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shuf: speed-up when limited output is requested

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A user noted that the following command was slower than they
expected:

   busybox shuf -i "1500000000-$(date +%s)" -n 5

At time of writing the range contains 128 million values.  On my
system this takes 7.7s whereas 'shuf' from coreutils takes a
handful of milliseconds.

Optimise BusyBox 'shuf' for cases where -n is specified by stopping
shuffling once the required number of lines have been processed.
On my system the time for the example is reduced to 0.4s.

function                                             old     new   delta
shuf_main                                            520     540     +20
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 20/0)               Total: 20 bytes

v2: Code shrink.  Since outlines <= numlines:
    - the loop in shuffle_lines() only needs to test the value of
      outlines;
    - shuffle_lines() can be called unconditionally.
    Update timing to allow for the 13 million seconds elapsed since v1.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Ron Yorston 2021-08-07 09:41:49 +01:00 committed by Denys Vlasenko
parent 74c4f356ae
commit 8817e285b7
1 changed files with 15 additions and 14 deletions
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29
coreutils/shuf.c
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@ -24,7 +24,7 @@
//usage: "\n -i L-H Treat numbers L-H as lines"
//usage: "\n -n NUM Output at most NUM lines"
//usage: "\n -o FILE Write to FILE, not standard output"
//usage: "\n -z End lines with zero byte, not newline"
//usage: "\n -z NUL terminated output"
#include "libbb.h"
@ -39,8 +39,10 @@
/*
* Use the Fisher-Yates shuffle algorithm on an array of lines.
* If the required number of output lines is less than the total
* we can stop shuffling early.
*/
static void shuffle_lines(char **lines, unsigned numlines)
static void shuffle_lines(char **lines, unsigned numlines, unsigned outlines)
{
unsigned i;
unsigned r;
@ -48,7 +50,7 @@ static void shuffle_lines(char **lines, unsigned numlines)
srand(monotonic_us());
for (i = numlines-1; i > 0; i--) {
for (i = numlines-1; outlines > 0; i--, outlines--) {
r = rand();
/* RAND_MAX can be as small as 32767 */
if (i > RAND_MAX)
@ -67,7 +69,7 @@ int shuf_main(int argc, char **argv)
char *opt_i_str, *opt_n_str, *opt_o_str;
unsigned i;
char **lines;
unsigned numlines;
unsigned numlines, outlines;
char eol;
opts = getopt32(argv, "^"
@ -128,24 +130,23 @@ int shuf_main(int argc, char **argv)
fclose_if_not_stdin(fp);
}
if (numlines != 0)
shuffle_lines(lines, numlines);
outlines = numlines;
if (opts & OPT_n) {
outlines = xatou(opt_n_str);
if (outlines > numlines)
outlines = numlines;
}
shuffle_lines(lines, numlines, outlines);
if (opts & OPT_o)
xmove_fd(xopen(opt_o_str, O_WRONLY|O_CREAT|O_TRUNC), STDOUT_FILENO);
if (opts & OPT_n) {
unsigned maxlines;
maxlines = xatou(opt_n_str);
if (numlines > maxlines)
numlines = maxlines;
}
eol = '\n';
if (opts & OPT_z)
eol = '\0';
for (i = 0; i < numlines; i++) {
for (i = numlines - outlines; i < numlines; i++) {
if (opts & OPT_i)
printf("%u%c", (unsigned)(uintptr_t)lines[i], eol);
else