chunk negative index bugfix

src/dexorder/ohlc.py

@@ -235,7 +235,7 @@ class Chunk:
             return None
         start = self.bars[0].start
         index = (time - start) // self.period
-        if index >= len(self.bars):
+        if index < 0 or index >= len(self.bars):
             return None
         bar = self.bars[index]
         assert bar.start == time
@@ -263,6 +263,16 @@ class Chunk:
                     time += self.period
                     self.bars.append(NativeOHLC(time, None, None, None, last_price))
                 assert index == len(self.bars)
+        elif index < 0:
+            # the incoming bar is prior to anything we have yet. rebase our bars on the incoming and pad
+            bars = [native]
+            for _ in range(-index-1):
+                time = bars[-1].start + self.period
+                price = bars[-1].close
+                bars.append(NativeOHLC(time, None, None, None, price))
+            assert bars[-1].start + self.period == self.bars[0].start
+            self.bars = [*bars, *self.bars]
+            return
         if index == len(self.bars):
             assert self.bars[-1].start + self.period == native.start
             self.bars.append(native)