Lazy AOC 2021

lazy-aoc-2021-irb.rb

# No gems, just plain Ruby 3. Assumes inputs are in an inputs folder or something.
# Each line is a line or multiline input in an IRB prompt.

# Day 1
values = IO.readlines("inputs/1.txt").map(&:to_i)

# Part 1
values.each_cons(2).filter_map { |(a, b)| b > a }.count

# Part 2
values.each_cons(3).each_slice(2).flat_map { |(a, b)| [a.sum, b.sum] }.each_cons(2).filter_map { |(a, b)| b > a }.count

# ----

# Day 2
values = IO.readlines("inputs/2.txt")

# Part 1
totals = values.map { |v| v.split(" ") }.each.with_object(Hash.new(0)) { |v, o| o[v[0]] += v[1].to_i }
totals["forward"] * (totals["down"] - totals["up"])

# Part 2
totals = values.each.with_object({x: 0, y: 0, a: 0}) do |s, o|
  i, v = s.split(" ", 2)
  v = v.to_i
  case i
  when "forward"
    o[:x] += v
    o[:y] += v * o[:a]
  when "up"
    o[:a] -= v
  when "down"
    o[:a] += v
  end
end
totals[:x] * totals[:y]

# ---

# Day 3
values = IO.readlines("inputs/3.txt", chomp: true).map { |v| v.chars.map(&:to_i) }


# Part 1
g = values.transpose.map { |v| v.sum > (v.size / 2.0) ? 1 : 0 }
e = g.map { |v| v.zero? ? 1 : 0 }
g.join.to_i(2) * e.join.to_i(2)

# Part 2
def search(data:, bit_criteria:, index: 0)
  counts = data.transpose[index].tally

  digit =
    if bit_criteria == :o2
      counts[0].to_i <= counts[1].to_i ? 1 : 0
    else
      counts[0].to_i <= counts[1].to_i ? 0 : 1
    end

  data = data.select do |val|
    val[index] == digit
  end

  return data.first if data.one?

  search(data: data, bit_criteria: bit_criteria, index: index + 1)
end

o2 = search(data: values, bit_criteria: :o2)
co2 = search(data: values, bit_criteria: :co2)

puts o2.join.to_i(2) * co2.join.to_i(2)

# ---

# Day 4

# Part 1