diff --git a/day17/sol.py b/day17/sol.py
index 30016a3..051e292 100644
--- a/day17/sol.py
+++ b/day17/sol.py
@@ -10,7 +10,6 @@ rocks = [
 wall = 0b10000000_10000000_10000000_10000000_10000000
 
 jets = ">>><<><>><<<>><>>><<<>>><<<><<<>><>><<>>"
-
 jets = open("input").read().strip()
 
 
@@ -20,10 +19,10 @@ def ceil(n):
     return n
 
 def drop(a, r, jets):
-    # start falling
     h = ceil(a.bit_length()) + 8*3
     numjets = 0
     while True:
+        # push left or right
         j = next(jets)
         if j == '<':
             x = (r<<1)
@@ -48,14 +47,28 @@ def drop(a, r, jets):
             return a, numjets
 
 def head(a):
+    # technically this isn't sufficient.
+    # you could imagine a stack of overhangs like
+    #
+    #  |#     #|
+    #  |####  #|
+    #  |#     #|
+    #  |#     #|
+    #  |#   ###|
+    #  |#     #|
+    #
+    # that a rock might be able to navigate through indefinitely,
+    # but our stacks are much messier than that, so no need to worry.
+    #
+    # we could probably just take the top 10 or so rows and call it good.
     h = ceil(a.bit_length())
-    x = 0
-    while h >= 8 and x < 0x7f:
+    x = 0x80
+    while h >= 8:
         h -= 8
         x |= (a>>h)&0xff
-    if x >= 0x7f:
-        return a>>h
-    return 0
+        if x == 0xff:
+            return a>>h
+    return a
 
 def height(a):
     return ceil(a.bit_length())//8
@@ -93,14 +106,15 @@ try:
         #for i in reversed(range(0,a.bit_length(),8)):
         #    print("{:08b}".format((a>>i)&0xff))
         #print()
-finally:
-    pass #print(i, j, stopped)
+except:
+    print(i, j, stopped)
+    raise
 
 b, old = seen[(i,j,head(a))]
 sdiff = stopped - old
 hdiff = height(a) - height(b)
 
-goal = 1000000000000
+goal = 10**12
 rounds, extra = divmod(goal - stopped, sdiff)
 for _ in range(extra):
     r = next(rocki)