histories = ARGF.readlines.map { _1.scan(/-?\d+/).map(&:to_i) }

# part one
p histories
  .map {|history|
    diffs = [history]
    until diffs.last.all?(&:zero?)
      diffs << diffs.last.each_cons(2).map { _2 - _1 }
    end

    diffs.sum(&:last)
  }
  .sum

# part two
p histories
  .map {|history|
    diffs = [history]
    until diffs.last.all?(&:zero?)
      diffs << diffs.last.each_cons(2).map { _2 - _1 }
    end

    diffs.map(&:first).reverse.inject { _2 - _1 }
  }
  .sum