advent-of-code/2020/08/part2.js

const fs = require('fs');

function parseLine(line) {
  let [ op, offset ] = line.split(' ');

  return { op: op, offset: Number(offset), visited: false };
}

function run(program) {
  let acc = 0;
  let pc = 0;

  while(true) {
    if (pc == program.length) {
      return acc;
    };

    let insn = program[pc];
    if (insn.visited) {
      return undefined;
    };

    insn.visited = true;

    switch (insn.op) {
    case 'nop':
      pc++;
      break;
    case 'jmp':
      pc = pc + insn.offset;
      break;
    case 'acc':
      acc = acc + insn.offset;
      pc++;
      break;
    }
  }
}

fs.readFile('input', (err, data) => {
  if (err) throw err;
  let program = data.toString().trim().split("\n").map( parseLine ).flat();

  for(i=0; i<program.length; i++) {
    let copy = JSON.parse(JSON.stringify(program))
    let insn = copy[i];

   if (insn.op == 'nop') {
      insn.op = 'jmp';
    } else if (insn.op == 'jmp') {
      insn.op = 'nop';
    } else {
      continue; // If we don't make any changes, the program can't terminate
    }

    if (result = run(copy)) {
      console.log(result);
      return;
    }
  }
});