advent_1_part2.py

if __name__ == '__main__':
  with open('input.txt') as data:
    last = None
    increased = 0
    sum_window = 0
    window = []
    count = 0
    for line in data:
      value = int(line)
      sum_window += value
      # If we don't have the initial window just yet just add the number to the window.
      if len(window) < 3:
        window.append(value)
        if len(window) == 3:
          last = sum_window
      else:
        # We need to substract the last added element from the window to the sum
        # so that we get the current sum.
        sum_window -= window[count % 3]
        # Override last added element on the sliding window with the current value.
        window[count % 3] = value
        if sum_window > last:
          increased += 1
        last = sum_window
      count += 1
    print(increased)