day19_an.rb

#!/usr/bin/env ruby

# http://adventofcode.com/2017/day/19

# slurp the input into a 2d array
ARGF.readlines.map(&:chars).tap{|m|
  # assign some variables:
  # v = visited cells
  # d = current direction
  # c = current position
  # s = number of steps
  v,d,c,s=[],[1,0],[0,m[0].find_index{|x|x!=" "}],1

  # get going
  loop{
    # visit the current cell
    v << m.dig(*c)

    # lots of ugly stuff happening here
    # 1. decide possible moves:
    #    (d[0].zero?? [d,[1,0],[-1,0]]:[d,[0,1],[0,-1]])
    #    if d[0] is zero, we're moving on the Y axis, otherwise the X axis,
    #    so we can plan our next possible moves
    # 2. Find the first move that contains a value in the cell
    #    find{|(a,b)|m.dig(c[0]+a,c[1]+b)!=" "}
    # 3. Break from our loop if no possible moves have a value
    #    and s+= 1 or break
    #    and also increment the step counter
    d=(d[0].zero?? [d,[1,0],[-1,0]]:[d,[0,1],[0,-1]]).find{|(a,b)|m.dig(c[0]+a,c[1]+b)!=" "}and s+=1 or break

    # move position
    c=[c[0] + d[0], c[1] + d[1]]
  }||

  # print the result
  p(v.grep(/[A-Z]+/).join,s)
}