As one can see easily, the running time inside the main loop is independent of the size of the array, hence, O(1), making the entire program run in linear time.

Yep. Linear, but twice as slow as my code above for the OPs stated requirements: 

c:\test>713977-javafan -N=1e6
Took 0.00029 seconds/iteration

c:\test>713977 -N=1e6
Took 0.00015 seconds/iteration

[download]

How much bigger than the requirements does the problem need to get before the additional complexity achieves parity?

And how much bigger still, before the additional complexity of your solution starts to save enough time that the user will actually notice?

Finally, what affect does that re-pick have upon the fairness of the shuffle? 

#713977-javafan.pl
#! perl -slw
use strict;
use Time::HiRes qw[ time ];

our $N ||= 1000;

my  @animals = qw[
    dog cat bird mouse rat snake horse cow
    pig lizard lamb zebra lion elephant monkey
];

my $start = time;
for( 1 .. $N ) {
    my @scratch = @animals;
    my @want;

    foreach my $animal (@animals) {
        for (my $j = @scratch - 1; $j >= @scratch - 3; $j --) {
            my $r = rand ($j + 1);
            @scratch[$j, $r] = @scratch[$r, $j];
            if ($scratch[$j] eq $animal) {
                my $r = rand ($j);
                @scratch[$j, $r] = @scratch[$r, $j];
            }
        }
        push @want, "$animal @scratch[ -3 .. -1 ]";
    }
}
printf "Took %7.5f seconds/iteration\n", ( time() - $start ) / $N;

#713977.pl
#! perl -slw
use strict;
use List::Util qw[ shuffle ];
use Time::HiRes qw[ time ];

our $N      ||= 1000;

my  @start = qw[
    dog cat bird mouse rat snake horse cow
    pig lizard lamb zebra lion elephant monkey
];

my $start = time;
for ( 1 .. $N ) {
    my  @want = map{
        join ' ', $start[ $_ ],
        ( shuffle @start[ 0 .. $_-1, $_+1 .. $#start ] )[ 0 .. 2 ]
    } 0 .. $#start;
}
printf "Took %7.5f seconds/iteration\n", ( time() - $start ) / $N;

[download]
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

In reply to Re^6: Array problem by BrowserUk
in thread Array problem by Anonymous Monk

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