Solve 2023-21 part 1

2023/21/aoc.in

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+...........
+.....###.#.
+.###.##..#.
+..#.#...#..
+....#.#....
+.##..S####.
+.##..#...#.
+.......##..
+.##.#.####.
+.##..##.##.
+...........

2023/21/sol1.rb

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+require 'set'
+
+$stdout = File.open('aoc.out', 'w')
+grid = File.open('aoc.in') do |f|
+  f.readlines.map do |line|
+    line.chomp.chars
+  end
+end
+
+def find_start(grid)
+  grid.size.times.each do |i|
+    grid[i].size.times.each do |j|
+      if grid[i][j] == 'S'
+        return [i, j]
+      end
+    end
+  end
+end
+
+start = find_start(grid)
+
+def bfs(grid, start)
+  reachable = Set.new
+  reachable << start
+  64.times do
+    new_reachable = Set.new
+    reachable.each do |x, y|
+      [[-1, 0], [1, 0], [0, -1], [0, 1]].each do |dx, dy|
+        next if grid[x + dx][y + dy] == '#'
+        next unless (0...grid.size).cover?(x + dx) && (0...grid[0].size).cover?(y + dy)
+        new_reachable << [x + dx, y + dy]
+      end
+    end
+    reachable = new_reachable
+  end
+
+  reachable.size
+end
+
+p bfs(grid, start)

2023/21/sol2.rb

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+require 'set'
+
+$stdout = File.open('aoc.out', 'w')
+grid = File.open('aoc.in') do |f|
+  f.readlines.map do |line|
+    line.chomp.chars
+  end
+end
+
+def find_start(grid)
+  grid.size.times.each do |i|
+    grid[i].size.times.each do |j|
+      if grid[i][j] == 'S'
+        return [i, j]
+      end
+    end
+  end
+end
+
+start = find_start(grid)
+N = 26501365
+
+# for repeats of the map that are not in the same (inter-map) row or column as
+# the starting point, it is always sufficient to move to the nearest corner from
+# the starting map, then move around in the sidewalks, then move from the corner
+# to the ending position
+
+def bfs(grid, start)
+  this = Set.new
+  this << start
+  last = Set.new                # to keep track of parity
+  frontier = Set.new
+  frontier << start
+
+  5000.times do
+    new_reachable = Set.new
+    reachable.each do |x, y|
+      [[-1, 0], [1, 0], [0, -1], [0, 1]].each do |dx, dy|
+        next if grid[(x + dx) % grid.size][(y + dy) % grid[0].size] == '#'
+        next if reached_last_time.include?([x + dx, y + dy])
+        new_reachable << [x + dx, y + dy]
+      end
+    end
+
+    # p new_reachable.size - reachable.size
+    reached_last_time = reachable
+    reachable = new_reachable
+    # p reachable.size
+  end
+
+  reachable.size
+end
+
+p bfs(grid, start)