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Replace snprintf with a branch-free writer for \uXXXX escapes

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dump_escaped called std::snprintf(..., "\u%04x", ...) once per escaped
code point in the string serialization hot path. snprintf re-parses
the format string and pulls in locale/printf machinery on every call,
which is far heavier than the fixed 6-/12-byte output warrants. This
is hot for any string containing control characters, and for all
non-ASCII text when ensure_ascii is set.

Replace it with write_u_escape, a small helper that writes the escape
directly into string_buffer via a nibble-to-hex lookup table, mirroring
the existing hand-rolled dump_integer fast path in the same file.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-07-04 22:42:55 +02:00
parent c034480c22
commit 4cad4e08a4
3 changed files with 86 additions and 18 deletions
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single_include/nlohmann/json.hpp
+30 -9
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@@ -19752,18 +19752,12 @@ class serializer
{
if (codepoint <= 0xFFFF)
{
// NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
static_cast<void>((std::snprintf)(string_buffer.data() + bytes, 7, "\\u%04x",
static_cast<std::uint16_t>(codepoint)));
bytes += 6;
bytes = write_u_escape(bytes, static_cast<std::uint16_t>(codepoint));
}
else
{
// NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
static_cast<void>((std::snprintf)(string_buffer.data() + bytes, 13, "\\u%04x\\u%04x",
static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)),
static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu))));
bytes += 12;
bytes = write_u_escape(bytes, static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)));
bytes = write_u_escape(bytes, static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu)));
}
}
else
@@ -19970,6 +19964,33 @@ class serializer
return result;
}
/*!
* @brief write a lowercase "\uXXXX" escape sequence into @a string_buffer
*
* Branch-free replacement for `snprintf(buf, 7, "\\u%04x", codeunit)` in the
* string escaping hot path. It writes exactly six characters ('\\', 'u' and
* four hex digits) at position @a pos of @a string_buffer via a nibble
* lookup table, avoiding the format-string parsing and locale machinery of
* `snprintf`.
*
* @param[in] pos position in @a string_buffer to write at; there must
* be at least 6 bytes of headroom
* @param[in] codeunit 16-bit value to encode
* @return position just after the written escape sequence (@a pos + 6)
*/
std::size_t write_u_escape(std::size_t pos, std::uint16_t codeunit) noexcept
{
JSON_ASSERT(string_buffer.size() - pos >= 6);
static constexpr const char nibble_to_hex[] = "0123456789abcdef";
string_buffer[pos + 0] = '\\';
string_buffer[pos + 1] = 'u';
string_buffer[pos + 2] = nibble_to_hex[(codeunit >> 12u) & 0x0Fu];
string_buffer[pos + 3] = nibble_to_hex[(codeunit >> 8u) & 0x0Fu];
string_buffer[pos + 4] = nibble_to_hex[(codeunit >> 4u) & 0x0Fu];
string_buffer[pos + 5] = nibble_to_hex[codeunit & 0x0Fu];
return pos + 6;
}
// templates to avoid warnings about useless casts
template <typename NumberType, enable_if_t<std::is_signed<NumberType>::value, int> = 0>
bool is_negative_number(NumberType x)