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Ingy döt Net 2023-07-01 11:58:00 -04:00
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Task/Quickselect-algorithm/Raku/quickselect-algorithm.raku Normal file
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my @v = <9 8 7 6 5 0 1 2 3 4>;
say map { select(@v, $_) }, 1 .. 10;
sub partition(@vector, $left, $right, $pivot-index) {
my $pivot-value = @vector[$pivot-index];
@vector[$pivot-index, $right] = @vector[$right, $pivot-index];
my $store-index = $left;
for $left ..^ $right -> $i {
if @vector[$i] < $pivot-value {
@vector[$store-index, $i] = @vector[$i, $store-index];
$store-index++;
}
}
@vector[$right, $store-index] = @vector[$store-index, $right];
return $store-index;
}
sub select( @vector,
\k where 1 .. @vector,
\l where 0 .. @vector = 0,
\r where l .. @vector = @vector.end ) {
my ($k, $left, $right) = k, l, r;
loop {
my $pivot-index = ($left..$right).pick;
my $pivot-new-index = partition(@vector, $left, $right, $pivot-index);
my $pivot-dist = $pivot-new-index - $left + 1;
given $pivot-dist <=> $k {
when Same {
return @vector[$pivot-new-index];
}
when More {
$right = $pivot-new-index - 1;
}
when Less {
$k -= $pivot-dist;
$left = $pivot-new-index + 1;
}
}
}
}