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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/detail/view/document_data.hpp
+13
View File
@@ -10,9 +10,11 @@
#include <array> // array
#include <cstddef> // size_t
#include <cstdint> // uint32_t
#include <cstring> // memcpy
#include <new> // operator new, placement new
#include <string> // string
#include <vector> // vector
#include <nlohmann/json.hpp>
#include <nlohmann/detail/view/macro_scope.hpp>
@@ -37,6 +39,17 @@ struct document_data
std::size_t inline_cap = 0;
std::string arena{}; ///< decoded strings that contained escapes // NOLINT(readability-redundant-member-init)
std::string owned{}; ///< owned copy of the input, if any // NOLINT(readability-redundant-member-init)
// hash indexes of large objects (see object_index.hpp)
static constexpr std::uint32_t index_min_members = 128;
struct object_index
{
std::size_t start; ///< first slot in index_slots
std::uint32_t mask; ///< slot count - 1 (a power of two minus one)
};
std::vector<object_index> indexes{}; // NOLINT(readability-redundant-member-init)
std::vector<std::uint32_t> index_slots{}; // NOLINT(readability-redundant-member-init)
std::vector<std::uint32_t> large_objects{}; ///< positions of the objects to index (noted while parsing) // NOLINT(readability-redundant-member-init)
std::array<const char*, 4> base = {{nullptr, nullptr, nullptr, nullptr}}; ///< string bases: source, arena (indexed by flags & node_flags::storage)
bool discarded = true;