2 files changed, 1070 insertions, 0 deletions

diff --git a/2020/day9/input b/2020/day9/input
new file mode 100644
index 0000000..c27e1a4
--- /dev/null
+++ b/2020/day9/input
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diff --git a/2020/day9/xmas_encoder.pl b/2020/day9/xmas_encoder.pl
new file mode 100644
index 0000000..4fdcb9b
--- /dev/null
+++ b/2020/day9/xmas_encoder.pl
@@ -0,0 +1,70 @@
+use strict;
+use warnings;
+use DDP;
+use Smart::Comments;
+use Tie::File;
+use List::Util qw(min max);
+
+tie my @xmas, 'Tie::File', "input" or die "no input present, $!";
+
+my %preamble;
+# because it's easier this way trust me
+my @also_queue;
+my $goalnum;
+
+foreach my $idx (0 .. $#xmas) {
+    if ($idx < 25) {
+        my $t = int($xmas[$idx]);
+        $preamble{$t} = 1;
+        push @also_queue, $t;
+    } else {
+        my $nextnum = int($xmas[$idx]);
+        my $tester = 0;
+        foreach my $num (keys %preamble) {
+            $tester++;
+            if (exists $preamble{$nextnum - $num}) {
+                my $old = shift @also_queue;
+                delete $preamble{$old};
+                push @also_queue, $nextnum;
+                $preamble{$nextnum} = 1;
+                last;
+            }
+        }
+
+        if (not scalar grep { $_ == $nextnum } @also_queue) {
+            $goalnum = $nextnum;
+            print("XMAS weak num: $goalnum\n");
+            last;
+        }
+    }
+}
+
+# find the contiguous set
+
+my @contiguous;
+my $total = 0;
+
+foreach my $curr (@xmas) {
+    if ($total + $curr < $goalnum) {
+        $total += $curr;
+        push @contiguous, $curr;
+    } elsif ($total + $curr > $goalnum) {
+        while ($total + $curr > $goalnum) {
+            my $evictee = shift @contiguous;
+            if (not defined $evictee) {
+                last;
+            }
+            $total -= $evictee;
+        }
+        push @contiguous, $curr;
+        $total += $curr;
+    }
+
+    if ($total == $goalnum) {
+        print(min(@contiguous) + max(@contiguous));
+        p @contiguous;
+        exit;
+    }
+}
+
+untie @xmas;