Day8b.rb

require 'set'

def group(obs_raw)
  obs = Hash.new {Set.new("abcdefg".chars)}
  obs_raw.each do |word|
    obs[word.length] &= Set.new(word.chars)
  end
  obs
end

def decode(obs)
  cf = obs[2]
  acf = obs[3]
  bcdf = obs[4]
  adg = obs[5]
  abfg = obs[6]
  abcdefg = obs[7]

  a = acf - cf
  b = bcdf - cf - adg
  d = bcdf - b - acf
  e = abcdefg - acf - bcdf - abfg
  f = abfg & cf
  c = cf - f
  g = adg - a - d

  {a.first => 'a', b.first => 'b', c.first => 'c', d.first => 'd',
   e.first => 'e', f.first => 'f', g.first => 'g'}
end

def char_set(s)
  Set.new(s.chars)
end

DISPLAY = {
  char_set('abcefg') => '0',
  char_set('cf') => '1',
  char_set('acdeg') => '2',
  char_set('acdfg') => '3',
  char_set('bcdf') => '4',
  char_set('abdfg') => '5',
  char_set('abdefg') => '6',
  char_set('acf') => '7',
  char_set('abcdefg') => '8',
  char_set('abcdfg') => '9',
}

total = 0
$<.each do |l|
  obs_raw, outs = l.chomp.split(' | ').map(&:split)
  tbl = decode(group(obs_raw))
  digits = outs.map do |out|
    real_out = out.chars.map {|c| tbl[c]}
    digit = DISPLAY[Set.new(real_out)]
  end
  total += digits.join.to_i
end
print total