[Advent of Code 2021] Day 2 | Dive!

Part 1

Now, you need to figure out how to pilot this thing.

It seems like the submarine can take a series of commands like forward 1, down 2, or up 3:

• forward X increases the horizontal position by X units.
• down X increases the depth by X units.
• up X decreases the depth by X units.

Note that since you're on a submarine, down and up affect your depth, and so they have the opposite result of what you might expect.

The submarine seems to already have a planned course (your puzzle input). You should probably figure out where it's going. For example:

forward 5
down 5
forward 8
up 3
down 8
forward 2

Your horizontal position and depth both start at 0. The steps above would then modify them as follows:

• forward 5 adds 5 to your horizontal position, a total of 5.
• down 5 adds 5 to your depth, resulting in a value of 5.
• forward 8 adds 8 to your horizontal position, a total of 13.
• up 3 decreases your depth by 3, resulting in a value of 2.
• down 8 adds 8 to your depth, resulting in a value of 10.
• forward 2 adds 2 to your horizontal position, a total of 15.

After following these instructions, you would have a horizontal position of 15 and a depth of 10. (Multiplying these together produces _150_.)

Calculate the horizontal position and depth you would have after following the planned course. What do you get if you multiply your final horizontal position by your final depth?

Proposed solution: iterate through the input while keeping track of position and depth

Time complexity: O(n)

Space complexity: O(1)

#!/usr/bin/env python3
import sys

if len(sys.argv) != 2:
    print("Usage: {} <input file>".format(sys.argv[0]))
    sys.exit(1)

file_input = open(sys.argv[1], "r").read().strip().split("\n")
data = list(map(lambda x: x.split(" "), file_input))

position, depth = 0, 0
for instruction in data:
    instruction_type = instruction[0]
    instruction_val = int(instruction[1])
    if instruction_type == "forward":
        position += instruction_val
    elif instruction_type == "up":
        depth -= instruction_val
    elif instruction_type == "down":
        depth += instruction_val

print("Final position * depth = " + str(position * depth))

In this case, I made a secondary list which contains space-delimited lists for each line. Memory is not so important just yet. This can be done in-place though.

#!/usr/bin/env python3
import sys

if len(sys.argv) != 2:
    print("Usage: {} <input file>".format(sys.argv[0]))
    sys.exit(1)

file_input = open(sys.argv[1], "r").read().strip().split("\n")

position, depth = 0, 0
for instruction in file_input:
    instruction_type, instruction_val = instruction.split()
    instruction_val = int(instruction_val)
    if instruction_type == "forward":
        position += instruction_val
    elif instruction_type == "up":
        depth -= instruction_val
    elif instruction_type == "down":
        depth += instruction_val

print("Final position * depth = " + str(position * depth))

❯ python3 solution2.py input2
Final position * depth = 2120749

Part 2

Based on your calculations, the planned course doesn't seem to make any sense. You find the submarine manual and discover that the process is actually slightly more complicated.

In addition to horizontal position and depth, you'll also need to track a third value, aim, which also starts at 0. The commands also mean something entirely different than you first thought:

• down X increases your aim by X units.
• up X decreases your aim by X units.
• forward X does two things:
• It increases your horizontal position by X units.
• It increases your depth by your aim multiplied by X.

Again note that since you're on a submarine, down and up do the opposite of what you might expect: "down" means aiming in the positive direction.

Now, the above example does something different:

• forward 5 adds 5 to your horizontal position, a total of 5. Because your aim is 0, your depth does not change.
• down 5 adds 5 to your aim, resulting in a value of 5.
• forward 8 adds 8 to your horizontal position, a total of 13. Because your aim is 5, your depth increases by 8*5=40.
• up 3 decreases your aim by 3, resulting in a value of 2.
• down 8 adds 8 to your aim, resulting in a value of 10.
• forward 2 adds 2 to your horizontal position, a total of 15. Because your aim is 10, your depth increases by 2*10=20 to a total of 60.

After following these new instructions, you would have a horizontal position of 15 and a depth of 60. (Multiplying these produces _900_.)

Using this new interpretation of the commands, calculate the horizontal position and depth you would have after following the planned course. What do you get if you multiply your final horizontal position by your final depth?

Proposed solution: same thing but new calculations while keeping track of aim

Time complexity: O(n)

Space complexity: O(1)

#!/usr/bin/env python3
import sys

if len(sys.argv) != 2:
    print("Usage: {} <input file>".format(sys.argv[0]))
    sys.exit(1)

file_input = open(sys.argv[1], "r").read().strip().split("\n")

position, depth, aim = 0, 0, 0
for instruction in file_input:
    instruction_type, instruction_val = instruction.split()
    instruction_val = int(instruction_val)
    if instruction_type == "forward":
        position += instruction_val
        depth += instruction_val * aim
    elif instruction_type == "up":
        aim -= instruction_val
    elif instruction_type == "down":
        aim += instruction_val

print("Final position * depth = " + str(position * depth))

Now our up and down instructions modify the aim instead of the depth directly. Only on the forward instruction do we adjust depth with aim as a factor.

❯ python3 solution2.py input2
Final position * depth = 2138382217