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Size the dump buffer of a small value by its own extent

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The source extent of a value was read from the next node, falling back to the rest of the document when that node held a decoded string. dump() of a small value could thus allocate a buffer as large as the document. Skip a few such nodes, cap the fallback estimate, and shrink a buffer that is much larger than its output.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Niels Lohmann committed 2026-10-09 16:11:14 +02:00
1 parent 8860bf6f3a
commit 2ba331e4d0
3 files changed
+62 -8
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include/nlohmann/json_view.hpp
+21 -7
View File
@@ -24,6 +24,7 @@
#ifndef INCLUDE_NLOHMANN_JSON_VIEW_HPP_
#define INCLUDE_NLOHMANN_JSON_VIEW_HPP_
#include <algorithm> // min
#include <cstddef> // size_t
#include <cstring> // memcpy, strlen
#include <iterator> // distance, input_iterator_tag, iterator_traits
@@ -682,20 +683,33 @@ class basic_json_view
}
/// the number of source bytes of this value (estimated for values with
/// decoded strings)
/// decoded strings); the estimate sizes the output buffer of dump()
std::size_t source_extent() const noexcept
{
const node* const next = document_data::after(m_node);
const bool in_source = (m_node->flags & detail::view::node_flags::storage) == 0;
if (!in_source)
const node* const end = m_doc->tape + m_doc->tape_size;
if ((m_node->flags & detail::view::node_flags::storage) != 0)
{
return m_node->len;
}
if (next != m_doc->tape + m_doc->tape_size && (next->flags & detail::view::node_flags::storage) == 0 && next->off >= m_node->off)
// the value ends where the next node in the source begins; nodes
// with decoded strings (their offset is in the arena) are skipped,
// but only a few of them, to keep the walk short
const node* next = document_data::after(m_node);
for (int skipped = 0; next != end && skipped < 16; ++skipped, ++next)
{
return next->off - m_node->off;
if ((next->flags & detail::view::node_flags::storage) == 0)
{
return next->off >= m_node->off ? next->off - m_node->off : 0;
}
}
return m_doc->size - m_node->off;
if (next == end)
{
return m_doc->size - m_node->off;
}
// the end is unknown: assume a few bytes per node, the output buffer
// grows should the value be larger
const auto nodes = static_cast<std::size_t>(document_data::after(m_node) - m_node);
return (std::min)(m_doc->size - m_node->off, static_cast<std::size_t>(1024) + nodes * 16);
}
/// the value of the first member with this key, or a discarded view