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79 lines
No EOL
2.5 KiB
Markdown
79 lines
No EOL
2.5 KiB
Markdown
# Day 5: Supply Stacks
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The expedition can depart as soon as the final supplies have been unloaded from the ships. Supplies are stored in stacks
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of marked **crates**, but because the needed supplies are buried under many other crates, the crates need to be
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rearranged.
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The ship has a **giant cargo crane** capable of moving crates between stacks. To ensure none of the crates get crushed
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or fall over, the crane operator will rearrange them in a series of carefully-planned steps. After the crates are
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rearranged, the desired crates will be at the top of each stack.
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The Elves don't want to interrupt the crane operator during this delicate procedure, but they forgot to ask her which
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crate will end up where, and they want to be ready to unload them as soon as possible, so they can embark.
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They do, however, have a drawing of the starting stacks of crates and the rearrangement procedure (your puzzle input).
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For example:
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```
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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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move 1 from 2 to 1
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move 3 from 1 to 3
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move 2 from 2 to 1
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move 1 from 1 to 2
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```
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In this example, there are three stacks of crates. Stack 1 contains two crates: crate `Z` is on the bottom, and
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crate `N` is on top. Stack 2 contains three crates; from bottom to top, they are crates `M`, `C`, and `D`. Finally,
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stack 3 contains a single crate, `P`.
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Then, the rearrangement procedure is given. In each step of the procedure, a quantity of crates is moved from one stack
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to a different stack. In the first step of the above rearrangement procedure, one crate is moved from stack 2 to stack
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1, resulting in this configuration:
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```
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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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```
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In the second step, three crates are moved from stack 1 to stack 3. Crates are moved **one at a time**, so the first
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crate to be moved (`D`) ends up below the second and third crates:
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```
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[Z]
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[N]
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[C] [D]
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[M] [P]
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1 2 3
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```
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Then, both crates are moved from stack 2 to stack 1. Again, because crates are **moved one at a time**, crate `C` ends
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up below crate `M`:
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```
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[Z]
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[N]
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[M] [D]
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[C] [P]
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1 2 3
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```
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Finally, one crate is moved from stack 1 to stack 2:
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```
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[Z]
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[N]
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[D]
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[C] [M] [P]
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1 2 3
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```
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The Elves just need to know **which crate will end up on top of each stack**; in this example, the top crates are `C` in
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stack 1, `M` in stack 2, and `Z` in stack 3, so you should combine these together and give the Elves the message `CMZ`.
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**After the rearrangement procedure completes, what crate ends up on top of each stack?** |