80 lines
3.2 KiB
Markdown
80 lines
3.2 KiB
Markdown
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## \-\-- Day 18: Duet \-\--
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You discover a tablet containing some strange assembly code labeled
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simply \"[Duet](https://en.wikipedia.org/wiki/Duet)\". Rather than
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bother the sound card with it, you decide to run the code yourself.
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Unfortunately, you don\'t see any documentation, so you\'re left to
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figure out what the instructions mean on your own.
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It seems like the assembly is meant to operate on a set of *registers*
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that are each named with a single letter and that can each hold a single
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[integer](https://en.wikipedia.org/wiki/Integer). You suppose each
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register should start with a value of `0`.
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There aren\'t that many instructions, so it shouldn\'t be hard to figure
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out what they do. Here\'s what you determine:
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- `snd X` *[plays a
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sound]{title="I don't recommend actually trying this."}* with a
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frequency equal to the value of `X`.
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- `set X Y` *sets* register `X` to the value of `Y`.
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- `add X Y` *increases* register `X` by the value of `Y`.
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- `mul X Y` sets register `X` to the result of *multiplying* the value
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contained in register `X` by the value of `Y`.
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- `mod X Y` sets register `X` to the *remainder* of dividing the value
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contained in register `X` by the value of `Y` (that is, it sets `X`
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to the result of `X`
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[modulo](https://en.wikipedia.org/wiki/Modulo_operation) `Y`).
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- `rcv X` *recovers* the frequency of the last sound played, but only
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when the value of `X` is not zero. (If it is zero, the command does
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nothing.)
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- `jgz X Y` *jumps* with an offset of the value of `Y`, but only if
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the value of `X` is *greater than zero*. (An offset of `2` skips the
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next instruction, an offset of `-1` jumps to the previous
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instruction, and so on.)
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Many of the instructions can take either a register (a single letter) or
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a number. The value of a register is the integer it contains; the value
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of a number is that number.
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After each *jump* instruction, the program continues with the
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instruction to which the *jump* jumped. After any other instruction, the
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program continues with the next instruction. Continuing (or jumping) off
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either end of the program terminates it.
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For example:
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set a 1
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add a 2
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mul a a
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mod a 5
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snd a
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set a 0
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rcv a
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jgz a -1
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set a 1
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jgz a -2
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- The first four instructions set `a` to `1`, add `2` to it, square
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it, and then set it to itself modulo `5`, resulting in a value of
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`4`.
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- Then, a sound with frequency `4` (the value of `a`) is played.
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- After that, `a` is set to `0`, causing the subsequent `rcv` and
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`jgz` instructions to both be skipped (`rcv` because `a` is `0`, and
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`jgz` because `a` is not greater than `0`).
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- Finally, `a` is set to `1`, causing the next `jgz` instruction to
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activate, jumping back two instructions to another jump, which jumps
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again to the `rcv`, which ultimately triggers the *recover*
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operation.
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At the time the *recover* operation is executed, the frequency of the
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last sound played is `4`.
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*What is the value of the recovered frequency* (the value of the most
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recently played sound) the *first* time a `rcv` instruction is executed
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with a non-zero value?
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To begin, [get your puzzle input](18/input).
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Answer:
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