[Bugfix] FlowPipe was not reading frames off the frame queue in the c…

…orrect order

src/flowty/flow_pipe.py

@@ -24,15 +24,15 @@ def run(self):
         frame_queue = deque()
         frame_iter = iter(self.src)
         while len(frame_queue) < self.dilation:
-            frame_queue.appendleft(next(frame_iter))
+            frame_queue.append(next(frame_iter))
         assert len(frame_queue) == self.dilation
 
         t = time.time()
         for i, target in enumerate(frame_iter):
             data_load_time = time.time() - t
             print("Data time (ms): ", data_load_time * 1e3)
             reference = frame_queue.popleft()
-            frame_queue.appendleft(target)
+            frame_queue.append(target)
             assert len(frame_queue) == self.dilation
             if i % self.stride == 0:
                 t = time.time()

tests/unit/test_flow_pipe.py

@@ -0,0 +1,45 @@
+from flowty.flow_pipe import FlowPipe
+import numpy as np
+
+
+class RecordingDestination:
+    def __init__(self):
+        self.flow = []
+
+    def write(self, flow):
+        self.flow.append(flow)
+
+
+class TestFlowPipe:
+    def test_1_flow_field_between_2_frames(self):
+        flow = self.compute_flow([1, 2])
+        assert len(flow) == 1
+        assert flow[0] == np.array([1])
+
+    def test_2_flow_fields_between_3_frames(self):
+        flow = self.compute_flow([1, 2, 4])
+        assert len(flow) == 2
+        assert flow == [np.array([1]), np.array([2])]
+
+    def test_1_flow_field_between_3_frames_when_dilation_is_2(self):
+        flow = self.compute_flow([1, 2, 3], dilation=2)
+        assert len(flow) == 1
+        assert flow[0] == np.array([2])
+
+    def test_2_flow_fields_between_4_frames_when_dilation_is_2(self):
+        flow = self.compute_flow([1, 2, 3, 5], dilation=2)
+        assert flow == [np.array([2]), np.array([3])]
+
+    def compute_flow(self, frames, dilation=1, stride=1):
+        src = [np.array([f]) for f in frames]
+
+        def flow_algorithm(reference, target):
+            print("target: {}, reference: {}".format(target ,reference))
+            return target - reference
+
+        dest = RecordingDestination()
+        pipe = FlowPipe(
+                src, flow_algorithm, dest, dilation=dilation, stride=stride
+        )
+        pipe.run()
+        return dest.flow