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Make diff() linear when an array shrinks (#5461)

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Signed-off-by: avionicharshit-byte <harshitavionic@gmail.com>
This commit is contained in:
Avionic Harshit
2026-09-05 09:52:53 +02:00
committed by GitHub
parent 137a40b9aa
commit 3c875683a5
3 changed files with 93 additions and 16 deletions
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include/nlohmann/json.hpp
+6 -8
View File
@@ -5128,19 +5128,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// We now reached the end of at least one array
// in a second pass, traverse the remaining elements
// remove my remaining elements
const auto end_index = static_cast<difference_type>(result.size());
while (i < source.size())
// remove my remaining elements, highest index first; appending
// in that order avoids the quadratic reinsertion done before
for (std::size_t j = source.size(); j > i; --j)
{
// add operations in reverse order to avoid invalid
// indices
result.insert(result.begin() + end_index, object(
result.push_back(object(
{
{"op", "remove"},
{"path", detail::concat<string_t>(path, '/', detail::to_string<string_t>(i))}
{"path", detail::concat<string_t>(path, '/', detail::to_string<string_t>(j - 1))}
}));
++i;
}
i = source.size();
// add other remaining elements
while (i < target.size())
single_include/nlohmann/json.hpp
+6 -8
View File
@@ -26547,19 +26547,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// We now reached the end of at least one array
// in a second pass, traverse the remaining elements
// remove my remaining elements
const auto end_index = static_cast<difference_type>(result.size());
while (i < source.size())
// remove my remaining elements, highest index first; appending
// in that order avoids the quadratic reinsertion done before
for (std::size_t j = source.size(); j > i; --j)
{
// add operations in reverse order to avoid invalid
// indices
result.insert(result.begin() + end_index, object(
result.push_back(object(
{
{"op", "remove"},
{"path", detail::concat<string_t>(path, '/', detail::to_string<string_t>(i))}
{"path", detail::concat<string_t>(path, '/', detail::to_string<string_t>(j - 1))}
}));
++i;
}
i = source.size();
// add other remaining elements
while (i < target.size())
tests/src/unit-json_patch.cpp
+81
View File
@@ -1388,3 +1388,84 @@ TEST_CASE("JSON patch - add to a primitive parent (regression #4292)")
CHECK_THROWS_AS(doc.patch(patch), json::out_of_range&);
}
}
TEST_CASE("JSON patch - diff emits array removals in descending index order")
{
SECTION("array shrunk to empty")
{
json const source = {0, 1, 2, 3, 4};
json const target = json::array();
json const patch = json::diff(source, target);
json const expected = R"(
[
{"op": "remove", "path": "/4"},
{"op": "remove", "path": "/3"},
{"op": "remove", "path": "/2"},
{"op": "remove", "path": "/1"},
{"op": "remove", "path": "/0"}
]
)"_json;
CHECK(patch == expected);
CHECK(source.patch(patch) == target);
}
SECTION("array partially shrunk, after a replacement at a common index")
{
json const source = {0, 1, 2, 3, 4};
json const target = {0, 9};
json const patch = json::diff(source, target);
// the replacement comes first, then the removals, highest index first
json const expected = R"(
[
{"op": "replace", "path": "/1", "value": 9},
{"op": "remove", "path": "/4"},
{"op": "remove", "path": "/3"},
{"op": "remove", "path": "/2"}
]
)"_json;
CHECK(patch == expected);
CHECK(source.patch(patch) == target);
}
SECTION("nested array shrunk")
{
json const source = {{"a", {0, 1, 2}}};
json const target = {{"a", json::array()}};
json const patch = json::diff(source, target);
json const expected = R"(
[
{"op": "remove", "path": "/a/2"},
{"op": "remove", "path": "/a/1"},
{"op": "remove", "path": "/a/0"}
]
)"_json;
CHECK(patch == expected);
CHECK(source.patch(patch) == target);
}
SECTION("many removals still round-trip")
{
json source = json::array();
for (int i = 0; i < 1000; ++i)
{
source.push_back(i);
}
json const target = json::array();
json const patch = json::diff(source, target);
CHECK(patch.size() == 1000);
CHECK(patch.front().at("path") == "/999");
CHECK(patch.back().at("path") == "/0");
CHECK(source.patch(patch) == target);
}
}