**
+     * Returns a function object which computes whether one BSONObj is equal to another under this
+     * comparator. This comparator must outlive the returned function object.
+     */
+    EqualTo makeEqualTo() const {
+        return EqualTo(this);
+    }
+
+    /**
+     * Construct an empty BSONObjSet whose ordering is given by this comparator. This comparator
+     * must outlive the returned set.
+     */
+    BSONObjSet makeOrderedBSONObjSet() const {
+        return BSONObjSet(LessThan(this));
+    }
+
+    /**
+     * Construct an empty BSONObjUnorderedSet whose equivalence classes are given by this
+     * comparator. This comparator must outlive the returned set.
+     */
+    BSONObjUnorderedSet makeUnorderedBSONObjSet() const {
+        // TODO SERVER-23990: Make the BSONObj hash collation-aware.
+        return BSONObjUnorderedSet(0, BSONObj::Hasher(), EqualTo(this));
+    }
+
+    /**
+     * Construct an empty ordered map from BSONObj to type T whose ordering is given by this
+     * comparator. This comparator must outlive the returned map.
+     */
+    template <typename T>
+    BSONObjMap<T> makeOrderedBSONObjMap() const {
+        return BSONObjMap<T>(LessThan(this));
+    }
+
+    /**
+     * Construct an empty unordered map from BSONObj to type T whose ordering is given by this
+     * comparator. This comparator must outlive the returned map.
+     */
+    template <typename T>
+    BSONObjIndexedMap<T> makeBSONObjIndexedMap() const {
+        // TODO SERVER-23990: Make the BSONObj hash collation-aware.
+        return BSONObjIndexedMap<T>(0, BSONObj::Hasher(), EqualTo(this));
+    }
+
+protected:
+    constexpr ComparatorInterface() = default;
+};
+
+}  // namespace mongo