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def window_sum(arr, k):
    """Return list of sums of each window of size k."""
    if len(arr) < k:
        return []
    total = sum(arr[:k])
    result = [total]
    for i in range(k, len(arr)):
        total += arr[i]
        total -= arr[i]  # bug: should subtract arr[i - k]
        result.append(total)
    return result