From 5e68fa489fc1df2d3207ce77b07add0a8bf0dbe3 Mon Sep 17 00:00:00 2001
From: PragmaTwice <twice@apache.org>
Date: Tue, 3 Dec 2024 16:44:00 +0800
Subject: [PATCH] feat: add merge_mode for field merging

---
 include/protopuf/field.h   | 26 +++++++++++++++++++++++---
 include/protopuf/message.h |  8 ++++----
 test/message.cpp           |  7 +++++++
 3 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/include/protopuf/field.h b/include/protopuf/field.h
index 26f309f..94249e6 100644
--- a/include/protopuf/field.h
+++ b/include/protopuf/field.h
@@ -290,12 +290,32 @@ namespace pp {
         }
     }
 
+    struct merge_mode {
+        enum class singular {
+            override,
+            assign_if_empty,
+        } singular_mode;
+
+        enum class repeated {
+            push_back,
+        } repeated_mode;
+
+        constexpr merge_mode(singular s = singular::override, repeated r = repeated::push_back)
+          : singular_mode(s), repeated_mode(r) {}
+    };
+
     /// Merge a field into another field: overwrite if it is singular and non-empty, merge to end otherwise
-    template <field_c D, typename S> requires field_c<std::remove_cvref_t<S>>
+    template <merge_mode mode = merge_mode{}, field_c D, typename S> requires field_c<std::remove_cvref_t<S>>
     constexpr void merge_field(D& f, S&& v) {
         if constexpr (D::attr == singular) {
-            if (!empty_field(v)) {
-                f = std::forward<S>(v);
+            if constexpr (mode.singular_mode == merge_mode::singular::override) {
+                if (!empty_field(v)) {
+                    f = std::forward<S>(v);
+                }
+            } else {
+                if (!empty_field(v) && empty_field(f)) {
+                    f = std::forward<S>(v);
+                }
             }
         } else {
             auto inserter = std::inserter(f, f.end());
diff --git a/include/protopuf/message.h b/include/protopuf/message.h
index 1e7e542..6906cca 100644
--- a/include/protopuf/message.h
+++ b/include/protopuf/message.h
@@ -226,9 +226,9 @@ namespace pp {
         /// @brief Merge another message into this message, for all fields: overwrite if it is singular and non-empty, merge to end otherwise
         ///
         /// same as `merge_field(field1, other.field1), ..., merge_field(fieldN, other.fieldN)`, ref to @ref merge_field
-        template <typename M> requires std::same_as<std::remove_cvref_t<M>, message>
+        template <merge_mode mode = merge_mode{}, typename M> requires std::same_as<std::remove_cvref_t<M>, message>
         constexpr void merge(M&& other) {
-            (merge_field(static_cast<T&>(*this), static_cast<type_forward<T, M&&>>(other)), ...);
+            (merge_field<mode>(static_cast<T&>(*this), static_cast<type_forward<T, M&&>>(other)), ...);
         }
     };
 
@@ -244,13 +244,13 @@ namespace pp {
     concept message_c = is_message<T>;
 
     /// Merge multiple messages into one message, and return the merged message
-    template <typename M1, typename... MN> 
+    template <merge_mode mode = merge_mode{}, typename M1, typename... MN>
     requires message_c<std::remove_cvref_t<M1>> && 
              are_same<std::remove_cvref_t<M1>, std::remove_cvref_t<MN>...>
     constexpr auto merge(M1&& msg1, MN&& ... msgN) {
         std::remove_cvref_t<M1> res = std::forward<M1>(msg1);
 
-        (res.merge(std::forward<MN>(msgN)), ...);
+        (res.template merge<mode>(std::forward<MN>(msgN)), ...);
 
         return res;
     }
diff --git a/test/message.cpp b/test/message.cpp
index 68a2406..31ad2fb 100644
--- a/test/message.cpp
+++ b/test/message.cpp
@@ -474,6 +474,13 @@ GTEST_TEST(message, merge) {
     EXPECT_EQ(merged["name"_f], "e");
     EXPECT_EQ(merged["titles"_f], (vector<string>{"b", "c", "d", "f"}));
     EXPECT_EQ(merged["age"_f], 124);
+
+    constexpr merge_mode mode{merge_mode::singular::assign_if_empty};
+    merged = merge<mode>(merged, Person{"a", {}, {}});
+    EXPECT_EQ(merged["name"_f], "e");
+
+    merged = merge<mode>(Person{{}, {}, {}}, Person{"a", {}, {}});
+    EXPECT_EQ(merged["name"_f], "a");
 }
 
 GTEST_TEST(message_coder, encode_with_insufficient_buffer_size) {