Advent of Code 2023 Day 14

So much for the breather. Today asks you to simulate the motion of round rocks in a grid. Each round rock should move upward until it hits another rock or the edge of the space. Part 1 is a little fiddly, but simple enough to solve using a dict of complex numbers to represent coordinates, with an enum type for the different types of rock.

Part 2 asks you to simulate a full “cycle” of rock movements - up, left, down, then right - 1000000000 times. When Advent of Code asks you to simulate something a billion times, it’s really asking you to simulate a few times until you detect a cycle, then do the arithmetic to determine the outcome after a billion iterations. I messed up some of the fiddly math at first, but got it eventually:

def total_load(grid, offset):
    grid = dict(grid)
    rounded = Coordinate.ROUND
    closed = {rounded, Coordinate.CUBE}
    load = 0
    keys = {
        1j: lambda x: (-x.imag, x.real),
        -1: lambda x: (x.real, x.imag),
        -1j: lambda x: (x.imag, x.real),
        1: lambda x: (-x.real, x.imag),
    }

    for coord in sorted(grid.keys(), key=keys[offset]):
        el = grid[coord]

        if el == rounded:
            current = coord
            grid[current] = Coordinate.OPEN
            while True:
                new = current + offset
                if new not in grid or grid[new] in closed:
                    grid[current] = rounded
                    load += current.imag + 1
                    break
                current = new

    return grid, int(load)


def predict(grid):
    results = OrderedDict()
    offsets = (1j, -1, -1j, 1)
    load = 0

    while True:
        for offset in offsets:
            grid, load = total_load(grid, offset)
        hash = frozenset(zip(grid.keys(), grid.values()))
        if hash in results:
            print(load)
            return results, tuple(i for i, k in enumerate(results.keys()) if k == hash)
        results[hash] = load

2022 day 17 was a harder version of this puzzle. And day 17 is in 3 days - on a weekend, no less…