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Speed up JSON pointer parsing, formatting, comparison and lookup (#5836)

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* Speed up JSON pointer parsing, formatting, comparison and lookup

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Restore unflatten() speed after the linear-time rewrite

The prefix tree introduced for linear unflatten() kept a std::map per prefix,
copied each token into it and looked every token up a second time while
creating the values. Replace the per-prefix maps by one open-addressing table
keyed by (parent, token) that refers to the parsed tokens, skip the lookups for
the prefix shared with the previous key (flattened objects are mostly sorted),
and derive the prefix numbers of the second pass from parent links instead of
lookups.

Time and memory stay linear in the pointer depth, and the result is still
independent of the iteration order of the flattened object.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Put [[noreturn]] before __attribute__ on the JSON pointer throw helpers

Clang 18 on Linux rejects a C++11 attribute list after a GNU
__attribute__ ("an attribute list cannot appear here").

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann authored and GitHub committed 2026-10-15 20:01:20 +02:00
1 parent 0c4e7c103c
commit 0a4b3a84ee
6 files changed
+939 -273
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include/nlohmann/json.hpp
+20 -26
View File
@@ -6562,24 +6562,18 @@ public:
};
// wrapper for "add" operation; add value at ptr
const auto operation_add = [&result](json_pointer & ptr, const basic_json & val)
const auto operation_add = [&result](json_pointer & ptr, basic_json val)
{
// adding to the root of the target document means replacing it
if (ptr.empty())
{
result = val;
result = std::move(val);
return;
}
// make sure the top element of the pointer exists
json_pointer const top_pointer = ptr.top();
if (top_pointer != ptr)
{
result.at(top_pointer);
}
// get reference to the parent of the JSON pointer ptr
const auto last_path = ptr.back();
// get reference to the parent of the JSON pointer ptr; a missing
// parent (or one of its ancestors) is reported by at()
auto last_path = std::move(ptr.reference_tokens.back());
ptr.pop_back();
// parent must exist when performing patch add per RFC6902 specs
basic_json& parent = result.at(ptr);
@@ -6590,7 +6584,7 @@ public:
case value_t::object:
{
// use operator[] to add value
parent[last_path] = val;
parent[std::move(last_path)] = std::move(val);
break;
}
@@ -6599,7 +6593,7 @@ public:
if (last_path == "-")
{
// special case: append to back
parent.push_back(val);
parent.push_back(std::move(val));
}
else
{
@@ -6611,7 +6605,7 @@ public:
}
// default case: insert add offset
parent.insert(parent.begin() + static_cast<difference_type>(idx), val);
parent.insert(parent.begin() + static_cast<difference_type>(idx), std::move(val));
}
break;
}
@@ -6634,7 +6628,8 @@ public:
const auto operation_remove = [this, & result](json_pointer & ptr)
{
// get reference to the parent of the JSON pointer ptr
const auto last_path = ptr.back();
static_cast<void>(ptr.back()); // throws out_of_range.405 if ptr is the root
const auto last_path = std::move(ptr.reference_tokens.back());
ptr.pop_back();
basic_json& parent = result.at(ptr);
@@ -6734,8 +6729,8 @@ public:
}
// collect mandatory members
const auto op = get_value("op", "op", true).template get<string_t>();
const auto path = get_value(op, "path", true).template get<string_t>();
const string_t& op = *get_value("op", "op", true).m_data.m_value.string;
const string_t& path = *get_value(op, "path", true).m_data.m_value.string;
json_pointer ptr(path);
switch (get_op(op))
@@ -6761,7 +6756,7 @@ public:
case patch_operations::move:
{
const auto from_path = get_value("move", "from", true).template get<string_t>();
const string_t& from_path = *get_value("move", "from", true).m_data.m_value.string;
json_pointer from_ptr(from_path);
if (JSON_HEDLEY_UNLIKELY(is_proper_prefix(from_ptr, ptr)))
@@ -6770,29 +6765,28 @@ public:
}
// the "from" location must exist - use at()
basic_json const v = result.at(from_ptr);
// a root "from" is rejected by operation_remove below, which must leave result intact
basic_json v = from_ptr.empty() ? basic_json(result) : std::move(result.at(from_ptr));
// The move operation is functionally identical to a
// "remove" operation on the "from" location, followed
// immediately by an "add" operation at the target
// location with the value that was just removed.
operation_remove(from_ptr);
operation_add(ptr, v);
operation_add(ptr, std::move(v));
break;
}
case patch_operations::copy:
{
const auto from_path = get_value("copy", "from", true).template get<string_t>();
const string_t& from_path = *get_value("copy", "from", true).m_data.m_value.string;
const json_pointer from_ptr(from_path);
// the "from" location must exist - use at()
basic_json const v = result.at(from_ptr);
// The copy is functionally identical to an "add"
// operation at the target location using the value
// specified in the "from" member.
operation_add(ptr, v);
// specified in the "from" member; the "from" location
// must exist - use at()
operation_add(ptr, result.at(from_ptr));
break;
}