diff --git a/streaming/base/spanner.py b/streaming/base/spanner.py
index 221491f72..0e186ad54 100644
--- a/streaming/base/spanner.py
+++ b/streaming/base/spanner.py
@@ -6,7 +6,6 @@
 import numpy as np
 from numpy.typing import NDArray
 
-
 class Spanner:
     """Given a list of shards, construct a mapping of global index to shard and relative index.
 
@@ -25,10 +24,34 @@ def __init__(self, shard_sizes: NDArray[np.int64], span_size: int = 1 << 10) ->
         underflow = span_size - overflow if overflow else 0
         self.shard_sizes[-1] += underflow
 
-        sample_shards = np.repeat(np.arange(len(shard_sizes)), self.shard_sizes)
-        sample_shards = sample_shards.reshape(-1, span_size)
-        span_lowest_shards = sample_shards.min(1)
-        span_highest_shards = sample_shards.max(1)
+        n_shards = len(shard_sizes)
+        current_shard = 0
+        current_position_in_shard = 0
+
+        span_lowest_shards = []
+        span_highest_shards = []
+
+        while current_shard < n_shards:
+            span_min_shard = current_shard
+            span_max_shard = current_shard
+
+            remaining_span_size = span_size
+            while remaining_span_size > 0 and current_shard < n_shards:
+                available_in_current_shard = shard_sizes[current_shard] - current_position_in_shard
+
+                if remaining_span_size >= available_in_current_shard:
+                    remaining_span_size -= available_in_current_shard
+                    current_shard += 1
+                    current_position_in_shard = 0
+                else:
+                    current_position_in_shard += remaining_span_size
+                    remaining_span_size = 0
+
+                if current_shard < n_shards:
+                    span_max_shard = current_shard
+
+            span_lowest_shards.append(span_min_shard)
+            span_highest_shards.append(span_max_shard)
 
         self.spans = []
         for low, high in zip(span_lowest_shards, span_highest_shards):