03.py

#!/usr/bin/env python3
from collections import namedtuple
from typing import List


# common functions
def read_data():
    with open(__file__, encoding="utf-8") as f:
        c = f.read()
        data = c[c.rindex("🎅") + 1: c.rindex("⛄")].rstrip()
    return data


def parse(_input: str):
    return [l for l in _input.split("\n") if len(l) > 0]


# solution
Slope = namedtuple('Slope', ['right', 'down'])


def count_trees(matrix: list, sl: Slope) -> int:
    # the same pattern repeats to the right many times - multiply each line by the slope
    _matrix = [str(line * int(sl.right * len(line) / sl.down)) for line in matrix]

    h = len(_matrix)
    w = len(_matrix[0])
    counter, i, j = 0, sl.down, sl.right

    while i < h and j < w:
        if _matrix[i][j] == '#':  # that's a tree
            counter += 1
        i += sl.down
        j += sl.right
    return counter


def count_trees_for_slopes(_matrix: List[str], _slopes: List[Slope]) -> int:
    counter = 1
    for sl in _slopes:
        counter *= count_trees(_matrix, sl)
    return counter


# test
test_input = """
..##.......
#...#...#..
.#....#..#.
..#.#...#.#
.#...##..#.
..#.##.....
.#.#.#....#
.#........#
#.##...#...
#...##....#
.#..#...#.#
"""
forest = parse(test_input)

# test 1
part_one = count_trees(forest, Slope(3, 1))
assert part_one == 7

# test 2
SLOPES_TWO = [Slope(1, 1), Slope(3, 1), Slope(5, 1), Slope(7, 1), Slope(1, 2)]
part_two = count_trees_for_slopes(forest, SLOPES_TWO)
assert part_two == 336

# given
forest = parse(read_data())
# solve
part_one = count_trees(forest, Slope(3, 1))
print(part_one)
assert part_one == 250

part_two = count_trees_for_slopes(forest, SLOPES_TWO)
print(part_two)
assert part_two == 1592662500

# input
"""🎅
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⛄"""