day6_take2.py

# Parse the input
data = open('input.txt').read().strip()

# Keep track of the last four characters received
last_four = []

# Process each character in the data
for i, ch in enumerate(data):
    # Add the character to the list of last four characters
    last_four.append(ch)
    # If we have more than four characters, remove the first one
    if len(last_four) > 4:
        last_four.pop(0)

    # If the last four characters are all different, we have found
    # a start-of-packet marker, so print the number of characters
    # processed and stop
    if len(set(last_four)) == 4:
        print(i+1)
        break