Day 3

day3.rb

MOVE_ARRAY = [[0,1],[-1,0],[0,-1],[1,0]]
ODD_SQUARES = (1..10000).select(&:odd?).map {|i| i * i}
# move the pointer in the spiral to the next position
def move(counter, x, y)
  # we should go right if we are a square number
  sqrt_counter = Math.sqrt(counter).to_i
  sqrt_counter = sqrt_counter.odd? ? sqrt_counter : sqrt_counter - 1
  return [x+1, y] if ODD_SQUARES.include? counter
  lowest_right_corner = sqrt_counter * sqrt_counter
  move_array_index = (1..4).detect do |i|
    counter < (lowest_right_corner + i * (sqrt_counter.to_i + 1))
  end
  # minus one, so that the math works out
  move_array_index -= 1
  MOVE_ARRAY[move_array_index].zip([x,y]).map {|v1,v2| v1 + v2}
end

# fill the newly calculated value in the spiral
def fill_value(spiral, x, y)
  new_value = 0
  # Get the eight surrounding values (we don't care that they are not filled in)
  (-1..1).each do |i|
    (-1..1).each do |j|
      new_value += get_value(spiral, x-i, y-j)
    end
  end
  set_value(spiral, x, y, new_value)
  new_value
end

def get_value(spiral, x, y)
  spiral[x][y]
end


def set_value(spiral, x, y, value)
  spiral[x][y] = value
end

#starting spiral
spiral = []
max_size = 100
(0...max_size).each do |i|
  spiral << Array.new(max_size, 0)
end

# initialize the middle of the spiral
x = max_size / 2
y = max_size / 2
counter = 1
set_value(spiral, x, y, 1)

loop do
  x, y = move(counter, x, y)
  value = fill_value(spiral, x, y)
  counter += 1
  if value > 289326
    puts value
    exit
  end
end