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#!/usr/bin/perl -s
use v5.24;
use Test2::V0;
use experimental qw(signatures);
our ($tests, $examples);
run_tests() if $tests || $examples; # does not return
die <<EOS unless @ARGV;
usage: $0 [-examples] [-tests] [-verbose] [--] [N...]
-examples
run the examples from the challenge
-tests
run some tests
N...
list of numbers
EOS
### Input and Output
main: {
local $" = ',';
local $, = ',';
say map "(@$_)", wave_array([sort {$b <=> $a} @ARGV], [], 1, [])->@*;
}
### Implementation
# Recursive implementation:
# - If there is only one element in the list, it is appended to the
# current wave if it agrees with the direction.
# - The first element of the wave array cannot be the smallest / largest
# from the list as there would not be a suitable second element
# otherwise.
# - Loop over unseen numbers only that agree with the direction.
# - On recursion to the next level, remove the currently selected
# element, reverse the remaining and flip the direction.
sub wave_array ($list, $prefix, $dir, $result) {
if (@$list == 1) {
push @$result, [@$prefix, @$list]
if @$prefix && ($list->[0] <=> $prefix->[-1]) * $dir >= 0;
return;
}
my %seen_p;
for my $p (0 .. $#$list - 1) {
my @l = @$list;
my $lp = splice @l, $p, 1;
next if $seen_p{$lp};
$seen_p{$lp} = 1;
next if @$prefix && ($lp <=> $prefix->[-1]) * $dir < 0;
wave_array([reverse @l], [@$prefix, $lp], -$dir, $result);
}
$result;
}
### Examples and tests
sub run_tests {
SKIP: {
skip "examples" unless $examples;
is wave_array([4, 3, 2, 1], [], 1, []),
bag {item [2, 1, 4, 3]; item [4,1,3,2]; etc;}, 'example';
}
SKIP: {
skip "tests" unless $tests;
}
done_testing;
exit;
}
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