cut: fix -s flag for newline delimiter and optimize memory allocation

[akervald]

Fixes & Improvements

• -slogic fix: Add the missing only_delimited check to properly suppress non-delimited lines.
• Field-Level Streaming: Replace whole-file split().collect() with a memchr-powered loop. This shifts memory complexity from O(Total File Size) to O(Max Field Size) - as "OOM-safe" as the specification allows.
• Zero-Allocation Skipping: Bypass unselected fields using BufReader::consume() to avoid heap copies.
• Sequential Pointer Tracking: Replace nested loops and segments.get() lookups with a single-pass range_idx pointer that synchronizes "Skip" and "Keep" paths in one linear sweep.
• Early Exit: Terminate I/O immediately once the highest requested field is processed.
• Edge Case Support: Correctly handle single lines lacking a trailing newline.

Benchmarks

10,000,000 records (seq 1 10000000 > bench_input.txt), base M1 Pro chip.

Case 1: Filtered Selection with Early Exit (-s -d $'\n' -f 2,1024,4096)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
gcut	330.8 ± 5.6	326.5	341.5	169.60 ± 30.28
./cut_old	437.1 ± 9.1	430.7	456.6	224.09 ± 40.10
./cut_new	2.0 ± 0.3	1.3	3.3	1.00

Result: ~224x faster than cut_old, ~170x faster than GNU cut.

Case 2: Full File Read / Base Throughput (-s -d $'\n' -f 1-10000000)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
gcut	676.7 ± 5.1	673.7	689.3	3.96 ± 0.13
./cut_old	527.0 ± 16.8	517.9	573.0	3.08 ± 0.14
./cut_new	171.1 ± 5.5	168.9	192.1	1.00

Result: ~3x faster than cut_old, ~4x faster than GNU cut.

References

• Fixes cut: incorrect delimiter handling when delimiter is newline #10012
• Supersedes cut: fix -s flag ignored when delimiter is newline #10037

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cut/cut. tests/cut/cut is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[codspeed-hq]

Merging this PR will improve performance by 45.19%

⚡ 1 improved benchmark
✅ 301 untouched benchmarks
🆕 2 new benchmarks
⏩ 42 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
🆕	Simulation	cut_fields_newline_delim	N/A	189.8 µs	N/A
🆕	Memory	cut_fields_newline_delim	N/A	67.8 KB	N/A
⚡	Memory	cut_fields_custom_delim	67.8 KB	46.7 KB	+45.19%

---

_{Comparing akervald:fix-cut-newline-s-flag (fe4e36b) with main (f335d14)}

Footnotes

1. 42 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cut/cut. tests/cut/cut is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cut/cut. tests/cut/cut is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cut/cut. tests/cut/cut is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/follow-name (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Note: The gnu test tests/pr/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cut/cut. tests/cut/cut is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/follow-name (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cut/cut. tests/cut/cut is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Congrats! The gnu test tests/timeout/timeout-group is no longer failing!
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cut/cut. tests/cut/cut is passing on 'main'. Maybe you have to rebase?
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.
Congrats! The gnu test tests/cp/link-heap is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Congrats! The gnu test tests/tail/retry is no longer failing!

[akervald]

Hi @cakebaker, the tests passed, but the benchmark failed due to an infrastructure issue. Could you please re-run that job? Thanks!

[akervald]

@sylvestre I noticed Attempt №3 was cancelled. Since I don't have permissions to trigger the CI/CD jobs myself, could you let me know if there’s a specific fix I need to make, or if you could re-run the checks when the environment is ready? Thanks!

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)

[akervald]

@cakebaker should be ready for a review/merge

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/retry is no longer failing!
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.

[cakebaker]

Thanks for your PR!