136 lines
3.7 KiB
Python
136 lines
3.7 KiB
Python
"""
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--- Part Two ---
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As you watch the crane operator expertly rearrange the crates, you
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notice the process isn't following your prediction.
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Some mud was covering the writing on the side of the crane, and you
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quickly wipe it away. The crane isn't a CrateMover 9000 - it's a
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CrateMover 9001.
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The CrateMover 9001 is notable for many new and exciting features: air
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conditioning, leather seats, an extra cup holder, and the ability to
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pick up and move multiple crates at once.
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Again considering the example above, the crates begin in the same
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configuration:
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[D]
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[N] [C]
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[Z] [M] [P]
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1 2 3
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Moving a single crate from stack 2 to stack 1 behaves the same as
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before:
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[D]
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[N] [C]
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[Z] [M] [P]
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1 2 3
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However, the action of moving three crates from stack 1 to stack 3
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means that those three moved crates stay in the same order, resulting
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in this new configuration:
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[D]
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[N]
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[C] [Z]
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[M] [P]
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1 2 3
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Next, as both crates are moved from stack 2 to stack 1, they retain
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their order as well:
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[D]
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[N]
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[C] [Z]
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[M] [P]
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1 2 3
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Finally, a single crate is still moved from stack 1 to stack 2, but now
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it's crate C that gets moved:
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[D]
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[N]
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[Z]
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[M] [C] [P]
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1 2 3
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In this example, the CrateMover 9001 has put the crates in a totally
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different order: MCD.
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Before the rearrangement process finishes, update your simulation so
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that the Elves know where they should stand to be ready to unload the
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final supplies. After the rearrangement procedure completes, what crate
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ends up on top of each stack?
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"""
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import re
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import string
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def main():
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stacks = []
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with open("input.txt", "r", encoding="utf-8") as f:
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# First, create the stacks.
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for line in f:
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if "[" in line and "]" in line:
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# A sample line could be:
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#
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# [Z] [M] [P]
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#
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# And we’re interested in the letters, meaning we need
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# the following indexes:
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#
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# [Z] [M] [P]
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# 1 5 9
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for i in range(1, 9999, 4):
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try:
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char = line[i]
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except IndexError:
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continue
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if char not in string.ascii_letters:
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continue
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stack_index = int(((i - 1) / 4))
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while len(stacks) <= stack_index:
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stacks.append([])
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stacks[stack_index].append(char)
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# Stacks are loaded top to bottom from the input file, meaning
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# the lowest character is now the last entry of each array. To
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# be able to ``pop()`` the top crates off the stacks, we need
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# the highest characters last in our arrays, so we reverse them.
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for i, stack in enumerate(stacks):
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stack.reverse()
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stacks[i] = stack
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# Now we need to replicate the movements described in the input
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# file.
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regex = re.compile(r"move (\d+) from (\d+) to (\d+)")
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f.seek(0)
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for line in f:
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match = regex.match(line)
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if match:
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amount = int(match[1])
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source_stack = int(match[2]) - 1
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target_stack = int(match[3]) - 1
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print(f"Moving {amount} crates from {source_stack} to {target_stack}")
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pickup_index = amount * -1
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stacks[target_stack].extend(stacks[source_stack][pickup_index:])
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stacks[source_stack] = stacks[source_stack][:pickup_index]
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answer = ""
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for stack in stacks:
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if len(stack):
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answer += stack.pop()
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print(f"The top crates are {answer}")
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if __name__ == "__main__":
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main()
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