Discussion on: Advent of Code 2019 Solution Megathread - Day 2: 1202 Program Alarm

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JavaScript checking in! I think it'll be interesting, seeing how we re-use this in future challenges. It did take me reading the end of part 2 a number of times before I understood what I was looking for! :)

I also feel like there should be a better way to process the second part...I saw folks on the subreddit talking about just changing the noun value until it's as high as can be while still below the desired output, then changing the verb, but it seems maybe there's unforeseen issues with that? I don't know, everyone I've seen so far is just nesting for loops so I'm not that worried about it. :)

// copy of initial input, so we can reset properly
let inputCopy = [...input];

// noun - input for address 1; verb - input for address 2
let noun = 12;
let verb = 2;

// for part 2 - desired output to match - output is final value at address 0 when program is done
const desiredOutput = 19690720;
// opcode 1 - get values at position 1&2 right after code, add together, store in position 3
// opcode 2 - get values at position 1&2 right after code, multiply, store in position 3
// opcode 99 - stop program
function intCode (op, a, b, c) {
  let valA = inputCopy[a];
  let valB = inputCopy[b];

  if (op === 1) {
    inputCopy[c] = valA + valB;
  } else if (op === 2) {
    inputCopy[c] = valA * valB;
  }
}

// run through memory input, following instructions until 99 is hit
function runProgram() {
  for (let i = 0; i < inputCopy.length; i += 4) {
    if (inputCopy[i] === 99) {
      break;
    }
    intCode(inputCopy[i], inputCopy[i+1], inputCopy[i+2], inputCopy[i+3]);
  }
}

// for part 1 - insert noun & verb provided
inputCopy[1] = noun;
inputCopy[2] = verb;

runProgram();
console.log(`Part 1: position 0 value is ${inputCopy[0]}`);

// part 2 - test different values for noun & verb, insert into memory, run program - when desired output is matched, log noun & verb used & end cycle

for (let n = 0; n < 100; n++) {
  noun = n;
  let found = false;
  for (let v = 0; v < 100; v++) {
    verb = v;

    // reset copy to initial input, then replace noun & verb
    inputCopy = [...input];
    inputCopy[1] = noun;
    inputCopy[2] = verb;

    runProgram();
    let currentOutput = inputCopy[0];

    if (currentOutput === desiredOutput) {
      found = true;
      break;
    }
  }
  if (found) {
    break;
  }
}

console.log(`desired noun and verb are ${noun} & ${verb}; output value is ${inputCopy[0]}`);
console.log(`Part 2 - computed value is ${100 * noun + verb}`);