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diff --git a/2020/day-11/day-11.partial.raku b/2020/day-11/day-11.partial.raku
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+++ b/2020/day-11/day-11.partial.raku
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+#!/usr/bin/env raku
+
+sub MAIN (
+    Int $part where * == 1|2 = 1 #= part to run (1 or 2)
+) {
+    my @input = "input".IO.lines;
+    my Int $x-max = @input[0].chars - 1;
+    my Int $row-length = $x-max + 1;
+
+    my @seats = @input.join.comb;
+    my $max-seats = @seats.end;
+
+    my @directions = (+$row-length, -$row-length); # down, up
+
+    # @non-left-corner contains directions that left corner can't
+    # follow. It should only be followed by non-left corner seats.
+    my @non-left-corner = (
+        -$row-length - 1, # up-left
+        -1, # left
+        +$row-length - 1, # down-left
+    );
+
+    # @non-right-corner contains directions that right corner can't
+    # follow. It should only be followed by non-right corner seats.
+    my @non-right-corner = (
+        -$row-length + 1, # up-right
+        +1, # right
+        +$row-length + 1, # down-right
+    );
+
+    my Int $round = 0;
+    loop {
+        my @changed;
+        my Bool $change = False;
+
+        INNER: for @seats.kv -> $idx, $seat {
+            @changed[$idx] = $seat;
+            given $seat {
+                when '.' { next INNER; }
+                when 'L' {
+                    if adjacent-occupied($idx, 1) == 0 {
+                        @changed[$idx] = '#';
+                        $change = True;
+                    }
+                }
+                when '#' {
+                    if adjacent-occupied($idx, 1) >= 4 {
+                        @changed[$idx] = 'L';
+                        $change = True;
+                    }
+                }
+            }
+        }
+        $round++;
+        print "Round $round.\r";
+
+        last unless $change;
+
+        for @changed.kv -> $idx, $changed_seat {
+            @seats[$idx] = $changed_seat;
+        }
+    }
+
+    my Int $occupied = @seats.comb('#').elems;
+    say "Part $part: ", $occupied;
+
+    # adjacent-occupied returns the number of adjacent cells that have
+    # been occupied by others. $visibility_range should be 1 if only
+    # directly adjacent seats are to be counted. Make it -1 for
+    # infinite visibility. It ignores floors ('.').
+    sub adjacent-occupied (
+        Int $idx, Int $visibility_range --> Int
+    ) {
+        my Int $occupied = 0;
+
+        for @directions -> $direction {
+            with @seats[$idx + $direction] {
+                $occupied++ if $_ eq '#';
+            }
+        }
+
+        # Elements in right corner can't follow @non-right-corner.
+        unless ($idx + 1) % 10 == 0 {
+            for @non-right-corner -> $direction {
+                with @seats[$idx + $direction] {
+                    $occupied++ if $_ eq '#';
+                }
+            }
+        }
+
+        # Elements in left corner can't follow @non-left-corner.
+        unless $idx % 10 == 0 {
+            for @non-left-corner -> $direction {
+                with @seats[$idx + $direction] {
+                    $occupied++ if $_ eq '#';
+                }
+            }
+        }
+
+        return $occupied;
+    }
+}