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#!/usr/bin/perl -s
use v5.16;
use Test2::V0;
use List::Gen;
use Math::Prime::Util 'divisor_sum';
our ($tests, $examples);
run_tests() if $tests || $examples; # does not return
die <<EOS unless @ARGV;
usage: $0 [-examples] [-tests] [N]
-examples
run the examples from the challenge
-tests
run some tests
N
find the first N amicable pairs
EOS
### Input and Output
gen_amicable_pairs()->map(sub{"(@$_)"})->say(shift);
### Implementation
# Build a generator for amicable pairs.
sub gen_amicable_pairs {
my %pairs;
<2..>->filter(sub {
# Skip seen pairs.
return if exists $pairs{$_};
my $s = divisor_sum($_) - $_;
# Skip perfect numbers.
return if $s == $_;
my $t = divisor_sum($s) - $s;
# Skip non-pairs.
return if $_ != $t;
# Remember the second.
$pairs{$s} = undef;
# Pair the second with the first and return "true".
$pairs{$_} = $s;
})
# Actually build the pairs.
->map(sub{[$_, $pairs{$_}]});
}
### Examples and tests
sub run_tests {
SKIP: {
skip "examples" unless $examples;
is gen_amicable_pairs()->take(10),
[[220, 284], [1184, 1210], [2620, 2924], [5020, 5564],
[6232, 6368], [10744, 10856], [12285, 14595],
[17296, 18416], [63020, 76084], [66928, 66992]],
'first ten pairs from Wiki';
}
SKIP: {
skip "tests" unless $tests;
}
done_testing;
exit;
}
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