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shuf: add a TODO, code shrink

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function                                             old     new   delta
shuf_main                                            501     500      -1

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2021-09-07 22:51:42 +02:00
parent 574b9c446d
commit 6a9b3f7acf
1 changed files with 14 additions and 10 deletions
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24
coreutils/shuf.c
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@ -44,21 +44,25 @@
*/
static void shuffle_lines(char **lines, unsigned numlines, unsigned outlines)
{
unsigned i;
unsigned r;
char *tmp;
srand(monotonic_us());
for (i = numlines - 1; outlines > 0; i--, outlines--) {
r = rand();
while (outlines != 0) {
char *tmp;
unsigned r = rand();
/* RAND_MAX can be as small as 32767 */
if (i > RAND_MAX)
if (numlines > RAND_MAX)
r ^= rand() << 15;
r %= i + 1;
tmp = lines[i];
lines[i] = lines[r];
r %= numlines;
//TODO: the above method is seriously non-uniform when numlines is very large.
//For example, with numlines of 0xf0000000,
//values of (r % numlines) in [0, 0x0fffffff] range
//are more likely: e.g. r=1 and r=0xf0000001 both map to 1,
//whereas only one value, r=0xefffffff, maps to 0xefffffff.
numlines--;
tmp = lines[numlines];
lines[numlines] = lines[r];
lines[r] = tmp;
outlines--;
}
}