2023: Day 11, part 2

2023/day11.rb

@@ -1,9 +1,10 @@
+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 == "." } }
+  data.each_with_index { |row, i| expand << i if row.all? { |c| c == "." || c == "," } }
   expand
 end
 
@@ -16,16 +17,17 @@ end
 # do expansion
 row_expand = needs_expand(by_row)
 row_expand.reverse.each do |i|
-  by_row.insert(i, by_row[i].dup)
+  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.insert(i, by_column[i].dup)
+  by_column[i] = ("," * by_column[i].length).split("") # add expansion marker
 end
 
 cosmos = by_column.transpose
+print_cosmos(cosmos)
 
 # find galaxies
 galaxies = []
@@ -39,4 +41,32 @@ def galaxy_distance(a, b)
   (a[0] - b[0]).abs + (a[1] - b[1]).abs
 end
 
-puts "Part 1: #{galaxies.combination(2).map { |a, b| galaxy_distance(a, b) }.sum}"
+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}"