Day15b.rb

require 'set'

# i had to learn dijkstra's algo for this...
class Cell
  def initialize(cave, x, y)
    @cave = cave
    @x = x
    @y = y
  end

  attr_reader :x, :y

  def risk
    @cave.risk_at(x, y)
  end

  def neighbors
    @cave.neighbors_at(x, y)
  end

  def unvisited?
    !@cave.total_risk_at(x, y)
  end
end

class Cave
  def initialize(map)
    @map = map
    @height = map.length
    @width = map[0].length
    @total_risk = {}
    @estimates = {[0, 0] => 0}
  end

  attr_reader :width, :height

  def [](x, y)
    Cell.new(self, x, y)
  end

  def risk_at(x, y)
    @map[y][x]
  end

  def neighbors_at(x, y)
    ns = []
    ns << self[x - 1, y] if x > 0
    ns << self[x + 1, y] if x + 1 < width
    ns << self[x, y - 1] if y > 0
    ns << self[x, y + 1] if y + 1 < height
    ns
  end

  def total_risk_at(x, y)
    @total_risk[[x, y]]
  end

  def explore
    until @estimates.empty?
      (x, y), tot = @estimates.min_by {|k, v| v}
      @estimates.delete [x, y]
      @total_risk[[x, y]] = tot
      cur = self[x, y]
      cur.neighbors.filter(&:unvisited?).each do |n|
        thru_cur = tot + n.risk
        @estimates[[n.x, n.y]] = [@estimates[[n.x, n.y]], thru_cur].compact.min
      end
    end
  end
end

def fiveify(grid)
  step1 = grid.map {|row| (0..4).flat_map {|o| row.map {|n| (n - 1) + o}}}
  step2 = (0..4).flat_map {|o| step1.map {|row| row.map {|n| (n + o) % 9 + 1}}}
end

grid = $<.map {|l| l.chomp.chars.map(&:to_i)}
grid = fiveify(grid)
cave = Cave.new(grid)
cave.explore
p cave.total_risk_at(cave.width - 1, cave.height - 1)