That weird shuffle sort (or desort might be a better term) does amazingly well on speed, and surprisingly, under at least some circumstances can actually also produce all possible combinations. Analyzing the distribution shows it real weakness. The story being told by the standard deviation figures below.

thraxil and aristotles corrected forms do much here, but at the expense of the loss of speed. I didn't see japhy's version till my tests were complete so I haven't included it.

However, a well crafted Fischer-Yates shuffles out does both of the "random sort" solutions for sheer performance by a good margin, being twice as fast with with arrays of 1000 elements and nearly 5 times as fast once you get to 100,000 elements.

It also seems to do the best on the distribution test, but that maybe simply down to the random number generator rather then the algorithm. That's too deep statistical voodoo for me to determine.

Update: Forgot to mention, the Fischer-Yates is an in-place sort so minimum memory usage too.

`#!/usr/bin/perl -w
use strict;
use Benchmark qw(cmpthese);
use Data::Dumper;
sub xform {
map {$_->[0]}
sort { $a->[1] <=> $b->[1]}
map {[$_, rand(1)]} @_;
}
sub slice {
my @random;
push @random, rand 1 for 0 .. $#_;
@_[ sort { $random[$a] <=> $random[$b] } 0 .. $#_ ];
}
sub shufl {
$a = $_ + rand @_ - $_ and @_[$_, $a] = @_[$a, $_] for (0..$#_);
return @_;
}
sub qshuf {
sort { .5 <=> rand(1) } @_;
}
my @array = 1 .. 1000;
cmpthese(10, {
slice => sub { slice @array },
xform => sub { xform @array },
shufl => sub { shufl @array },
qshuf => sub { qshuf @array },
});
my (%buckets, %d, @temp);;
my @set = qw(A B C D);
for (1 .. 100_000 ) {
$buckets{"@{[slice @temp=@set]}"}{slice}++;
$buckets{"@{[xform @temp=@set]}"}{xform}++;
$buckets{"@{[shufl @temp=@set]}"}{shufl}++;
$buckets{"@{[qshuf @temp=@set]}"}{qshuf}++;
}
print "\npermutation | slice | xform | shufl | qshuf \n";
print "--------------------------------------------------\n";
for my $key (sort keys %buckets) {
printf "%8.8s: | %4d | %4d | %4d | %4d\n", $key,
$buckets{$key}{slice},
$buckets{$key}{xform},
$buckets{$key}{shufl},
$buckets{$key}{qshuf};
$d{slice}{Ex} += $buckets{$key}{slice}; $d{slice}{Ex2} += $buckets
+{$key}{slice}**2;
$d{xform}{Ex} += $buckets{$key}{xform}; $d{xform}{Ex2} += $buckets
+{$key}{xform}**2;
$d{shufl}{Ex} += $buckets{$key}{shufl}; $d{shufl}{Ex2} += $buckets
+{$key}{shufl}**2;
$d{qshuf}{Ex} += $buckets{$key}{qshuf}; $d{qshuf}{Ex2} += $buckets
+{$key}{qshuf}**2;
}
print "---------------------------------------------------\n";
printf "Std. Dev. | %0.3f | %0.3f | %0.3f | %0.3f\n",
sqrt( ($d{slice}{Ex2} - ($d{slice}{Ex}**2/24))/23 ),
sqrt( ($d{xform}{Ex2} - ($d{xform}{Ex}**2/24))/23 ),
sqrt( ($d{shufl}{Ex2} - ($d{shufl}{Ex}**2/24))/23 ),
sqrt( ($d{qshuf}{Ex2} - ($d{qshuf}{Ex}**2/24))/23 );
__END__
C:\test>199981
Benchmark: timing 10000 iterations of qshuf, shufl, slice, xform...
qshuf: 3 wallclock secs ( 3.04 usr + 0.01 sys = 3.04 CPU) @ 32
+84.07/s (n=10000)
shufl: 217 wallclock secs (209.08 usr + 0.01 sys = 209.09 CPU) @
+ 47.83/s (n=10000)
slice: 435 wallclock secs (429.57 usr + 0.01 sys = 429.58 CPU) @
+ 23.28/s (n=10000)
xform: 716 wallclock secs (693.68 usr + 0.00 sys = 693.68 CPU) @
+ 14.42/s (n=10000)
Rate xform slice shufl qshuf
xform 14.4/s -- -38% -70% -100%
slice 23.3/s 61% -- -51% -99%
shufl 47.8/s 232% 105% -- -99%
qshuf 3284/s 22681% 14008% 6767% --
permutation | slice | xform | shufl | qshuf
--------------------------------------------------
A B C D: | 4322 | 4277 | 4127 | 12320
A B D C: | 4127 | 4115 | 4143 | 6134
A C B D: | 4284 | 4185 | 4156 | 6430
A C D B: | 4246 | 4083 | 4272 | 3094
A D B C: | 4205 | 4192 | 4062 | 3167
A D C B: | 4182 | 4128 | 4125 | 1597
B A C D: | 4143 | 4287 | 4246 | 12478
B A D C: | 4146 | 4156 | 4154 | 6273
B C A D: | 4027 | 4133 | 4133 | 6354
B C D A: | 4171 | 4153 | 4163 | 3092
B D A C: | 4191 | 4128 | 4201 | 3170
B D C A: | 4187 | 4233 | 4143 | 1546
C A B D: | 4088 | 4163 | 4170 | 6217
C A D B: | 4044 | 4197 | 4127 | 3190
C B A D: | 4214 | 4228 | 4114 | 6261
C B D A: | 4169 | 4021 | 4260 | 3080
C D A B: | 4069 | 4075 | 4185 | 1480
C D B A: | 4120 | 4102 | 4185 | 1533
D A B C: | 4177 | 4151 | 4199 | 3037
D A C B: | 4248 | 4207 | 4198 | 1608
D B A C: | 4175 | 4252 | 4203 | 3087
D B C A: | 4135 | 4173 | 4198 | 1641
D C A B: | 4203 | 4157 | 4098 | 1620
D C B A: | 4127 | 4204 | 4138 | 1591
---------------------------------------------------
Std. Dev. | 70.671 | 64.372 | 50.640 | 3127.055
`

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