Solution to problem 2 in python

src/Year_2021/P2.py

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+# --- Day 2: Dive! ---
+
+# 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?
+
+with open("files/P2.txt", "r") as f:
+    instructions = [line for line in f.read().strip().split("\n")]
+
+
+def part_1() -> None:
+    position, depth = 0, 0
+    for instruction in instructions:
+        pos, amount = instruction.split(" ")
+        if pos == "forward":
+            position += int(amount)
+        if pos == "down":
+            depth += int(amount)
+        if pos == "up":
+            depth -= int(amount)
+
+    print(f"Planned course for part 1 is {position * depth}")
+
+
+# --- Part Two ---
+
+# 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?
+
+
+def part_2() -> None:
+    position, depth, aim = 0, 0, 0
+    for instruction in instructions:
+        pos, amount = instruction.split(" ")
+        if pos == "forward":
+            position += int(amount)
+            depth += aim * int(amount)
+        if pos == "down":
+            aim += int(amount)
+        if pos == "up":
+            aim -= int(amount)
+
+    print(f"Planned course for part 2 is {position * depth}")
+
+
+if __name__ == "__main__":
+    part_1()
+    part_2()