PERF: Refactor AdjustedArrayWindow.

Make `__next__` and `seek` share code instead of seek() calling
`__next__`.  This avoids having to make a large number of integer
comparisons and `asanyarray` calls when seeking more than one tick
forward.

zipline/lib/_windowtemplate.pxi

@@ -15,6 +15,10 @@ from numpy cimport ndarray
 from numpy import asanyarray
 
 
+class Exhausted(Exception):
+    pass
+
+
 cdef class AdjustedArrayWindow:
     """
     An iterator representing a moving view over an AdjustedArray.
@@ -34,11 +38,11 @@ cdef class AdjustedArrayWindow:
         readonly databuffer data
         readonly dict view_kwargs
         readonly Py_ssize_t window_length
-        Py_ssize_t anchor, next_anchor, max_anchor, next_adj
+        Py_ssize_t anchor, max_anchor, next_adj
         Py_ssize_t perspective_offset
         dict adjustments
         list adjustment_indices
-        ndarray last_out
+        ndarray output
 
     def __cinit__(self,
                   databuffer data not None,
@@ -52,7 +56,7 @@ cdef class AdjustedArrayWindow:
         self.adjustments = adjustments
         self.adjustment_indices = sorted(adjustments, reverse=True)
         self.window_length = window_length
-        self.anchor = window_length + offset
+        self.anchor = window_length + offset - 1
         if perspective_offset > 1:
             # Limit perspective_offset to 1.
             # To support an offset greater than 1, work must be done to
@@ -63,11 +67,10 @@ cdef class AdjustedArrayWindow:
                             "is perspective_offset={0}".format(
                                 perspective_offset))
         self.perspective_offset = perspective_offset
-        self.next_anchor = self.anchor
         self.max_anchor = data.shape[0]
 
         self.next_adj = self.pop_next_adj()
-        self.last_out = None
+        self.output = None
 
     cdef pop_next_adj(self):
         """
@@ -82,54 +85,61 @@ cdef class AdjustedArrayWindow:
         return self
 
     def __next__(self):
+        try:
+            self._tick_forward(1)
+        except Exhausted:
+            raise StopIteration()
+
+        self._update_output()
+        return self.output
+
+    def seek(self, Py_ssize_t target_anchor):
+        cdef:
+            Py_ssize_t anchor = self.anchor
+
+        if target_anchor < anchor:
+            raise Exception('Can not access data after window has passed.')
+
+        if target_anchor == anchor:
+            return self.output
+
+        self._tick_forward(target_anchor - anchor)
+        self._update_output()
+
+        return self.output
+
+    cdef inline _tick_forward(self, int N):
         cdef:
             object adjustment
-            ndarray out
-            Py_ssize_t start, anchor
-            dict view_kwargs
+            Py_ssize_t anchor = self.anchor
+            Py_ssize_t target = anchor + N
 
-        anchor = self.anchor = self.next_anchor
-        if anchor > self.max_anchor:
-            raise StopIteration()
+        if target > self.max_anchor:
+            raise Exhausted()
 
         # Apply any adjustments that occured before our current anchor.
         # Equivalently, apply any adjustments known **on or before** the date
         # for which we're calculating a window.
-        while self.next_adj < anchor + self.perspective_offset:
+        while self.next_adj < target + self.perspective_offset:
 
             for adjustment in self.adjustments[self.next_adj]:
                 adjustment.mutate(self.data)
 
             self.next_adj = self.pop_next_adj()
 
-        start = anchor - self.window_length
+        self.anchor = target
 
-        # If our data is a custom subclass of ndarray, preserve that subclass
-        # by using asanyarray instead of asarray.
-        out = asanyarray(self.data[start:self.anchor])
-        view_kwargs = self.view_kwargs
+    cdef inline _update_output(self):
+        cdef:
+            ndarray new_out
+            Py_ssize_t anchor = self.anchor
+            dict view_kwargs = self.view_kwargs
+
+        new_out = asanyarray(self.data[anchor - self.window_length:anchor])
         if view_kwargs:
-            out = out.view(**view_kwargs)
-        out.setflags(write=False)
-
-        self.next_anchor = self.anchor + 1
-        self.last_out = out
-        return out
-
-    def seek(self, target_anchor):
-        cdef ndarray out = None
-
-        if target_anchor < self.anchor:
-            raise Exception('Can not access data after window has passed.')
-
-        if target_anchor == self.anchor:
-            return self.last_out
-
-        while self.anchor < target_anchor:
-            out = next(self)
-
-        self.last_out = out
-        return out
+            new_out = new_out.view(**view_kwargs)
+        new_out.setflags(write=False)
+        self.output = new_out
 
     def __repr__(self):
         return "<%s: window_length=%d, anchor=%d, max_anchor=%d, dtype=%r>" % (