Fix weighted sequential strategy

Author: Hironsan

backend/examples/assignment/strategies.py

@@ -45,16 +45,10 @@ def __init__(self, dataset_size: int, weights: List[int]):
 
     def assign(self) -> List[Assignment]:
         assignments = []
-        proba = np.array(self.weights) / 100
-        counts = np.round(proba * self.dataset_size).astype(int)
-        reminder = self.dataset_size - sum(counts)
-        for i in np.random.choice(range(len(self.weights)), size=reminder, p=proba):
-            counts[i] += 1
-
-        start = 0
-        for user, count in enumerate(counts):
-            assignments.extend([Assignment(user=user, example=example) for example in range(start, start + count)])
-            start += count
+        ratio = list(np.round(np.cumsum(self.weights) / 100 * self.dataset_size).astype(int))
+        ratio = [0] + ratio[:-1] + [self.dataset_size]
+        for user, (start, end) in enumerate(zip(ratio, ratio[1:])):  # Todo: use itertools.pairwise
+            assignments.extend([Assignment(user=user, example=example) for example in range(start, end)])
         return assignments