2 files changed, 134 insertions, 0 deletions
diff --git a/src/buildtool/common/TARGETS b/src/buildtool/common/TARGETS
index 4c75627b..19bad797 100644
--- a/src/buildtool/common/TARGETS
+++ b/src/buildtool/common/TARGETS
@@ -158,6 +158,20 @@
     ]
   , "stage": ["src", "buildtool", "common"]
   }
+, "tree_overlay":
+  { "type": ["@", "rules", "CC", "library"]
+  , "name": ["tree_overlay"]
+  , "hdrs": ["tree_overlay.hpp"]
+  , "deps":
+    [ "action_description"
+    , "artifact_description"
+    , "common"
+    , ["@", "fmt", "", "fmt"]
+    , ["@", "json", "", "json"]
+    , ["src/buildtool/crypto", "hash_function"]
+    ]
+  , "stage": ["src", "buildtool", "common"]
+  }
 , "config":
   { "type": ["@", "rules", "CC", "library"]
   , "name": ["config"]
diff --git a/src/buildtool/common/tree_overlay.hpp b/src/buildtool/common/tree_overlay.hpp
new file mode 100644
index 00000000..1b7faaed
--- /dev/null
+++ b/src/buildtool/common/tree_overlay.hpp
@@ -0,0 +1,120 @@
+// Copyright 2025 Huawei Cloud Computing Technology Co., Ltd.
+//
+// Licensed 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.
+
+#ifndef INCLUDED_SRC_BUILDTOOL_COMMON_TREE_OVERLAY_HPP
+#define INCLUDED_SRC_BUILDTOOL_COMMON_TREE_OVERLAY_HPP
+
+#include <cstddef>
+#include <functional>
+#include <memory>
+#include <optional>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "fmt/core.h"
+#include "nlohmann/json.hpp"
+#include "src/buildtool/common/action.hpp"
+#include "src/buildtool/common/action_description.hpp"
+#include "src/buildtool/common/artifact_description.hpp"
+#include "src/buildtool/crypto/hash_function.hpp"
+
+class TreeOverlay {
+
+  public:
+    using to_overlay_t = std::vector<ArtifactDescription>;
+    using Ptr = std::shared_ptr<TreeOverlay>;
+    explicit TreeOverlay(to_overlay_t const& trees, bool disjoint)
+        : id_{ComputeId(trees, disjoint)}, trees_{trees}, disjoint_{disjoint} {}
+
+    [[nodiscard]] auto Id() const& -> std::string const& { return id_; }
+    [[nodiscard]] auto Id() && -> std::string { return std::move(id_); }
+
+    [[nodiscard]] auto ToJson() const -> nlohmann::json {
+        return ComputeDescription(trees_, disjoint_);
+    }
+
+    [[nodiscard]] auto Inputs() const -> ActionDescription::inputs_t {
+        return AsInputs(trees_);
+    }
+
+    [[nodiscard]] auto Action() const -> ActionDescription {
+        return {{/*unused*/},
+                {/*unused*/},
+                Action::CreateTreeOverlayAction(id_, disjoint_),
+                AsInputs(trees_)};
+    }
+
+    [[nodiscard]] auto Output() const -> ArtifactDescription {
+        return ArtifactDescription::CreateTreeOverlay(id_);
+    }
+
+    [[nodiscard]] static auto FromJson(HashFunction::Type hash_type,
+                                       std::string const& id,
+                                       nlohmann::json const& json)
+        -> std::optional<TreeOverlay::Ptr> {
+        bool disjoint = json["disjoint"];
+        auto trees = to_overlay_t{};
+        for (auto const& entry : json["trees"]) {
+            auto artifact_desc =
+                ArtifactDescription::FromJson(hash_type, entry);
+            if (not artifact_desc) {
+                return std::nullopt;
+            }
+            trees.emplace_back(std::move(*artifact_desc));
+        }
+        return std::make_shared<TreeOverlay>(id, std::move(trees), disjoint);
+    }
+
+    TreeOverlay(std::string id, to_overlay_t&& trees, bool disjoint)
+        : id_{std::move(id)}, trees_{std::move(trees)}, disjoint_{disjoint} {}
+
+  private:
+    std::string id_;
+    to_overlay_t trees_;
+    bool disjoint_;
+
+    static auto ComputeDescription(to_overlay_t const& trees,
+                                   bool disjoint) -> nlohmann::json {
+        auto json = nlohmann::json::object();
+        auto tree_descs = nlohmann::json::array();
+        for (auto const& tree : trees) {
+            tree_descs.emplace_back(tree.ToJson());
+        }
+        json["trees"] = tree_descs;
+        json["disjoint"] = disjoint;
+        return json;
+    }
+
+    static auto ComputeId(to_overlay_t const& trees,
+                          bool disjoint) -> std::string {
+        // The type of HashFunction is irrelevant here. It is used for
+        // identification of trees. SHA256 is used.
+        HashFunction const hash_function{HashFunction::Type::PlainSHA256};
+        return hash_function
+            .PlainHashData(ComputeDescription(trees, disjoint).dump())
+            .HexString();
+    }
+
+    static auto AsInputs(to_overlay_t const& trees)
+        -> ActionDescription::inputs_t {
+        ActionDescription::inputs_t as_inputs{};
+        for (size_t i = 0; i < trees.size(); i++) {
+            as_inputs.emplace(fmt::format("{:010d}", i), trees[i]);
+        }
+        return as_inputs;
+    }
+};
+
+#endif