# --- Day 5: Binary Boarding ---

# You board your plane only to discover a new problem: you dropped your
# boarding pass! You aren't sure which seat is yours, and all of the flight
# attendants are busy with the flood of people that suddenly made it through
# passport control.

# You write a quick program to use your phone's camera to scan all of the
# nearby boarding passes (your puzzle input); perhaps you can find your seat
# through process of elimination.

# Instead of zones or groups, this airline uses binary space partitioning to
# seat people. A seat might be specified like FBFBBFFRLR, where F means
# "front", B means "back", L means "left", and R means "right".

# The first 7 characters will either be F or B; these specify exactly one of
# the 128 rows on the plane (numbered 0 through 127). Each letter tells you
# which half of a region the given seat is in. Start with the whole list of
# rows; the first letter indicates whether the seat is in the front (0 through
# 63) or the back (64 through 127). The next letter indicates which half of
# that region the seat is in, and so on until you're left with exactly one row.

# For example, consider just the first seven characters of FBFBBFFRLR:

#     Start by considering the whole range, rows 0 through 127.
#     F means to take the lower half, keeping rows 0 through 63.
#     B means to take the upper half, keeping rows 32 through 63.
#     F means to take the lower half, keeping rows 32 through 47.
#     B means to take the upper half, keeping rows 40 through 47.
#     B keeps rows 44 through 47.
#     F keeps rows 44 through 45.
#     The final F keeps the lower of the two, row 44.

# The last three characters will be either L or R; these specify exactly one of
# the 8 columns of seats on the plane (numbered 0 through 7). The same process
# as above proceeds again, this time with only three steps. L means to keep the
# lower half, while R means to keep the upper half.

# For example, consider just the last 3 characters of FBFBBFFRLR:

#     Start by considering the whole range, columns 0 through 7.
#     R means to take the upper half, keeping columns 4 through 7.
#     L means to take the lower half, keeping columns 4 through 5.
#     The final R keeps the upper of the two, column 5.

# So, decoding FBFBBFFRLR reveals that it is the seat at row 44, column 5.

# Every seat also has a unique seat ID: multiply the row by 8, then add the
# column. In this example, the seat has ID 44 * 8 + 5 = 357.

# Here are some other boarding passes:

#     BFFFBBFRRR: row 70, column 7, seat ID 567.
#     FFFBBBFRRR: row 14, column 7, seat ID 119.
#     BBFFBBFRLL: row 102, column 4, seat ID 820.

# As a sanity check, look through your list of boarding passes. What is the
# highest seat ID on a boarding pass?

with open("files/P5.txt", "r") as f:
    boarding_passes = [code for code in f.read().strip().split("\n")]


def binary_search(
    arr: str, upper_limit: int, var_lower: str, var_upper: str
) -> int:
    range_l = 0
    range_h = upper_limit
    range_m = 0
    for char in arr:
        range_m = (range_h + range_l) // 2
        if char == var_lower:
            range_h = range_m
        elif char == var_upper:
            range_l = range_m + 1
    return int(range_l)


def part_1() -> None:
    seat_ID: int = 0
    for code in boarding_passes:
        row, col = code[:7], code[-3:]

        _row = binary_search(row, 127, "F", "B")
        _col = binary_search(col, 7, "L", "R")
        _seat_ID: int = _row * 8 + _col
        seat_ID = max(seat_ID, _seat_ID)
    print(f"The highest seat ID on a boarding pass is {seat_ID}")


if __name__ == "__main__":
    part_1()