Guarantee set ordering in to_json (#1784)

Fixes: #1783
Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

src/core/json/include/sourcemeta/core/json_auto.h

@@ -3,6 +3,7 @@
 
 #include <sourcemeta/core/json_value.h>
 
+#include <algorithm>  // std::sort
 #include <concepts>   // std::same_as, std::constructible_from
 #include <functional> // std::function
 #include <optional>   // std::optional
@@ -75,6 +76,16 @@ concept json_auto_map_like =
     !json_auto_has_method<T> &&
     std::is_same_v<typename T::key_type, JSON::String>;
 
+/// @ingroup json
+template <typename, typename = void>
+struct json_auto_has_reverse_iterator : std::false_type {};
+
+/// @ingroup json
+template <typename T>
+struct json_auto_has_reverse_iterator<T,
+                                      std::void_t<typename T::reverse_iterator>>
+    : std::true_type {};
+
 /// @ingroup json
 template <typename T>
 concept json_auto_tuple_mono = requires {
@@ -151,6 +162,11 @@ auto to_json(typename T::const_iterator begin, typename T::const_iterator end)
     result.push_back(to_json(*iterator));
   }
 
+  // To guarantee ordering across implementations
+  if constexpr (!json_auto_has_reverse_iterator<T>::value) {
+    std::sort(result.as_array().begin(), result.as_array().end());
+  }
+
   return result;
 }
 
@@ -166,6 +182,11 @@ auto to_json(
     result.push_back(callback(*iterator));
   }
 
+  // To guarantee ordering across implementations
+  if constexpr (!json_auto_has_reverse_iterator<T>::value) {
+    std::sort(result.as_array().begin(), result.as_array().end());
+  }
+
   return result;
 }
 

test/json/json_to_json_test.cc

@@ -2,8 +2,10 @@
 
 #include <map>
 #include <optional>
+#include <set>
 #include <string>
 #include <tuple>
+#include <unordered_set>
 #include <utility>
 #include <vector>
 
@@ -176,6 +178,28 @@ TEST(JSON_to_json, map_without_iterators_and_transform) {
   EXPECT_EQ(result, expected);
 }
 
+TEST(JSON_to_json, set_without_iterators) {
+  const std::set<std::string> value{"foo", "bar", "baz"};
+  const auto result{sourcemeta::core::to_json(value)};
+
+  const auto expected{sourcemeta::core::parse_json(R"JSON([
+    "bar", "baz", "foo"
+  ])JSON")};
+
+  EXPECT_EQ(result, expected);
+}
+
+TEST(JSON_to_json, unordered_set_without_iterators) {
+  const std::unordered_set<std::string> value{"foo", "bar", "baz"};
+  const auto result{sourcemeta::core::to_json(value)};
+
+  const auto expected{sourcemeta::core::parse_json(R"JSON([
+    "bar", "baz", "foo"
+  ])JSON")};
+
+  EXPECT_EQ(result, expected);
+}
+
 TEST(JSON_to_json, pair) {
   const std::pair<std::string, std::size_t> value{"foo", 1};
   const auto result{sourcemeta::core::to_json(value)};