2023: Day 11, part 2

2023/day11.rb

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+MULTIPLIER = ENV["MULTIPLIER"].to_i || 2
+input = ARGF.read.strip
+by_row = input.lines.map(&:rstrip).map { |line| line.split("") }
+
+def needs_expand(data)
+  expand = []
+  data.each_with_index { |row, i| expand << i if row.all? { |c| c == "." || c == "," } }
+  expand
+end
+
+def print_cosmos(cosmos)
+  cosmos.each do |line|
+    puts line.join("")
+  end
+end
+
+# do expansion
+row_expand = needs_expand(by_row)
+row_expand.reverse.each do |i|
+  by_row[i] = ("," * by_row[i].length).split("") # add expansion marker
+end
+
+by_column = by_row.transpose
+col_expand = needs_expand(by_column)
+col_expand.reverse.each do |i|
+  by_column[i] = ("," * by_column[i].length).split("") # add expansion marker
+end
+
+cosmos = by_column.transpose
+print_cosmos(cosmos)
+
+# find galaxies
+galaxies = []
+(0...cosmos.first.length).each do |x|
+  (0...cosmos.length).each do |y|
+    galaxies << [x, y] if cosmos[y][x] == "#"
+  end
+end
+
+def galaxy_distance(a, b)
+  (a[0] - b[0]).abs + (a[1] - b[1]).abs
+end
+
+def cosmos_at(x, y, cosmos)
+  cosmos[y][x]
+end
+
+distances = galaxies.combination(2).map do |combo|
+  a, b = combo.sort
+  distance = galaxy_distance(a, b)
+
+  # see how many millions are crossed in x
+  x_millions = (a[0]..b[0]).filter_map do |x|
+    loc = cosmos_at(x, a[1], cosmos)
+    loc if loc == ","
+  end
+
+  # now y component
+  range = if a[1] > b[1]
+      b[1]..a[1]
+    else
+      a[1]..b[1]
+    end
+  y_millions = range.filter_map do |y|
+    loc = cosmos_at(b[0], y, cosmos)
+    loc if loc == ","
+  end
+
+  distance + ((x_millions.length + y_millions.length) * MULTIPLIER) - x_millions.length - y_millions.length
+end
+
+puts "Sum of distances with multiplier #{MULTIPLIER}: #{distances.sum}"

2023/input/day11.sample

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+...#......
+.......#..
+#.........
+..........
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+..........
+.......#..
+#...#.....
+

2023/input/day11.txt

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