Solves Problem 28. Easy after all

Excellent processing time

Still have to perform some good work on prime numbers! 

Signed-off-by: Julien Lengrand-Lambert <julien@lengrand.fr>

e_28.py

@@ -17,14 +17,30 @@ It can be verified that the sum of the numbers on the diagonals is 101.
 
 What is the sum of the numbers on the diagonals in a 1001 by 1001 spiral formed in the same way?
 ''' 
-def sum_power():
+def sum_diag(max_lines):
     """
-    Finds
+    Returns the sum of both diagonals of the square of max_lines size
     """
+    dsum = 1 # sum of diagonals
+    cpt = 1 # number of lines processed
+    val = 1 # value of the current place in the square
+    inc = 0 # the increment between number for one line
+    
+    while cpt < max_lines:
+        cpt += 2
+        inc += 2
+        
+        for corner in range(4):
+            val += inc
+            dsum += val
 
-
-    return 1
+    return dsum
 
 if __name__ == '__main__':
-    # The major problem in there is to find the upper limit.  
-    print "Answer : %d " % (sum_power())
+    # The major problem in there is to find the logic
+    #n = 1 gives 1 
+    #n = 3 gives 3, 5, 7, 9 => +2
+    #n = 5 gives 13, 17, 21, 25 => +4
+    #n = 7 gives 31, 37, 43, 49 => +6
+    #n = 9 gives 57, . . .      
+    print "Answer : %d " % (sum_diag(1001))