Advent Of Code 2017 - 03(2) http://adventofcode.com/2017/day/3

aoc2017-03-2.js

// data spiral
// 147  142  133  122   59
// 304    5    4    2   57
// 330   10    1    1   54
// 351   11   23   25   26
// 362  747  806--->   ...

function getStepsToData (target)
{
  // a grid to keep track of the spiral
  const grid = [[1]];
  
  // helper to GET grid values (for convenience, yet undefined values default to 0)
  const getEntry = (x, y) => grid[-y] && grid[-y][x] || 0;
  
  // helper to SET grid values
  const setEntry = (x, y, value) => {
    if(!grid[-y])
    {
      grid[-y] = [];
    }
	  grid[-y][x] = value;
  };

  // helper to get the sum of all adjacent cells
  const sumNeighbours = (x, y) => [
    getEntry(x-1, y-1),
    getEntry(x, y-1),
    getEntry(x+1, y-1),
    getEntry(x-1, y),
    getEntry(x+1, y),
    getEntry(x-1, y+1),
    getEntry(x, y+1),
    getEntry(x+1, y+1)
  ].reduce((a, b) => a + b);
  
  // helper function to print the spiral/grid for debugging
  const printGrid = () => {
    for (let y of Object.keys(grid).map(Number).sort((a, b) => a > b))
    {
      const row = Object.keys(grid[y]).map(Number).sort((a, b) => a > b).map(k => grid[y][k]);
      console.log(row.join('\t'));
    }    
  };

  // same as in part one
  const directions = [[1, 0], [0, 1], [-1, 0], [0, -1]];
  let directionPointer = 0;
  let x = 0;
  let y = 0;
  let stepsToNextCorner = 1;
  let stepsTaken = 0;
  let cornersTaken = 0;
  let currentNumber = 0;

  // build up the spiral until we reach the target
  while (currentNumber < target) 
  {
    x += directions[directionPointer][0];
    y += directions[directionPointer][1];
    
    // for each step we sum up the already existing neighbours
    currentNumber = sumNeighbours(x, y);
    // and store the result in its place in the spiral
    setEntry(x, y, currentNumber);

    // the rest is like in part one
    stepsTaken += 1;
    if (stepsTaken === stepsToNextCorner)
    {
      directionPointer = (directionPointer + 1) % directions.length;
      stepsTaken = 0;
      cornersTaken += 1;
      if (cornersTaken % 2 === 0)
      {
        stepsToNextCorner += 1;
      }
    }  
  }

  //printGrid();
  return currentNumber;
}