Advent of code 2017 Day 3 part 2

day3.js

// Advent of Code 2017 - Day 3 Part 2
// by @sclark39

var add = ( a, b ) => ([ a[0] + b[0], a[1] + b[1] ])
var turn = (a) => ( a[0] == 0 ? [ -a[1], 0 ] : [ 0, a[0] ] )

function buildSpiral(input)
{
	var spiral = [ [1] ]
	spiral.get = ( c ) => ( spiral[c[0]] && spiral[c[0]][c[1]] )
	spiral.set = ( c, v ) => { 
		spiral[c[0]] = spiral[c[0]] || []
		spiral[c[0]][c[1]] = v
	}

	var d = [1,0], c = [0,0]

	while ( true )
	{
		// Move cursor
		c = add( c, d )

		// Find sum of adjacent
		var sum = 0
		for ( var i = -1; i <= 1; i++ )
			for ( var j = -1; j <= 1; j++ )
				sum += spiral.get( add( c, [i,j] ) ) || 0

		// Add to spiral
		spiral.set( c, sum )

		// Time to return?
		if ( sum > input )
			return sum

		// Should we change direction?
		var l = add( c, turn(d) )
		if ( ! spiral.get( l ) )
			d = turn(d)
	}
}

console.log( buildSpiral(289326) )