Fixes #856 for std::nth_element and std::ranges::nth_element. This implements a fallback to the median-of-medians-of-five algorithm when the quickselect algorithm seems to be making too little progress.

The median-of-medians algorithm is mostly the textbook version, with two minor tweaks:

• If the processed sequence doesn't cleanly divide into groups of five elements, the remainder group with less than five elements isn't considered for the median computation. (This reduces the amount of code and doesn't make any difference in the asymptotics. I couldn't observe any practical difference in running time, too.)
• When the pivot (=median-of-medians) has been computed, all (greater) medians located after the pivot are moved to the very end of the processed sequence and the pivot is swapped into the middle of the sequence. This is because all of these elements are guaranteed to be moved by the pivot partitioning algorithm, so this step immediately moves them into an appropriate position (or the pivot probably closer to it). This way, the medians can also be excluded from the sequence on which the partitioning algorithm is applied, avoiding some unnecessary comparisons. (In practice, the benchmarks suggested that this makes the algorithm a few percent faster, but the difference is minor.)

Benchmark results

bm_uniform just applies nth_element to an integer array of the given length. The integer array is uniformly sampled from a fixed seed. This is to check that the worst-case fallback does not noticeably worsen the processing time on such a sequence.

bm_tunkey_adversary applies nth_element to a sequence on which the implemented quickselect algorithm performs terribly.

Before:

--------------------------------------------------------------------------------
Benchmark                                      Time             CPU   Iterations
--------------------------------------------------------------------------------
bm_uniform<alg_type::std_fn>/1024           1845 ns         1803 ns       407273
bm_uniform<alg_type::std_fn>/2048           3966 ns         3990 ns       172308
bm_uniform<alg_type::std_fn>/4096           7702 ns         7673 ns        89600
bm_uniform<alg_type::std_fn>/8192          18090 ns        18032 ns        40727
bm_uniform<alg_type::rng>/1024              1759 ns         1758 ns       373333
bm_uniform<alg_type::rng>/2048              3985 ns         4011 ns       179200
bm_uniform<alg_type::rng>/4096              7694 ns         7847 ns        89600
bm_uniform<alg_type::rng>/8192             18015 ns        17997 ns        37333
bm_tunkey_adversary<alg_type::std_fn>      12995 ns        13393 ns        56000
bm_tunkey_adversary<alg_type::rng>         12714 ns        12835 ns        56000

After:

--------------------------------------------------------------------------------
Benchmark                                      Time             CPU   Iterations
--------------------------------------------------------------------------------
bm_uniform<alg_type::std_fn>/1024           1599 ns         1604 ns       448000
bm_uniform<alg_type::std_fn>/2048           3626 ns         3610 ns       194783
bm_uniform<alg_type::std_fn>/4096           7068 ns         7150 ns        89600
bm_uniform<alg_type::std_fn>/8192          16469 ns        16044 ns        44800
bm_uniform<alg_type::rng>/1024              1701 ns         1709 ns       448000
bm_uniform<alg_type::rng>/2048              3841 ns         3931 ns       194783
bm_uniform<alg_type::rng>/4096              7447 ns         7324 ns        74667
bm_uniform<alg_type::rng>/8192             17024 ns        16741 ns        37333
bm_tunkey_adversary<alg_type::std_fn>       6075 ns         5929 ns        89600
bm_tunkey_adversary<alg_type::rng>          6270 ns         6278 ns       112000

As expected, the fallback greatly improves the running time for bm_tunkey_adversary. The timings for bm_uniform are about on par, or more precisely even a bit better with this PR on my machine.

The fallback heuristic

std::sort switches to its fallback when the recursion depth exceeds some logarithmic threshold. We could use the same heuristic as well, however, this would not guarantee linear time in the worst case but "only" an $O(n \log n)$ bound. Alternatively, we could limit the recursion depth to some constant, but that's likely a pessimization for large sequences.

So I opted for an adaptive depth limit: Like the heuristic for std::sort, it assumes that each iteration should reduce the range of inspected elements by 25 %. But while std::sort derives a maximum recursion depth from this assumption, this heuristic falls back to the median-of-medians algorithm when the actual size of the processed sequence exceeds the desired size by some constant tolerance factor (currently about 2) during some iteration. Thus, the total number of processed elements over all quickselect iterations is bounded by a multiple of the sequence length times a geometric sum, ensuring worst-case linear time overall. At the same time, the tolerance factor introduces some leeway so that one or two bad iterations (especially at the beginning) don't trigger the fallback immediately.

Obviously, there are many possible choices for the desired percentage reduction per iteration and the tolerance factor. But the benchmarks seem to suggest that the chosen values aren't too bad; a smaller percentage reduction or a larger margin factor noticeably worsen the bm_tunkey_adversary benchmark, but result in little difference for bm_uniform. Besides, the implementation of std::sort already sets a precedent for a desired reduction of 25 % per iteration.

Test

The newly added test applies nth_element to the same worst-case sequence as the benchmark. This makes sure that the fallback is actually exerted by the test. (I think it's also the first test that exerts the quickselect algorithm and not the just the insertion sort fallback.)