Advent of Code Day 24

require 'set'

DIRS = [ [1,0], [0,1], [-1,0], [0,-1], [0,0] ]
WRAP = [ [:min,:same], [:same, :min], [:max,:same], [:same,:max] ]
ARROW = [ '>', 'v', '<', '^']

def show(grid, pos)
  msg = nil
  grid.each_with_index do |row, y|
    row.each_with_index do |cell, x|
      if pos == [x,y]
        c = 'E'
        msg = 'OVERLAP?!?!' unless cell.empty?
      elsif cell == '#'
        c = '#'
      elsif cell.size == 0
        c = '.'
      elsif cell.size == 1
        c = ARROW[cell[0]]
      else
        c = cell.size
      end
      print c
    end
    puts
  end
  puts msg unless msg.nil?
end

def step(grid)
  new_grid = Array.new(grid.size){ Array.new(grid[0].size){[]}}
  max = [new_grid[0].size - 2, new_grid.size - 2]

  grid.each_with_index do |row, y|
    row.each_with_index do |cell, x|
      if cell == '#'
        new_grid[y][x] = '#'
        next
      end

      cell.each do |b|
        n = [x + DIRS[b][0], y + DIRS[b][1]]
        if grid[n[1]][n[0]] == '#'
          [0,1].each do |i|
            case WRAP[b][i]
            when :min then n[i] = 1
            when :max then n[i] = max[i]
            end
          end
        end
        new_grid[n[1]][n[0]] << b
      end
    end
  end

  new_grid
end

def valid_moves(grid, pos)
  x,y = pos
  maxx, maxy = [grid[0].size-1, grid.size-1]
  valids = []
  DIRS.each do |d|
    nx,ny = x+d[0], y+d[1]
    next if nx < 0 || ny < 0 || nx > maxx || ny > maxy
    valids << [nx,ny] if grid[ny][nx].empty?
  end

  Set.new(valids)
end

grid = File.open('input_day24.txt', 'r').readlines.map do |line|
  line.strip.split('').map do |cell|
    if cell == '#'
      t = cell
    else
      t = []
      t << ARROW.index(cell) if ARROW.include? cell
    end
    t
  end
end

goal = [grid[0].size-2, grid.size-1]
pos = [[1,0]]

goals = [goal, [1,0], goal]

n = 0
while !goals.empty?
  grid = step(grid)
  newpos = Set.new
  pos.each {|p| newpos = newpos.union(valid_moves(grid, p))}

  if newpos.reduce(false){|v,p| v || p == goals[0]}
    puts "FOUND A WAY TO GOAL #{goals[0]}! (TURN #{n+1})"
    pos = [goals.shift]
    n += 1
    next
  end

  newpos.each do |np|
    x,y = np
    puts "WAIT A MINUTE, THERE'S AN OVERLAP" unless grid[y][x].empty?
  end

  pos = newpos
  n += 1
end