Advent of code 2021 - Day 3

day03.rb

# frozen_string_literal: true

require 'minitest/autorun'
require_relative 'input_data'

class InputData
  @file = __FILE__
  def parse
    raw_data.lines.map { |line| line.scan(/\d/) }
  end
end


# This is horrible and confusing
def part_one(parsed)
  freq = parsed.transpose.map { |row| row.group_by { |x| x }.transform_values(&:size).invert }

  something = freq.map do |f|
    f.minmax.map(&:last)
  end.transpose.map { |v| v.join.to_i(2) }.reduce(:*)
end

def part_two(parsed)
  oxygen_rating(parsed) * co2_scrubber_rating(parsed)
end

def oxygen_rating(parsed)
  candidates = parsed.dup
  guard = candidates.first.size
  n = 0

  while candidates.size > 1 && guard
    # Find the most common bit in position n
    mcv = most_common_value(candidates, n, tiebreak: "1")
    candidates.select! { |candidate| candidate[n] == mcv }
    n += 1
    guard -= 1
  end

  candidates.first.join.to_i(2)
end

def co2_scrubber_rating(parsed)
  candidates = parsed.dup
  guard = candidates.first.size
  n = 0

  while candidates.size > 1 && guard
    # Find the most common bit in position n
    mcv = least_common_value(candidates, n)
    candidates.select! { |candidate| candidate[n] == mcv }
    n += 1
    guard -= 1
  end

  candidates.first.join.to_i(2)
end

def most_common_value(parsed, position, tiebreak: "1")
  ones = parsed.count { |row| row[position] == "1" }
  zeros = parsed.size - ones
  { -1 => "0", 0 => tiebreak, 1 => "1"}[ones <=> zeros]
end

def least_common_value(parsed, position, tiebreak: "0")
  ones = parsed.count { |row| row[position] == "1" }
  zeros = parsed.size - ones
  { 1 => "0", 0 => tiebreak, -1 => "1"}[ones <=> zeros]
end

class TestDay < Minitest::Test
  EXAMPLE_ONE = <<~LINES
  00100
  11110
  10110
  10111
  10101
  01111
  00111
  11100
  10000
  11001
  00010
  01010
  LINES

  def test_part_one_example
    data = InputData.new(EXAMPLE_ONE).parse
    assert_equal 198, part_one(data)
  end

  def test_part_one_answer
    data = InputData.new.parse
    assert_equal 775304, part_one(data)
  end

  def test_example_oxygen_rating
    data = InputData.new(EXAMPLE_ONE).parse
    assert_equal 23, oxygen_rating(data)
  end

  def test_example_co2_scrubber_rating
    data = InputData.new(EXAMPLE_ONE).parse
    assert_equal 10, co2_scrubber_rating(data)
  end

  def test_part_two_example
    data = InputData.new(EXAMPLE_ONE).parse
    assert_equal 230, part_two(data)
  end

  def test_part_two_answer
    data = InputData.new.parse
    assert_equal 1370737, part_two(data)
  end
end