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Index the large objects of json_view

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Lookups in objects are linear, as for ordered_json. Objects with 128
members or more now get a hash table after parsing (open addressing; the
first of duplicate keys is kept, as for the linear search), so that
operator[], at(), find(), contains(), count(), value(), and JSON pointers
take constant time on average in them; the idea of switching to a hash
table for large objects is Boost.JSON's. The parser notes such objects when
it closes them (out of line, so that the parse loop only has a call for
it), and the object node keeps the number of its table.

Looking up each key of an object with 10,000 members: 59.8 ms -> 0.16 ms.
Parsing (json_document::parse, best of 7, separate processes): most files
within 1%; canada +5%, mesh.pretty +3%, citm +3%.

Tests: objects with 127, 128, 129, and 10,000 members (escaped, empty,
and duplicate keys, missing keys, comparisons), nested large objects, and
documents reused with read().

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-29 14:03:07 +02:00
parent 57b5d56522
commit 61b104606d
14 changed files with 421 additions and 6 deletions
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include/nlohmann/json_view.hpp
+9 -1
View File
@@ -25,6 +25,7 @@
#define INCLUDE_NLOHMANN_JSON_VIEW_HPP_
#include <cstddef> // size_t
#include <cstdint> // uint32_t
#include <cstring> // memcpy, strlen
#include <iterator> // distance, input_iterator_tag, iterator_traits
#include <map> // map
@@ -56,6 +57,7 @@
#include <nlohmann/detail/view/macro_scope.hpp>
#include <nlohmann/detail/view/materialize.hpp>
#include <nlohmann/detail/view/node.hpp>
#include <nlohmann/detail/view/object_index.hpp>
#include <nlohmann/detail/view/pointer.hpp>
#include <nlohmann/detail/view/serializer.hpp>
#include <nlohmann/detail/view/string_ref.hpp>
@@ -925,7 +927,9 @@ class basic_json_document
}
return sizeof(document_data) + (m_data->inline_cap * sizeof(detail::view::node))
+ (m_data->tape != m_data->inline_tape ? m_data->tape_cap * sizeof(detail::view::node) : 0)
+ m_data->arena.capacity() + m_data->owned.capacity();
+ m_data->arena.capacity() + m_data->owned.capacity()
+ (m_data->indexes.capacity() * sizeof(document_data::object_index)) + (m_data->index_slots.capacity() * sizeof(std::uint32_t))
+ (m_data->large_objects.capacity() * sizeof(std::uint32_t));
}
/// release unused capacity of the index and the decoded strings; like
@@ -995,6 +999,9 @@ class basic_json_document
d.size = size;
d.tape_size = 0;
d.arena.clear();
d.indexes.clear();
d.index_slots.clear();
d.large_objects.clear();
d.discarded = true;
detail::view::parse_failure failure;
bool ok = false;
@@ -1010,6 +1017,7 @@ class basic_json_document
{
d.base[0] = d.src;
d.base[1] = d.arena.data();
detail::view::build_object_indexes(d);
d.discarded = false;
return;
}