--- a/python/utils.py	Fri Dec 04 13:47:22 2009 -0500
+++ b/python/utils.py	Sat Dec 05 11:04:03 2009 -0500
@@ -8,6 +8,46 @@
 
 commentChar = '#';
 
+# maths
+
+def segmentIntersection(s1, s2):
+    '''Returns the intersecting point, None otherwise
+    format for a segment is [[x1, x2], [y1,y2]]
+    
+    >>> segmentIntersection([[0, 1], [0,1]], [[0, 1], [1,2]])
+    >>> segmentIntersection([[0, 1], [1,0]], [[0, 1], [2,1]])
+    >>> segmentIntersection([[0, 2], [0,0]], [[1, 1], [-1,1]])
+    [1.0, 0.0]
+    >>> segmentIntersection([[0, 2], [0,0]], [[1, 1], [1,2]])
+    '''
+    from numpy import matrix
+    from numpy.linalg import linalg, det
+
+    ds1 = [s1[0][1]-s1[0][0], s1[1][1]-s1[1][0]]
+    ds2 = [s2[0][1]-s2[0][0], s2[1][1]-s2[1][0]]
+
+    A = matrix([[ds1[1], -ds1[0]],
+                [ds2[1], -ds2[0]]])
+    B = matrix([[ds1[1]*s1[0][0]-ds1[0]*s1[1][0]],
+                [ds2[1]*s2[0][0]-ds2[0]*s2[1][0]]])
+
+    if linalg.det(A) == 0:#crossProduct(ds1, ds2) == 0:
+        return None
+    else:
+        intersection = linalg.solve(A,B)
+        if (intersection[0,0] >= s1[0][0] and intersection[0,0] <= s1[0][1]
+            and intersection[0,0] >= s2[0][0] and intersection[0,0] <= s2[0][1]
+            and intersection[1,0] >= s1[1][0] and intersection[1,0] <= s1[1][1]
+            and intersection[1,0] >= s2[1][0] and intersection[1,0] <= s2[1][1]):
+            return [intersection[0,0], intersection[1,0]]
+        else:
+            return None
+
+def crossProduct(l1, l2):
+    return l1[0]*l2[1]-l1[1]*l2[0]
+
+# file I/O
+
 def openCheck(filename, option = 'r', quit = False):
     '''Open file filename in read mode by default
     and checks it is open