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Make diff() account for member order in ordered_json objects

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diff() compared source/target objects purely by key set, ignoring
relative member order. For ordered_json (insertion-ordered, vector-
backed object_t), two objects that differ only in member order are
unequal via operator==, but diff() never emitted any patch operation
to fix the order, so source.patch(diff(source, target)) == target
could fail to hold.

Fix by detecting when common keys appear in a different relative
order in source vs. target (or when a new key would need to land
somewhere other than the end), and in that case removing and
re-adding the affected keys in target's order, which relies on
patch()'s "add" op appending new keys at the end of an ordered_map.
For plain json (std::map-backed, always key-sorted iteration) this
is a no-op and the original minimal per-key diff path is unchanged.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-05 22:01:40 +02:00
committed by GitHub
parent 3bb551f46f
commit d82ab21724
3 changed files with 327 additions and 30 deletions
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include/nlohmann/json.hpp
+123 -15
View File
@@ -5159,21 +5159,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
case value_t::object:
{
// first pass: traverse this object's elements
// first pass: find keys that were deleted (i.e., in source but not in target)
for (auto it = source.cbegin(); it != source.cend(); ++it)
{
// escape the key name to be used in a JSON patch
const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
if (target.find(it.key()) != target.end())
if (target.find(it.key()) == target.end())
{
// recursive call to compare object values at key it
auto temp_diff = diff(it.value(), target[it.key()], path_key);
result.insert(result.end(), temp_diff.begin(), temp_diff.end());
}
else
{
// found a key that is not in o -> remove it
// found a key that is not in target -> remove it
const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
result.push_back(object(
{
{"op", "remove"}, {"path", path_key}
@@ -5181,12 +5173,108 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
}
}
// second pass: traverse other object's elements
// determine whether the relative order of the keys common to
// source and target already matches. For an object_t whose
// iteration order is a pure function of the key set (e.g.,
// the default std::map, which always iterates in sorted key
// order), this is always true, so this check (and the "else"
// branch below) is effectively a no-op for a regular `json`
// object; it only matters for a reorderable object_t such as
// the one backing `ordered_json`.
std::vector<typename object_t::key_type> order_in_source;
for (auto it = source.cbegin(); it != source.cend(); ++it)
{
if (target.find(it.key()) != target.end())
{
order_in_source.push_back(it.key());
}
}
std::vector<typename object_t::key_type> order_in_target;
for (auto it = target.cbegin(); it != target.cend(); ++it)
{
if (source.find(it.key()) == source.end())
if (source.find(it.key()) != source.end())
{
order_in_target.push_back(it.key());
}
}
// The fast path below (like the original implementation)
// only ever recurses into common keys -- without touching
// their relative position -- and appends brand-new keys at
// the very end. That reproduces target's order only if the
// common keys already appear in target in the same relative
// order as in source, AND every brand-new key comes after
// every common key in target (i.e., the new keys form a
// suffix of target's key order); otherwise a new key would
// need to be inserted somewhere other than the end.
bool new_keys_form_suffix = true;
{
bool seen_new_key = false;
for (auto it = target.cbegin(); it != target.cend(); ++it)
{
if (source.find(it.key()) == source.end())
{
seen_new_key = true;
}
else if (seen_new_key)
{
new_keys_form_suffix = false;
break;
}
}
}
bool reordered = false;
if (order_in_source == order_in_target && new_keys_form_suffix)
{
// fast path: order of common keys already matches (or the
// object_t's iteration order does not depend on
// insertion history), so a plain per-key recursive diff
// is correct and minimal, as before
for (auto it = source.cbegin(); it != source.cend(); ++it)
{
if (target.find(it.key()) != target.end())
{
const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
auto temp_diff = diff(it.value(), target[it.key()], path_key);
result.insert(result.end(), temp_diff.begin(), temp_diff.end());
}
}
}
else
{
// slow path: the common keys are in a different relative
// order in source and target (only possible for a
// reorderable object_t like ordered_map). Building a
// minimal reordering patch is a nontrivial (LCS-like)
// problem; instead, remove every common key and re-add it
// (with its final target value) in target's order, which
// is enough to guarantee source.patch(diff(source,
// target)) == target. basic_json::patch()'s "add"
// operation on an object uses operator[], which appends
// at the end for a vector-backed insertion-ordered map
// when the key does not already exist -- so removing a
// key and then adding it moves it to the end, fixing its
// position.
reordered = true;
for (const auto& key : order_in_source)
{
const auto path_key = detail::concat<string_t>(path, '/', detail::escape(key));
result.push_back(object(
{
{"op", "remove"}, {"path", path_key}
}));
}
// add every key that is either common (just removed
// above) or brand new, in target's iteration order, so
// that the final order after applying the patch matches
// target exactly
for (auto it = target.cbegin(); it != target.cend(); ++it)
{
// found a key that is not in this -> add it
const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
result.push_back(
{
@@ -5196,6 +5284,26 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
}
}
// second pass: find keys that were added (i.e., in target but
// not in source); already handled above when reordering, to
// keep them correctly interleaved with the moved keys
if (!reordered)
{
for (auto it = target.cbegin(); it != target.cend(); ++it)
{
if (source.find(it.key()) == source.end())
{
// found a key that is not in source -> add it
const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
result.push_back(
{
{"op", "add"}, {"path", path_key},
{"value", it.value()}
});
}
}
}
break;
}