Index the large objects of json_view

[nlohmann]

Part of the stack for the zero-copy view (#5295).

Summary

Lookups in objects are linear, as for ordered_json. Objects with 128 members or more now get a hash table after parsing:

• open addressing;
• of duplicate keys, the first is kept, as for the linear search;
• operator[], at(), find(), contains(), count(), value(), and JSON pointers take constant time on average in such objects.

The idea of switching to a hash table for large objects comes from Boost.JSON.

The parser notes a large object when it closes it. This happens out of line, so the parse loop only has a call for it; done inline, it made canada.json 37% slower. The object node keeps the number of its table.

The node index section of the architecture page describes the tables and what extra holds for objects.

Benchmarks

• Looking up every 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, duplicate, and missing keys, and comparisons.
• Nested large objects, and documents reused with read().

Public API

No change (faster lookups).

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Written by Claude Code.

🤖 Generated with Claude Code

[nlohmann]

CI fixes for this PR (commits fa58c6e and 448c9dd, plus the lower fixes merged in). CI had not run yet, so these were found locally with the CI's compilers:

• ci_test_gcc: -Werror=useless-cast on static_cast<std::size_t>(key_hash(...)) (twice) in detail/view/object_index.hpp. The call returns a std::uint64_t prvalue, which is std::size_t on Linux x86-64. The hash is now stored in a variable and cast from there.
• old clang (ci_test_compilers_clang 3.x): with the new nested struct object_index, clang 4 rejects document_data() noexcept = default; with "default member initializer for 'indexes' needed within definition of enclosing class". Nothing depends on that constructor being noexcept, so it now takes the implicit exception specification (document_data() = default;). Reproduced with silkeh/clang:4. The same error reappeared for every upper branch before this fix.

Verified with gcc:16 (linux/amd64, CI flags, C++11/C++20, plus a JSON_NOEXCEPTION build run as --no-throw) and with silkeh/clang:4 (syntax check of all view tests, up to #5635).

— posted by Claude Code on behalf of @nlohmann

[nlohmann]

The zero-copy view stack (#5295) was rebased onto develop and regrouped from 20 PRs into 10, one squashed commit each. The changes of this PR are now in #5628; its review comments stay here for reference.

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Written by Claude Code.