Changes for version 0.281 - 2026-08-03
- `median` (LikeR.xs) — the same answers in about an eighth of the time. On the `benchmark.pl` case (10,000 normals in one array ref) a call went from 0.83 ms to 0.097 ms, which puts it ahead of the two implementations it was behind: `numpy.median` at 0.105 ms and R's `median` at 0.196 ms, measured on the same machine. Small samples — a per-group median under `agg` or `group_by`, which is where most calls to it come from — went from 647 ns to 251 ns.
- A median is the middle one or two values, so most of the sort the function used to do was wasted work: `qsort` orders all n elements at a cost of n log n comparisons, every one an indirect call through a function pointer the compiler cannot see into, to answer a question that depends on one or two of them. Those values are now selected instead, and the sample is walked once rather than twice.
- The selection is introselect, the same shape numpy's `partition` uses: quickselect with a median-of-three pivot, an insertion sort once a range is small, and a heapsort fallback past a depth limit, so an input crafted to defeat the pivot choice degrades to O(n log n) rather than O(n²). The awkward data people actually have comes out faster than random data rather than slower — sorted, reversed, all-equal and organ-pipe samples of 100,000 values each take about 0.25 ms against 0.90 ms for random ones. For an even count the lower of the middle pair is the largest value left below the upper one, which a scan of that side finds without a second selection.
- The counting pass is gone. It walked every element through `av_fetch` before any arithmetic, only to size the buffer; the array lengths give the same count, exactly, because an undef anywhere still dies.
- The pass that remains reads cells through `AvARRAY` instead of `av_fetch`, with tied arrays kept on the `av_fetch` path, since only it sees their values.
- A sample of 256 values or fewer is copied to the C stack rather than the heap, so the common small call no longer pays for a malloc and free at all: 10,000 of them now grow RSS by nothing. Larger samples still copy n values, which is what leaves the caller's array in its original order — the selection reorders whatever it works on, and `t/median.t` checks that the input comes back untouched.
- Error messages that carry an index or a count were unreadable on older perls, in nineteen places across LikeR.xs, and now are not. `croak` runs perl's own formatter rather than the C library's, and that formatter does not understand C99's `z` length modifier: it printed the conversion literally, so `median(1, undef)` on perl 5.10 or 5.12 said `undefined value at argument index %zu` instead of naming the argument that was undefined — the one thing the message existed to say. `min`, `max`, `mode`, `sum`, `sd`, `var`, `median`, `mcnemar_test`, `friedman_test`, `hoa2hoh` and `oneway_test` were all affected. They now use `UVuf`, as the rest of the file already did. The `snprintf` calls elsewhere in the file are unaffected and unchanged: those do go to the C library, where `%zu` means what it says.
- New `t/croak.messages.t` covers every one of those messages: each is triggered, checked for the number it should name, and then swept for any conversion left unexpanded, which is what will catch the next one written with `%zu`. Run against the code as it stood before this change, thirty-two of its assertions fail on perl 5.10.
- New `t/median.t`: every length from 1 to 25 and from 254 to 258 — either side of the insertion-sort cutoff and of the point where the buffer moves off the stack — across sorted, reversed, all-equal, two-valued, duplicate-heavy, organ-pipe and median-of-three-killer samples, each checked against a plain Perl sort, together with the error messages and `Test::LeakTrace` over the stack, heap, mixed-argument and croak paths.
- fix for threaded Perls https://www.cpantesters.org/cpan/report/2dbacf8f-7138-1014-a1ab-f0f91cf3b922
Modules
Get basic statistical functions, like in R, but with Perl using XS for performance