88 lines
3.5 KiB
Markdown
88 lines
3.5 KiB
Markdown
# Day 2: 1202 Program Alarm
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## Problem
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### Part One
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On the way to your gravity assist around the Moon, your ship computer beeps
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angrily about a "1202 program alarm". On the radio, an Elf is already
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explaining how to handle the situation: "Don't worry, that's perfectly norma--"
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The ship computer bursts into flames.
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You notify the Elves that the computer's magic smoke seems to have escaped.
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"That computer ran Intcode programs like the gravity assist program it was
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working on; surely there are enough spare parts up there to build a new Intcode
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computer!"
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An Intcode program is a list of integers separated by commas (like 1,0,0,3,99).
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To run one, start by looking at the first integer (called position 0). Here,
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you will find an opcode - either 1, 2, or 99. The opcode indicates what to do;
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for example, 99 means that the program is finished and should immediately halt.
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Encountering an unknown opcode means something went wrong.
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Opcode 1 adds together numbers read from two positions and stores the result in
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a third position. The three integers immediately after the opcode tell you
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these three positions - the first two indicate the positions from which you
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should read the input values, and the third indicates the position at which the
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output should be stored.
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For example, if your Intcode computer encounters 1,10,20,30, it should read the
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values at positions 10 and 20, add those values, and then overwrite the value
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at position 30 with their sum.
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Opcode 2 works exactly like opcode 1, except it multiplies the two inputs
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instead of adding them. Again, the three integers after the opcode indicate
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where the inputs and outputs are, not their values.
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Once you're done processing an opcode, move to the next one by stepping forward
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4 positions.
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For example, suppose you have the following program:
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1,9,10,3,2,3,11,0,99,30,40,50
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For the purposes of illustration, here is the same program split into multiple
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lines:
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1,9,10,3,
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2,3,11,0,
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99,
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30,40,50
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The first four integers, 1,9,10,3, are at positions 0, 1, 2, and 3. Together,
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they represent the first opcode (1, addition), the positions of the two inputs
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(9 and 10), and the position of the output (3). To handle this opcode, you
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first need to get the values at the input positions: position 9 contains 30,
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and position 10 contains 40. Add these numbers together to get 70. Then, store
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this value at the output position; here, the output position (3) is at position
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3, so it overwrites itself. Afterward, the program looks like this:
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1,9,10,70,
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2,3,11,0,
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99,
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30,40,50
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Step forward 4 positions to reach the next opcode, 2. This opcode works just
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like the previous, but it multiplies instead of adding. The inputs are at
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positions 3 and 11; these positions contain 70 and 50 respectively. Multiplying
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these produces 3500; this is stored at position 0:
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3500,9,10,70,
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2,3,11,0,
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99,
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30,40,50
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Stepping forward 4 more positions arrives at opcode 99, halting the program.
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Here are the initial and final states of a few more small programs:
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* 1,0,0,0,99 becomes 2,0,0,0,99 (1 + 1 = 2).
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* 2,3,0,3,99 becomes 2,3,0,6,99 (3 * 2 = 6).
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* 2,4,4,5,99,0 becomes 2,4,4,5,99,9801 (99 * 99 = 9801).
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* 1,1,1,4,99,5,6,0,99 becomes 30,1,1,4,2,5,6,0,99.
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Once you have a working computer, the first step is to restore the gravity
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assist program (your puzzle input) to the "1202 program alarm" state it had
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just before the last computer caught fire. To do this, before running the
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program, replace position 1 with the value 12 and replace position 2 with the
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value 2. What value is left at position 0 after the program halts? |