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# eliminate duplicates in history and keep only the most recent
for(my $i=scalar(@history)-1;$i>=0;$i--) {
    $_ = $history[$i];
    !$history{$_} ? $history{$_}=1 : splice(@history,$i--,1);
  }

{
  my (%history, $histcount);   # Should I initialise $histcount = 0 or
+ is that "Cargo cult"?
...
... # deside we want the $n'th previous value
my $last_nth = getHist( $n );
...

for ( my $i=$#history; $i >=0; $i-- ) {  # for $i = (last index of @hi
+story) to 0
  $_ = $history[$i];                     # get the value from the posi
+tion in @history
...
    : splice @history, $i--, 1)          # if it has remove it from @h
+istory 
                                         # (reducing $i to compensate 
+for its remove;
}

for my $i ($#history .. 0 ) {
  !exists $history{ history[$i] }        # use exists as testing its v
+alue against zero will 
...
    : splice @history, $i--, 1);         # the -- in this version woul
+d be bad!!  
                                         # Thou shalt not modify the i
+terator in a for loop!
}

for ($#history .. 0) {
    my $temp = pop @history;
...
        unshift @history;
    } # else nothing to do we've already removed it.
}

for ($#history .. 0) {
    my $temp = pop @history;
...
    $history{ $temp } ++;
    unshift @history;
}

for ( push @history, '~~SOME UNLIKELY VALUE~~', my $value= shift @hist
+ory; 
    $value ne '~~SOME UNLIKELY VALUE~~';
...
{
    push @history, $value, $history{ $value }++ unless exists $history
+{ $value }
}

use constant SENTINEL => '~~SOME UNLIKELY VALUE~~';

...
{
    push @history, $value, $history{ $value }++ unless exists $history
+{ $value }
}

my %history;
....
# get a new value here and only add it if its not there.
$history{ $newvalue } = 1 unless exists $history{ $newvalue };
...

my (%history, $histcount);
....
# get a new value here and only add it if its not there.

$history{ $newvalue } = $histcount++ unless exists $history{ $newvalue
+ };

$history{ $newvalue } = $histcount++;  
...

for my $i  (keys %history) {
    last if $history{ $i } == ($histcount - $n);         
}
my $last_nth = $history{ $i };

 # Can't use multiple modifiers. Basic Plus2 was nice!
my last_nth = do { $_ for (keys %history) } while ( $history{ $_ } == 
+($histcount - $n) );  

my $last_nth = $_ if $history{ $_ } == n for (keys %history); # Again 
+$_ is local

my $last_nth;
for my $i  (keys %history) {
    last if ($last_nth = $history{ $i }) == ($histcount - $n); 
}

my last_nth = grep{ $history{ $_ } == ($histcount - $n } keys %history
+;

my ($last_nth) = grep{ $history{ $_ } == ($histcount - $n); } keys %hi
+story;

my (%history, $histcount);

...

# To retrieve the last-but-$n'th 
my ($last_nth) = grep{ $history{ $_ } == ($histcount - $n); } keys %hi
+story;

my (%history, $histcount);     # Should I initialise $histcount = 0 or
+ is that "Cargo cult"?

...
... # deside we want the $n'th previous value
my $last_nth = getHist( $n );
...

my (%history, $histcount);     # Should I initialise $histcount = 0 or
+ is that "Cargo cult"?

...
    #validate
    return = grep{ $hashref->{ $_ } == ($histcount - $n); } keys %hist
+ory; 
}

... # get $newvalue from somewhere
addHist( \%history, $histcount, $newvalue );
...
... # deside we want the $n'th previous value
my $last_nth = getHist( \%history, $histcount, $n );
...

{
    my (%history, $histcount);     # Should I initialise $histcount = 
+0 or is that "Cargo cult"?
...
    sub addHist { $history{ $_[0] } = $histcount++; }
    sub getHist { return = grep{ $history{ $_ } == ($histcount - $_[0]
+); } keys %history; }
}

... # get $newvalue from somewhere
addHist( $newvalue );
...
... # deside we want the $n'th previous value
my $last_nth = getHist( $n );
...