Dive!

Part 1

At first read, this seems very simple. Turns out, it is! There’s probably a clever way to do it, but for something like this, I find that “clear” is best.

We need 2 variables: depth and horiz to track our position. Then we go through each line and split on the space. A little if/else block lets us choose which value to edit and in which direction. Then we multiply the results:

depth = 0
horiz = 0

for ins in self.input:
    direction, value_str = ins.split(" ")
    value = int(value_str)
    if direction == "forward":
        horiz += value
    elif direction == "up":
        depth -= value
    elif direction == "down":
        depth += value
    else:
        raise ValueError(f'Unrecognized direction: "{direction}"')

return depth * horiz

The one thing to note is that we could have done else instead of elif direction == "down". Our input only has the 3 directions, so we’d be safe doing that. In the context of the puzzle, that’s totally fine. In real life, I’ve found that pattern to be a common source of errors. If our input ever did add a 4th direction, then it would silently fall through to down’s block and cause weird bugs. So, if we get something we don’t have an explicit case for, I’d rather error out than build on incomplete assumptions.

Part 2

In an unusual move, part 2 needs only a couple of lines changed from part 1 to work. There’s not much to say here- we’ll add a third variable and slightly change what we do on each action. I’ve marked the lines that changed:

depth = 0
horiz = 0
aim = 0 # added

for ins in self.input:
    direction, value_str = ins.split(" ")
    value = int(value_str)
    if direction == "forward":
        horiz += value
        depth += aim * value # added
    elif direction == "up":
        aim -= value # changed
    elif direction == "down":
        aim += value # changed
    else:
        raise ValueError(f'Unrecognized direction: "{direction}"')

return depth * horiz

A final note - if you’re running Python 3.10+, you can take advantage of “structural pattern matching” (lengthy description). It doesn’t change much about today’s solution, but instead of saying “split the string and give me the direction and value”, you can match against the shape of the input. So it’s more like “if ins.split is the word “forward” followed by any second item, do this”. Here’s that in practice:

...

for ins in self.input:
    match ins.split(" "):
        case "forward", value:
            horiz += int(value)
        case "up", value:
            ...

Again, not super life changing here. But, hopefully we’ll get a chance to do something else with it in a future day!