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Derive the lexer position lazily for seekable input (#5809)

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* Derive the lexer position lazily for seekable input

The lexer counted the read position on every character: two increments
and a newline check per get(), all on members kept in memory. For
seekable input (pointers, strings, other random-access byte ranges) the
position can be derived instead: chars_read_total is the adapter's read
count (including reads at EOF) minus a pending unget, and line and
column are recomputed from the consumed input only when get_position()
is called, i.e. on parse errors. JSON_DIAGNOSTIC_POSITIONS uses the new
O(1) get_chars_read_total(). Stream, file, wide-string and user-defined
adapters keep the eager counters.

Parsing pretty-printed input gets 8-12% faster and accept() 12-27%
(canada, citm_catalog, twitter; clang 21 arm64 and GCC 13 x86-64).
Error messages, byte offsets, SAX positions and diagnostic positions are
unchanged (differential comparison against develop on 548k inputs).

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

* Move the std::vector<std::byte> binary type tests out of unit-regression3

The #4804/#4209 tests instantiated a second basic_json specialization
(with_binary_t<std::vector<std::byte>>), including its parser and CBOR
reader. With clang 18, the C++20 object of unit-regression3.cpp had
32656 COFF sections on develop; the lazy position helpers pushed it to
32975, past what the MinGW linker of the Windows clang jobs handles
("relocation truncated to fit: IMAGE_REL_AMD64_REL32 against .rdata").
unit-custom-binary-type.cpp already instantiates the same type, so the
tests move there; unit-regression3.cpp drops to 27616 sections.

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

* Report the parse errors of the position test via SAX

The test caught json::parse_error, so it aborted in the
ci_test_noexceptions build, where JSON_NOEXCEPTION turns the throw into
std::abort(). sax_parse() hands the error position and the exception
object to parse_error() without throwing, so the test now records them
there and runs in every configuration.

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

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann authored and GitHub committed 2026-10-13 17:16:52 +02:00
1 parent 40a62947d3
commit f23830b65e
9 files changed
+380 -105
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tests/src/unit-regression3.cpp
-48
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@@ -58,11 +58,6 @@ using ordered_json = nlohmann::ordered_json;
#include <optional>
#elif __has_include(<experimental/optional>)
#endif
/////////////////////////////////////////////////////////////////////
// for #4804
/////////////////////////////////////////////////////////////////////
using json_4804 = nlohmann::json::with_binary_t<std::vector<std::byte>>;
#endif
#ifdef JSON_HAS_CPP_20
@@ -458,49 +453,6 @@ TEST_CASE("regression tests 3")
#endif
#ifdef JSON_HAS_CPP_17
SECTION("issue #4804: from_cbor incompatible with std::vector<std::byte> as binary_t")
{
const std::vector<std::uint8_t> data = {0x80};
const auto decoded = json_4804::from_cbor(data);
CHECK((decoded == json_4804::array()));
}
#ifndef SKIP_TESTS_FOR_ENUM_SERIALIZATION
SECTION("discussion #4209 - custom BinaryType direct assignment and round-tripping")
{
// Test that assigning a custom BinaryType directly creates a binary value, not an array
const std::vector<std::byte> original{std::byte{1}, std::byte{2}, std::byte{3}};
const json_4804 j = original;
CHECK(j.is_binary());
CHECK(!j.is_array());
// Test round-tripping: extracting the binary value back as the custom container type
const auto extracted = j.get<std::vector<std::byte>>();
CHECK(extracted == original);
// Test that the default json alias behavior is unchanged: std::vector<uint8_t> -> array
const json default_json = std::vector<std::uint8_t> {1, 2, 3};
CHECK(default_json.is_array());
CHECK(!default_json.is_binary());
}
SECTION("discussion #4209 - custom BinaryType extraction from parsed array")
{
// Test that extracting a custom BinaryType from a parsed JSON array still works
// (not just from a binary-typed node)
const auto j = json_4804::parse("[1,2,3]");
CHECK(j.is_array());
CHECK(!j.is_binary());
// Extracting as custom BinaryType should work from arrays
const auto extracted = j.get<std::vector<std::byte>>();
CHECK(extracted.size() == 3);
CHECK(extracted[0] == std::byte{1});
CHECK(extracted[1] == std::byte{2});
CHECK(extracted[2] == std::byte{3});
}
#endif
SECTION("issue #5046 - implicit conversion of return json to std::optional no longer implicit")
{
const json jval{};