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Speedup; check for the expected separator before the lexer's token switch (#5592)

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* Check for the expected separator before the lexer's token switch

After a key the parser expects ':', after a value usually ','. Test for
that character first instead of going through scan()'s switch, which
compiles to an indirect jump. Any other character takes the old path,
so tokens and error messages are unchanged.

Parsing 6.3% faster with GCC 15.2 and 2.7% with Clang 22.1 (geomean of
the ParseString, ParseFile and ParseIndented benchmarks).

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>

* Improvement: address PR comments

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>

* fix: address comments

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>

* Fix clang-tidy bugprone-signed-char-misuse in scan_expecting

Convert the expected separator through unsigned char before storing it as
char_int_type. The generated code is unchanged.

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>

* Use raw string literals in the separator comment tests

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>

---------

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>
Co-authored-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>
This commit is contained in:
Michiel van SlobbeandMichiel van Slobbe authored and GitHub committed 2026-10-06 08:36:40 +02:00
1 parent 0490778fc3
commit 5ecb704f6b
4 files changed
+140 -10
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include/nlohmann/detail/input/parser.hpp
+11 -4
View File
@@ -260,7 +260,7 @@ class parser
}
// parse separator (:)
if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
@@ -423,7 +423,7 @@ class parser
{
// comma -> next value
// or end of array (ignore_trailing_commas = true)
if (get_token() == token_type::value_separator)
if (get_token_expecting(token_type::value_separator))
{
// parse a new value
get_token();
@@ -463,7 +463,7 @@ class parser
// comma -> next value
// or end of object (ignore_trailing_commas = true)
if (get_token() == token_type::value_separator)
if (get_token_expecting(token_type::value_separator))
{
get_token();
@@ -484,7 +484,7 @@ class parser
}
// parse separator (:)
if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
@@ -528,6 +528,13 @@ class parser
return last_token = m_lexer.scan();
}
/// get next token from lexer; true if it is the separator @a expected_type
/// (name_separator or value_separator), which it usually is
bool get_token_expecting(token_type expected_type)
{
return (last_token = m_lexer.scan_expecting(expected_type)) == expected_type;
}
std::string exception_message(const token_type expected, const std::string& context)
{
std::string error_msg = "syntax error ";