The prior commit's ~1.5x "unexplained floor" was measured in debug mode,
because release-mode test builds hit an unrelated PureYAMLJSON test-target
issue. sample(1) profiling of that debug build pointed at consumePlainContent,
a mutating throws method, which is a known trigger for defensive retain/copy
traffic under -Onone that the optimizer removes in release.
Worked around the release-test-build issue with -Xswiftc -enable-testing and
re-measured: the isFinal: true control now shows 1.04x, essentially batch
parity, so the checkpoint-skip fix was a full fix for that case, not the
small, unexplained one debug mode suggested.
What remains is now correctly explained rather than guessed at: a chunked
run's isFinal only becomes true once, on the last chunk, so for any input
that arrives incrementally, the checkpoint stays live for virtually the
entire scan regardless of chunk size (release-mode ratios: 11.70x at 16
scalars down to 1.75x at 1,000,000, never reaching the 1.04x floor). Closing
that needs the deferred persisted mid-token suspension, which removes the
need for a checkpoint rather than making it cheaper; isFinal: true is the
only condition under which skipping it is provably safe, since a single
scalar can span past any finite "probably buffered enough" margin.
Removed the temporary sample(1) profiling scratch test and documented how to
re-run the measurement in release mode, which the doc comment previously
did not call out as necessary.
Add StreamingScannerPerformanceTests (issue #4), gated behind
PUREYAML_STREAMING_PERF_MEASURE=1, measuring scanStep() against scanAll() on
the largest real-world fixture (9.5MB) across a spread of chunk sizes.
Small chunk sizes (16-256 scalars) show a real, chunk-size-dependent rescan
cost, up to 6x at 16, consistent with the checkpoint-and-retry design
rescanning a fetch attempt once per chunk it spans. A control run with
isFinal: true from construction, where ranDry can never be set and no
rollback ever happens, still cost ~1.5x over scanAll(), proving a separate,
chunk-size-independent cost dominates at any realistic chunk size (4KB+): the
checkpoint was being copied on every fetch even when it could never be used.
Fixed by skipping the checkpoint entirely when reader.isFinal is true.
Re-measured rather than assumed fixed: the isolated control improved from
1.49x to 1.38x, a real but modest gain, while the large-chunk-size ratios
were unchanged within measurement noise. The remaining floor's cause is not
identified and is left open on issue #4 rather than patched further on
speculation; closing it needs a profiler, not another guess.
Whole-suite conformance (402/402) and the chunk-size differential proof both
still pass unchanged.
