Day13a.rb

require 'set'

def fold(line, val)
  if val < line
    val
  elsif val > line
    line - (val - line)
  else
    raise "dot on line!"
  end
end

def print_dots(dots)
  width, height =
    dots.reduce([0, 0]) {|(px, py), (x, y)| [[px, x].max, [py, y].max]}
  (0..height).each do |y|
    (0..width).each do |x|
      print(dots.include?([x, y]) ? '#' : '.')
    end
    puts
  end
end

dots = Set.new

while gets =~ /(\d+),(\d+)/
  dots << [$1.to_i, $2.to_i]
end

while gets =~ /fold along ([xy])=(\d+)/
  line = $2.to_i
  case $1
  when 'x'
    dots.map! {|x, y| [fold(line, x), y]}
  when 'y'
    dots.map! {|x, y| [x, fold(line, y)]}
  end
end

print_dots(dots)