test-mergesort: add unriffle_skewed mode

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>

t/helper/test-mergesort.c

@@ -178,6 +178,33 @@ static void mode_unriffle(int *arr, int n)
 	free(tmp);
 }
 
+static unsigned int prev_pow2(unsigned int n)
+{
+	unsigned int pow2 = 1;
+	while (pow2 * 2 < n)
+		pow2 *= 2;
+	return pow2;
+}
+
+static void unriffle_recursively_skewed(int *arr, int n, int *tmp)
+{
+	if (n > 1) {
+		int pow2 = prev_pow2(n);
+		int rest = n - pow2;
+		unriffle(arr + pow2 - rest, rest * 2, tmp);
+		unriffle_recursively_skewed(arr, pow2, tmp);
+		unriffle_recursively_skewed(arr + pow2, rest, tmp);
+	}
+}
+
+static void mode_unriffle_skewed(int *arr, int n)
+{
+	int *tmp;
+	ALLOC_ARRAY(tmp, n);
+	unriffle_recursively_skewed(arr, n, tmp);
+	free(tmp);
+}
+
 #define MODE(name) { #name, mode_##name }
 
 static struct mode {
@@ -191,6 +218,7 @@ static struct mode {
 	MODE(sort),
 	MODE(dither),
 	MODE(unriffle),
+	MODE(unriffle_skewed),
 };
 
 static const struct mode *get_mode_by_name(const char *name)