SERVER-48834 Implement $project expressions in grammar for agg

1 files changed, 131 insertions, 0 deletions

diff --git a/src/mongo/db/cst/path.h b/src/mongo/db/cst/path.h
new file mode 100644
index 00000000000..4d8885fb3e6
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+++ b/src/mongo/db/cst/path.h
@@ -0,0 +1,131 @@
+/**
+ *    Copyright (C) 2020-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 "mongo/platform/basic.h"
+
+#include <numeric>
+#include <string>
+#include <vector>
+
+#include "mongo/stdx/variant.h"
+
+namespace mongo {
+
+/**
+ * A path occurring as a fieldname in a $project stage or a find project which indicates source/
+ * destination object fields for inclusion/exclusion and destination fields for computed projection.
+ * Of the syntactic form: "a" or "a.b.c".
+ */
+struct ProjectionPath {
+    auto operator==(const ProjectionPath& other) const {
+        return components == other.components;
+    }
+    auto operator!=(const ProjectionPath& other) const {
+        return !(*this == other);
+    }
+
+    std::vector<std::string> components;
+};
+
+/**
+ * A path occurring as a fieldname in a find project indicating predicate application to array
+ * elements. Of the syntactic form: "a.$" or "a.b.c.$".
+ */
+struct PositionalProjectionPath {
+    auto operator==(const PositionalProjectionPath& other) const {
+        return components == other.components;
+    }
+    auto operator!=(const PositionalProjectionPath& other) const {
+        return !(*this == other);
+    }
+
+    std::vector<std::string> components;
+};
+
+/**
+ * A path occurring as a value in aggregation expressions acting as a field reference. Of the
+ * syntactic form: "$a" or "$a.b.c".
+ */
+struct AggregationPath {
+    auto operator==(const AggregationPath& other) const {
+        return components == other.components;
+    }
+    auto operator!=(const AggregationPath& other) const {
+        return !(*this == other);
+    }
+
+    std::vector<std::string> components;
+};
+
+/**
+ * A path occurring as a value in aggregation expressions acting as variable access. Of the
+ * syntactic form: "$$a" or "$$a.b.c".
+ */
+struct AggregationVariablePath {
+    auto operator==(const AggregationVariablePath& other) const {
+        return components == other.components;
+    }
+    auto operator!=(const AggregationVariablePath& other) const {
+        return !(*this == other);
+    }
+
+    std::vector<std::string> components;
+};
+
+namespace path {
+
+template <typename StringType>
+inline auto vectorToString(const std::vector<StringType>& vector) {
+    return std::accumulate(
+        std::next(vector.cbegin()),
+        vector.cend(),
+        std::string{vector[0]},
+        [](auto&& pathString, auto&& element) { return pathString + "." + element; });
+}
+
+template <typename PathType>
+inline auto vectorToString(const PathType& path) {
+    return vectorToString(path.components);
+}
+
+}  // namespace path
+
+/**
+ * A path in the fieldname position in input BSON syntax. Such as "a.b" in '{"a.b": ""}'.
+ */
+using FieldnamePath = stdx::variant<ProjectionPath, PositionalProjectionPath>;
+
+/**
+ * A path in the value position in input BSON syntax. Such as "$a.b" in '{"": "$a.b"}'.
+ */
+using ValuePath = stdx::variant<AggregationPath, AggregationVariablePath>;
+
+}  // namespace mongo