Add json_document and json_view: node index, parser, and document

Add json_document and json_view, a read-only, zero-copy index of a
JSON text, as the first public slice of the zero-copy view (#5295).

A parse produces a flat array of 16-byte nodes in document order,
one per value and one per object key. Strings stay in the source
text; escaped strings are decoded into an arena. Integers are
converted while their digits are in the cache; floats keep only
their digit layout and are converted on read. Containers store the
size of their subtree, so a reader can step over one in constant
time. A document makes a handful of allocations, however many
values it has.

The parser accepts exactly what json::parse accepts, with every
combination of ignore_comments and ignore_trailing_commas, with and
without a trailing NUL, and under JSON_STRICT_NUL_HANDLING. It is
portable C++11 and does not depend on byte order.

basic_json_document adds parse, parse_copy, accept, read (reuses a
document's memory), root, is_discarded, source, owns_source,
node_count, memory_usage, and shrink_to_fit. basic_json_view adds
type, the is_* queries, operator bool, size, empty, materialize,
and source_offset. A parse error throws the same exception
basic_json::parse would throw for the same input, message and
position included.

detail::abi_config keeps JSON_STRICT_NUL_HANDLING and
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON readable after json.hpp
undefines them, in the ABI namespace so they always match the
basic_json in use.

A NUL byte that ends a // comment is the end of the input, as in
parse() since #5696.

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

include/nlohmann/detail/view/document_data.hpp

@@ -0,0 +1,130 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <array> // array
+#include <cstddef> // size_t
+#include <cstring> // memcpy
+#include <new> // operator new, placement new
+#include <string> // string
+
+#include <nlohmann/json.hpp>
+#include <nlohmann/detail/view/macro_scope.hpp>
+#include <nlohmann/detail/view/node.hpp>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+namespace view
+{
+
+/// storage of a parsed document; heap-allocated (header and an initial node
+/// array in one block) so that views survive moves of the owning document
+struct document_data
+{
+    const char* src = nullptr;
+    std::size_t size = 0;
+    node* tape = nullptr;
+    std::size_t tape_size = 0;
+    std::size_t tape_cap = 0;
+    node* inline_tape = nullptr; ///< node array allocated together with this header
+    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)
+    std::array<const char*, 4> base = {{nullptr, nullptr, nullptr, nullptr}}; ///< string bases: source, arena (indexed by flags & node_flags::storage)
+    bool discarded = true;
+
+    /// one allocation for the header and room for `nodes` nodes; large
+    /// documents get a separate node array instead (so it can be trimmed)
+    static document_data* create(std::size_t nodes)
+    {
+        nodes = nodes <= 256 ? nodes : 0;
+        void* mem = ::operator new (sizeof(document_data) + (nodes * sizeof(node)));
+        auto* d = new (mem) document_data(); // NOLINT(cppcoreguidelines-owning-memory): owned by the returned pointer, freed by deleter
+        // (aligned: sizeof is a multiple of the alignment; through void*, as GCC's -Wcast-align wants)
+        d->inline_tape = static_cast<node*>(static_cast<void*>(static_cast<char*>(mem) + sizeof(document_data))); // NOLINT(bugprone-casting-through-void)
+        d->inline_cap = nodes;
+        d->tape = d->inline_tape;
+        d->tape_cap = nodes;
+        return d;
+    }
+
+    struct deleter
+    {
+        void operator()(document_data* d) const noexcept
+        {
+            d->~document_data();
+            ::operator delete (d);
+        }
+    };
+
+    document_data() noexcept = default;
+    document_data(const document_data&) = delete;
+    document_data(document_data&&) = delete;
+    document_data& operator=(const document_data&) = delete;
+    document_data& operator=(document_data&&) = delete;
+    ~document_data()
+    {
+        release();
+    }
+
+    void release() noexcept
+    {
+        if (tape != inline_tape)
+        {
+            ::operator delete (tape);
+        }
+        tape = inline_tape;
+        tape_cap = inline_cap;
+    }
+
+    /// make room for n nodes; keeps the first tape_size nodes
+    void reserve(std::size_t n)
+    {
+        if (n <= tape_cap)
+        {
+            return;
+        }
+        node* fresh = static_cast<node*>(::operator new (n * sizeof(node)));
+        if (tape_size != 0)
+        {
+            std::memcpy(fresh, tape, tape_size * sizeof(node));
+        }
+        release();
+        tape = fresh;
+        tape_cap = n;
+    }
+
+    const char* str(const node& n) const noexcept
+    {
+        return base[n.flags & node_flags::storage] + n.off;
+    }
+
+    /// the node after n's subtree (containers span `next` nodes, scalars one)
+    static NLOHMANN_VIEW_ALWAYS_INLINE const node* after(const node* n) noexcept
+    {
+        return n + (is_container(*n) ? n->next : 1u);
+    }
+
+    /// first element (array) or first key (object) of a container
+    static NLOHMANN_VIEW_ALWAYS_INLINE const node* first_child(const node* n) noexcept
+    {
+        return n + 1;
+    }
+
+    /// end of the elements of a container
+    static NLOHMANN_VIEW_ALWAYS_INLINE const node* child_end(const node* n) noexcept
+    {
+        return n + n->next;
+    }
+};
+
+}  // namespace view
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END