shuf: fix random line selection. Closes 9971

"""
For example, given input file:

    foo
    bar
    baz

after shuffling the input file, foo will never end up back on the first line.
This came to light when I ran into a use-case where someone was selecting
a random line from a file using shuf | head -n 1, and the results on busybox
were showing a statistical anomaly (as in, the first line would never ever
be picked) vs the same process running on environments that had gnu coreutils
installed.

On line https://git.busybox.net/busybox/tree/coreutils/shuf.c#n56 it uses
r %= i, which will result in 0 <= r < i, while the algorithm specifies
0 <= r <= i.
"""

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/coreutils/shuf.c b/coreutils/shuf.c
index 9f61f2f7de3d10ac54b98a0dd4d698b094b407f8..217f15c977afc2b6041f1eb95d82bbbf8372c7e4 100644 (file)
--- a/coreutils/shuf.c
+++ b/coreutils/shuf.c
@@ -53,7 +53,7 @@ static void shuffle_lines(char **lines, unsigned numlines)
                /* RAND_MAX can be as small as 32767 */
                if (i > RAND_MAX)
                        r ^= rand() << 15;
-               r %= i;
+               r %= i + 1;
                tmp = lines[i];
                lines[i] = lines[r];
                lines[r] = tmp;