amorfo77/json
1
0 Fork 0
mirror of https://github.com/nlohmann/json.git synced 2026-10-09 16:07:14 +00:00
Code Issues Packages Projects Releases Wiki Activity

Unflatten in time and memory linear in the pointer depth

Browse Source 
#5443 made unflatten() decide between arrays and objects independently
of the iteration order by collecting the pointer prefixes that have a
reference token 0 below them in a std::set<std::vector<string_t>>.
Every such prefix was stored as a copy of all its reference tokens, and
get_and_create() compared whole prefix vectors at every step, so
unflattening a pointer of depth d took time and memory quadratic in d:
a 10,000-level array pointer took 18 s and 1.3 GB, a 100,000-level one
did not finish.

The prefixes are now numbered nodes of a tree, so each is stored once
and get_and_create() follows the tree token by token. The result is
unchanged, including its independence of the iteration order.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-10-09 09:49:05 +02:00
1 parent 3f6a037b9c
commit 49ea918969
3 files changed
+129 -44
Download Patch File Download Diff File Expand all files Collapse all files

No files matched your search

single_include/nlohmann/json.hpp
+42 -16
View File
@@ -19928,8 +19928,8 @@ NLOHMANN_JSON_NAMESPACE_END
#include <iosfwd> // ostream
#endif // JSON_NO_IO
#include <limits> // max
#include <map> // map
#include <numeric> // accumulate
#include <set> // set
#include <string> // string
#include <utility> // move
#include <vector> // vector
@@ -20277,33 +20277,45 @@ class json_pointer
private:
/*!
@brief the reference token sequences that denote arrays
@brief the pointer prefixes of a flattened object, and which of them denote arrays
@ref unflatten collects the pointer prefixes that have a reference token 0
among their children; @ref get_and_create creates arrays exactly below
those prefixes and objects everywhere else. Deciding this up front keeps
the result independent of the order in which the flattened object is
iterated, which is unspecified for some object types.
The prefixes form a tree and are numbered, so each of them is stored only
once (as a node) rather than as a copy of all of its reference tokens.
*/
using array_parents_t = std::set<std::vector<string_t>>;
struct prefix_tree
{
// children[id] maps a reference token to the number of the prefix
// extended by that token; number 0 is the empty prefix
std::vector<std::map<string_t, std::size_t>> children;
// is_array[id] is true iff some flattened key has the reference token
// 0 directly below the prefix with number id
std::vector<bool> is_array;
};
/*!
@brief create and return a reference to the pointed to value
Complexity: Linear in the number of reference tokens.
Complexity: Linear in the number of reference tokens (times the logarithm
of the number of siblings for the prefix lookup).
@throw parse_error.106 if an array index begins with '0'
@throw parse_error.109 if array index is not a number
@throw type_error.313 if value cannot be unflattened
*/
template<typename BasicJsonType>
BasicJsonType& get_and_create(BasicJsonType& j, const array_parents_t& array_parents) const
BasicJsonType& get_and_create(BasicJsonType& j, const prefix_tree& tree) const
{
auto* result = &j;
// the reference tokens that have been consumed so far; used to look up
// whether the value to be created below is an array or an object
std::vector<string_t> prefix;
// the number of the prefix consumed so far; used to look up whether
// the value to be created below is an array or an object
std::size_t id = 0;
// in case no reference tokens exist, return a reference to the JSON value
// j which will be overwritten by a primitive value
@@ -20313,7 +20325,7 @@ class json_pointer
{
case detail::value_t::null:
{
if (array_parents.find(prefix) != array_parents.end())
if (tree.is_array[id])
{
// some reference token below this position is 0, so the
// value is an array
@@ -20358,7 +20370,9 @@ class json_pointer
JSON_THROW(detail::type_error::create(313, "invalid value to unflatten", &j));
}
prefix.push_back(reference_token);
const auto it = tree.children[id].find(reference_token);
JSON_ASSERT(it != tree.children[id].end());
id = it->second;
}
return *result;
@@ -20944,19 +20958,31 @@ class json_pointer
// collect the pointer prefixes that have a reference token 0 among
// their children; the values below them are arrays, all others are
// objects (see array_parents_t)
array_parents_t array_parents;
// objects (see prefix_tree)
prefix_tree tree;
tree.children.emplace_back();
tree.is_array.push_back(false);
for (const auto& element : *value.m_data.m_value.object)
{
json_pointer ptr(element.first);
std::vector<string_t> prefix;
std::size_t id = 0;
for (auto& reference_token : ptr.reference_tokens)
{
if (reference_token == "0")
{
array_parents.insert(prefix);
tree.is_array[id] = true;
}
// do not keep a reference into tree.children across the
// push_back below, as it may reallocate
const std::size_t next = tree.children.size();
const auto inserted = tree.children[id].emplace(std::move(reference_token), next);
id = inserted.first->second;
if (inserted.second)
{
tree.children.emplace_back();
tree.is_array.push_back(false);
}
prefix.push_back(std::move(reference_token));
}
}
@@ -20972,7 +20998,7 @@ class json_pointer
// that if the JSON pointer is "" (i.e., points to the whole value),
// function get_and_create returns a reference to the result itself.
// An assignment will then create a primitive value.
json_pointer(element.first).get_and_create(result, array_parents) = element.second;
json_pointer(element.first).get_and_create(result, tree) = element.second;
}
return result;