advent_3_2.py

from math import sqrt, ceil, floor

MOVE_START = 0
MOVE_UP = 1
MOVE_DOWN = 2
MOVE_LEFT = 3
MOVE_RIGHT = 4

target, number = 361527, 361527
#target, number = 23, 23

# setup matrix
w = int(ceil(sqrt(number))) # w = h

matrix = [[0 for i in range(w)] for j in range(w)]

x0 = int(round(w/2.0)) - 1
y0 = w/2 + 1 - 1

x = x0
y = y0
matrix[y][x] = 1
move = MOVE_START

for i in range(2, number + 1):	
	if move == MOVE_START:
		move = MOVE_RIGHT
		x = x + 1
	elif move == MOVE_RIGHT:
		if matrix[y-1][x] == 0:
			y = y -1
			move = MOVE_UP
		else:
			x = x + 1
	elif move == MOVE_UP:
		if matrix[y][x-1] == 0:
			x = x -1
			move = MOVE_LEFT
		else:
			y = y - 1
	elif move == MOVE_LEFT:
		if matrix[y+1][x] == 0:
			y = y + 1
			move = MOVE_DOWN
		else:
			x = x - 1
	elif move == MOVE_DOWN:
		if matrix[y][x+1] == 0:
			x = x + 1
			move = MOVE_RIGHT
		else:
			y = y + 1

	value = 0			
	
	if y - 1 >= 0 and x - 1 >= 0:	
		value = value + matrix[y-1][x-1] 
	if y - 1 >= 0:
		value = value + matrix[y-1][x] 
	if y - 1 >= 0 and x + 1 < w:
		value = value + matrix[y-1][x+1] 
	
	if x - 1 >= 0:
		value = value + matrix[y][x-1]
	if x + 1 < w:		
		value = value + matrix[y][x+1] 
	
	if y + 1 < w and x - 1 >= 0:
		value = value + matrix[y+1][x-1] 
	if y + 1 < w:
		value = value + matrix[y+1][x]
	if y + 1 < w and x + 1 < w:
		value = value + matrix[y+1][x+1] 
	
	if value > 361527:
		print 'answer is', value
		break
	matrix[y][x] = value