aoc2021_day9_part2.js

(function(inp)
{
  function getNeighbours(x,y)
  {
    let deltas = [
        [-1,0],
        [0,-1],
        [0,1],
        [1,0],
     ];
      
     let neighbours = [];
        
     for(let delta of deltas)
     {
         let [xd, yd] = delta;
         neighbours.push([xd+x, yd+y]);
     }
     return neighbours;
  }
   
  let points = inp.split('\n').map(line => line.split('')).filter(line => line.length > 0).flat().map(v => parseInt(v,10));
  
  const width = 100;
  
  let lowpoints = [];
  
  for(let i = 0; i < points.length; i++)
  {
    let x = i % width;
    let y = Math.floor(i / width);
    
    let neighbours = getNeighbours(x,y);
    
    let highercount = 0;
    
    for(let neigh of neighbours)
    {
      let [nx, ny] = neigh;
      if(nx >= width || nx < 0 || ny >= width || ny < 0)
      {
        highercount++;
        continue;
      }
      let ni = nx + (ny * width);
      if(points[ni] > points[i])
      {
        highercount++;
      }
    }
    
    if(highercount == 4)
    {
      lowpoints.push(i);
    }
  }
  
  let basins = new Map();
  
  for(let index of lowpoints)
  {
    basins.set(index, new Set([index]));
  }
  
  for(let index of lowpoints)
  {
    let lx = index%width;
    let ly = Math.floor(index/width);
    let neighbours = getNeighbours(lx, ly);
    let pointsToCheck = [];
    for(let neigh of neighbours)
    {
      let [nx, ny] = neigh;
      if(nx >= width || nx < 0 || ny >= width || ny < 0)
      {
        continue;
      }
      
      let ni = nx + (ny*width);
      
      if(points[ni] < 9)
      {
        let b = basins.get(index)
        b.add(ni);
        pointsToCheck.push(ni);
      }
    }
    let npi = 0;
    while(npi <= pointsToCheck.length)
    {
      let pindex = pointsToCheck[npi];
      
      let px = pindex%width;
      let py = Math.floor(pindex/width);
      
      let pneighbours = getNeighbours(px,py);
      
      for(let neigh of pneighbours)
      {
        let [nx, ny] = neigh;
        if(nx >= width || nx < 0 || ny >= width || ny < 0)
        {
          continue;
        }
      
        let ni = nx + (ny*width);
      
        if(points[ni] < 9)
        {
          let b = basins.get(index)
          b.add(ni);
          if(!pointsToCheck.includes(ni))
          {
            pointsToCheck.push(ni);
          }
        }
      }
      
      npi++;
    }
  }
  
  let longests=[new Set(),new Set(),new Set()];
  
  for(let [index, basin] of basins)
  {
    if(basin.size > longests[0].size)
    {
      longests[2] = longests[1];
      longests[1] = longests[0];
      longests[0] = basin;
    }
    else if(basin.size > longests[1].size)
    {
      longests[2] = longests[1];
      longests[1] = basin;
    }
    else if(basin.size > longests[2].size)
    {
      longests[2] = basin;
    }
  }
  
  return longests[0].size * longests[1].size * longests[2].size;
})(document.querySelector('pre').textContent);