advent_2_part1.py

if __name__ == '__main__':
  with open('input.txt') as data:
    one_counts = []
    total = 0
    for line in data:
      line = line.strip()
      if len(one_counts) == 0:
        one_counts = [0] * len(line)
      for idx, char in enumerate(line):
        if char == '1':
          # One counts is considered to be aligned with less significant bit
          # to most significant. Since input is from most significant to less
          # significant we need to flip the direction when filling one_counts.
          one_counts[len(one_counts) - 1 - idx] += 1
      total += 1
    gamma = 0
    # one_counts contains the counts of 1s on each bit position.
    # To calculate gamma we need to get a binary number formed by the
    # most common digit on each position, so if one_count at index X
    # is >= than half the numbers the bit on that position is 1, otherwise
    # we skip it since the most common digit is 0.
    for idx, count in enumerate(one_counts):
      if count >= total / 2:
        gamma += 2 ** idx
    # Epsilon composed by the less common bits, so we only need to negate
    # gamma and apply a mask on the bit leght (2^length - 1).
    epsilon = ~gamma & (2 ** (len(one_counts)) - 1)
    print(gamma, epsilon, gamma * epsilon)