1 files changed, 48 insertions, 0 deletions
diff --git a/2020/day-10/day-10.raku b/2020/day-10/day-10.raku
new file mode 100755
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+++ b/2020/day-10/day-10.raku
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+#!/usr/bin/env raku
+
+sub MAIN (
+    Int $part where * == 1|2 = 1 #= part to run (1 or 2)
+) {
+    my $solution;
+
+    # Initialize @adapters with charging outlet joltage. @adapters
+    # will hold joltages of each adapter.
+    my @adapters = 0;
+    append @adapters, "input".IO.lines>>.Int.sort;
+    push @adapters, @adapters[*-1] + 3; # push the built in joltage.
+
+    if $part == 1 {
+        my Int ($diff_1, $diff_3);
+
+        for 0 .. @adapters.end - 1 -> $idx {
+            # joltage difference.
+            my $diff = @adapters[$idx + 1] - @adapters[$idx];
+            $diff_1++ if $diff == 1;
+            $diff_3++ if $diff == 3;
+        }
+        $solution = $diff_1 * $diff_3;
+    } elsif $part == 2 {
+        my @memoize;
+        # complete-chain returns the number of ways to complete the
+        # chain given that you're currently at @adapters[$idx]. This
+        # is taken from Jonathan Paulson's solution.
+        sub complete-chain (
+            Int $idx
+        ) {
+            return 1 if $idx == @adapters.end;
+            return @memoize[$idx] if @memoize[$idx];
+
+            my Int $ways;
+            for $idx + 1 .. @adapters.end {
+                if @adapters[$_] - @adapters[$idx] <= 3 {
+                    $ways += complete-chain($_);
+                }
+            }
+            @memoize[$idx] = $ways;
+            return $ways;
+        }
+        $solution = complete-chain(0);
+    }
+
+    say "Part $part: ", $solution;
+}