147 lines
7.4 KiB
Plaintext
147 lines
7.4 KiB
Plaintext
Advent of Code
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--- Day 2: 1202 Program Alarm ---
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On the way to your gravity assist around the Moon, your ship computer beeps angrily about a "1202 program alarm".
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On the radio, an Elf is already 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. "That computer ran Intcode programs
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like the gravity assist program it was working on; surely there are enough spare parts up there to build a new
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Intcode computer!"
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An Intcode program is a list of integers separated by commas (like 1,0,0,3,99). To run one, start by looking at
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the first integer (called position 0). Here, you will find an opcode - either 1, 2, or 99. The opcode indicates
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what to do; for example, 99 means that the program is finished and should immediately halt. Encountering an
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unknown opcode means something went wrong.
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Opcode 1 adds together numbers read from two positions and stores the result in a third position. The three
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integers immediately after the opcode tell you these three positions - the first two indicate the positions from
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which you should read the input values, and the third indicates the position at which the output should be stored.
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For example, if your Intcode computer encounters 1,10,20,30, it should read the values at positions 10 and 20, add
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those values, and then overwrite the value at position 30 with their sum.
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Opcode 2 works exactly like opcode 1, except it multiplies the two inputs instead of adding them. Again, the three
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integers after the opcode indicate 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 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 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, they represent the first opcode (1,
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addition), the positions of the two inputs (9 and 10), and the position of the output (3). To handle this opcode,
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you first need to get the values at the input positions: position 9 contains 30, and position 10 contains 40. Add
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these numbers together to get 70. Then, store this value at the output position; here, the output position (3) is
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at position 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 like the previous, but it multiplies
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instead of adding. The inputs are at positions 3 and 11; these positions contain 70 and 50 respectively.
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Multiplying 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 assist program (your puzzle input) to
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the "1202 program alarm" state it had 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 value 2. What value is left at
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position 0 after the program halts?
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Your puzzle answer was 4930687.
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--- Part Two ---
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"Good, the new computer seems to be working correctly! Keep it nearby during this mission - you'll probably use it
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again. Real Intcode computers support many more features than your new one, but we'll let you know what they are
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as you need them."
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"However, your current priority should be to complete your gravity assist around the Moon. For this mission to
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succeed, we should settle on some terminology for the parts you've already built."
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Intcode programs are given as a list of integers; these values are used as the initial state for the computer's
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memory. When you run an Intcode program, make sure to start by initializing memory to the program's values. A
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position in memory is called an address (for example, the first value in memory is at "address 0").
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Opcodes (like 1, 2, or 99) mark the beginning of an instruction. The values used immediately after an opcode, if
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any, are called the instruction's parameters. For example, in the instruction 1,2,3,4, 1 is the opcode; 2, 3, and
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4 are the parameters. The instruction 99 contains only an opcode and has no parameters.
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The address of the current instruction is called the instruction pointer; it starts at 0. After an instruction
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finishes, the instruction pointer increases by the number of values in the instruction; until you add more
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instructions to the computer, this is always 4 (1 opcode + 3 parameters) for the add and multiply instructions.
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(The halt instruction would increase the instruction pointer by 1, but it halts the program instead.)
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"With terminology out of the way, we're ready to proceed. To complete the gravity assist, you need to determine
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what pair of inputs produces the output 19690720."
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The inputs should still be provided to the program by replacing the values at addresses 1 and 2, just like before.
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In this program, the value placed in address 1 is called the noun, and the value placed in address 2 is called the
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verb. Each of the two input values will be between 0 and 99, inclusive.
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Once the program has halted, its output is available at address 0, also just like before. Each time you try a pair
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of inputs, make sure you first reset the computer's memory to the values in the program (your puzzle input) - in
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other words, don't reuse memory from a previous attempt.
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Find the input noun and verb that cause the program to produce the output 19690720. What is 100 * noun + verb?
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(For example, if noun=12 and verb=2, the answer would be 1202.)
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Your puzzle answer was 5335.
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Both parts of this puzzle are complete! They provide two gold stars: **
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At this point, you should return to your Advent calendar and try another puzzle.
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If you still want to see it, you can get your puzzle input.
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You can also [Shareon Twitter Mastodon] this puzzle.
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References
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Visible links
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. https://adventofcode.com/
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. https://adventofcode.com/2019/about
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. https://adventofcode.com/2019/events
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. https://adventofcode.com/2019/settings
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. https://adventofcode.com/2019/auth/logout
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. Advent of Code Supporter
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https://adventofcode.com/2019/support
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. https://adventofcode.com/2019
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. https://adventofcode.com/2019
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. https://adventofcode.com/2019/support
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. https://adventofcode.com/2019/sponsors
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. https://adventofcode.com/2019/leaderboard
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. https://adventofcode.com/2019/stats
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. https://adventofcode.com/2019/sponsors
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. https://www.hq.nasa.gov/alsj/a11/a11.landing.html#1023832
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. https://en.wikipedia.org/wiki/Gravity_assist
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. https://en.wikipedia.org/wiki/Halt_and_Catch_Fire
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. https://en.wikipedia.org/wiki/Magic_smoke
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. https://en.wikipedia.org/wiki/Integer
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. https://adventofcode.com/2019
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. https://adventofcode.com/2019/day/2/input
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