Refactor propagation in a function

That's better! I want more test cases to check my algorythm !

technical_tests/sand_grain_drop.py

@@ -36,11 +36,33 @@ def main(pile, n):
     """
 
     size = len(pile)
-    x_max, y_max = size - 1, size - 1
     center = int((size - 1) / 2)
 
     while n != 0:
         pile[center][center] += 1
+        do_propagation(pile)
+        n -= 1
+
+    # All sand grain are dropped, let's finish propagations
+    while True in [4 in row for row in pile]:
+        do_propagation(pile)
+
+    return pile
+
+
+def do_propagation(pile):
+    """
+    Walk through the pile to propagate sand grains
+
+    :tests:
+    >>> test_case = [[0,0,4,0,0],[0,2,0,2,0],[4,0,4,0,4],[0,2,0,2,0],[0,0,4,0,0]]
+    >>> do_propagation(test_case)
+    >>> print(test_case)
+    [[0, 1, 0, 1, 0], [1, 2, 2, 2, 1], [0, 2, 0, 2, 0], [1, 2, 2, 2, 1], [0, 1, 0, 1, 0]]
+    """
+
+    size = len(pile)
+    x_max, y_max = size - 1, size - 1
 
     # find cells > 3
     for x in range(size):
@@ -63,10 +85,6 @@ def main(pile, n):
                 if y > 0:
                     pile[x][y - 1] += 1
 
-        n -= 1
-
-    return pile
-
 
 if __name__ == "__main__":
     import doctest