Add newline_delimited, slice_at, and skip documents by delimiter - #2803

next_document() reaches the next document with skip_child(0), which loads the input byte behind every remaining structural and branches on it. When the caller reads only part of each document, that walk covers the rest of it.

For a delimiter-framed stream the document ends at the next delimiter, so memchr plus a galloping search of the structural index gets there without touching the structurals in between. This applies to json_sequence and to a new stream_format::newline_delimited, where the caller guarantees one document per line. whitespace_delimited cannot use it: it permits several documents on one line and line feeds inside a document, so a line feed is a document boundary but not necessarily the next one.

The shortcut is gated on depth() > 0, which means the caller left the document part-read, so there is a walk worth avoiding, and the iterator is still inside the document, so the next delimiter terminates it.

simdjson::slice_at cuts a padded_string_view into document-aligned slices. Documents are independent, so callers can parse the slices on as many threads as they like instead of relying on the built-in stage-1 thread, which caps out near a factor of two. doc/iterate_many.md shows the pattern.

document_stream also allocated a stage1_worker, which holds a thread, a mutex and a condition variable, on every construction even with threading off. It is now allocated only when a stage-1 thread is started.

Benchmarks

benchmark/bench_stream_formats.cpp now runs whitespace_delimited and newline_delimited side by side, plus a bench_sliced variant using slice_at across threads. On a 64-core Xeon Gold 6548N, small-document NDJSON:

Testing

131/131 tests pass on x86-64 (Xeon Gold 6548N, GCC 14) and 118/118 on arm64 (Apple Silicon, clang). New tests cover partial and full document reads, blank lines and CRLF, scalar documents, agreement with whitespace_delimited across batch sizes, and slice_at reassembling its input exactly for a range of block sizes.

Notes

document_stream now befriends token_iterator to reach peek() and set_position(). Narrow accessors may be preferable.

slice_at returns an empty view when a block falls entirely inside one document, which happens only if that document is longer than block_size. Callers should iterate while index * block_size < size and skip empty slices rather than stopping at the first one.