advent-of-code/2020/14/solution2.js

const fs = require("fs");

const input = fs.readFileSync("input", "utf-8").trim().split("\n");

function main() {
  // original mask string
  let sum = 0n;
  let mask = null;
  const memory = [];
  for (const line of input) {
    if (line.startsWith("mask")) {
      mask = line.slice(7);
    } else {
      const sourceAddress = BigInt(parseInt(line.slice(4)));
      const num = BigInt(parseInt(line.split(" = ")[1]));
      const addresses = apply(sourceAddress, mask);
      for (const address of addresses) {
        if (typeof memory[address] !== "undefined") {
          sum -= memory[address];
        }
        memory[address] = num;
        sum += num;
      }
      // debug(sourceAddress, mask, addresses);
    }
  }
  console.log("Fill memory done");
  console.log("Solution:");
  console.log(sum.toString());
}

function debug(address, mask, addresses) {
  console.log("=".repeat(18), " DEBUG ", "=".repeat(18));
  console.log(`address:  ${numToBin(address)}  (decimal ${address})`);
  console.log(`mask:     ${mask}`);
  console.log(`result: `);
  for (const address of addresses) {
    console.log(`${numToBin(address)}  (decimal ${address})`);
  }
}

function numToBin(num) {
  return num.toString(2).padStart(36, "0");
}

function binToNum(bin) {
  return BigInt(parseInt(bin, 2));
}

function apply(address, mask) {
  address = address | binToNum(mask.replaceAll("X", 0));
  address = numToBin(address);
  let addresses = [""];
  for (let i = 0; i < address.length; i++) {
    const bit = address[i];
    const maskBit = mask[i];
    if (maskBit === "X") {
      const zeroConcated = addresses.map((adr) => adr + "0");
      const oneConcated = addresses.map((adr) => adr + "1");
      addresses = [...zeroConcated, ...oneConcated];
    } else {
      addresses = addresses.map((adr) => adr + bit);
    }
  }
  return addresses.map(binToNum);
}

main();