Add a mode that turns a sorted list into adversarial input for a
bottom-up mergesort implementation that doubles the length of sorted
sublists at each level -- like our llist_mergesort().
While unriffle mode splits the list in half at each recursion step,
unriffle_skewed splits it into 2^l items and the rest, with 2^l being
the highest power of two smaller than the number of items and thus
2^l >= rest. The rest is unriffled with the tail of the first half to
require a merge to compare the maximum number of elements.
It complements the unriffle mode, which targets balanced merges. If
the number of elements is a power of two then both actually produce the
same result, as 2^l == rest == n/2 at each recursion step in that case.
Here are the results:
$ t/helper/test-tool mergesort test | awk '
$7 > max[$3] {max[$3] = $7; line[$3] = $0}
END {for (n in line) print line[n]}
'
distribut mode n m get_next set_next compare verdict
sawtooth unriffle_skewed 100 128 1184 700 589 OK
sawtooth unriffle_skewed 1023 1024 16373 10230 9207 OK
sawtooth unriffle 1024 1024 16384 10240 9217 OK
sawtooth unriffle_skewed 1025 2048 18454 11275 10241 OK
The sawtooth distribution with m>=n produces a sorted list and
unriffle_skewed mode turns it into adversarial input for unbalanced
merges, which it wins in all cases except for n=1024 -- the resulting
list is the same, but unriffle is tested before unriffle_skewed, so its
result is selected by the AWK script.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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