day6_final.py

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

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

# Keep track of whether the start-of-packet marker and the
# start-of-message marker have been found
packet_marker_found = False
message_marker_found = False

# 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)
    last_fourteen.append(ch)
    # If we have more than four characters, remove the first one
    if len(last_four) > 4:
        last_four.pop(0)
    if len(last_fourteen) > 14:
        last_fourteen.pop(0)

    # If the last four characters are all different and we haven't
    # already found the start-of-packet marker, we have found
    # a start-of-packet marker, so print the number of characters
    # processed and mark that we have found it
    if len(set(last_four)) == 4 and not packet_marker_found:
        print(f"First start-of-packet marker after {i+1} characters")
        packet_marker_found = True

    # If the last 14 characters are all different and we haven't
    # already found the start-of-message marker, we have found
    # a start-of-message marker, so print the number of characters
    # processed and mark that we have found it
    if len(set(last_fourteen)) == 14 and not message_marker_found:
        print(f"First start-of-message marker after {i+1} characters")
        message_marker_found = True

    # If we have found both markers, stop processing the input
    if packet_marker_found and message_marker_found:
        break