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

dots = Set.new

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

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

print dots.count