Fix partition count assertion (#1597)

The actual asserted bound is a bit more complex because the last partial
partition is merged into multiple existing ones instead of choosing to
have one smaller partition.

vortex-datafusion/src/persistent/execution.rs

@@ -168,23 +168,29 @@ fn repartition_by_size(
         curr_partition_size += file.object_meta.size;
         curr_partition.push(file);
 
-        if curr_partition_size > target_partition_size {
+        if curr_partition_size >= target_partition_size {
             curr_partition_size = 0;
             partitions.push(std::mem::take(&mut curr_partition));
         }
     }
 
-    // if there's anything left, we shove it into existing partitions
-    for (idx, file) in curr_partition.into_iter().enumerate() {
-        let part_idx = idx % partitions.len();
-        partitions[part_idx].push(file);
+    // If we we're still missing the last partition
+    if !curr_partition.is_empty() && partitions.len() != desired_partitions {
+        partitions.push(std::mem::take(&mut curr_partition));
+    // If we already have enough partitions
+    } else if !curr_partition.is_empty() {
+        for (idx, file) in curr_partition.into_iter().enumerate() {
+            let new_part_idx = idx % partitions.len();
+            partitions[new_part_idx].push(file);
+        }
     }
 
+    // Assert that we have the correct number of partitions and that the total number of files is right
     assert_eq!(
         partitions.len(),
-        usize::min(total_file_count, desired_partitions),
-        "The final number of partitions should be smallest between the total number of files and the desired partition count."
+        usize::min(desired_partitions, total_file_count)
     );
+    assert_eq!(total_file_count, partitions.iter().flatten().count());
 
     partitions
 }
@@ -203,8 +209,12 @@ mod tests {
             PartitionedFile::new("e", 50),
         ]];
 
-        let output = repartition_by_size(file_groups, 2);
+        repartition_by_size(file_groups, 2);
+
+        let file_groups = vec![(0..100)
+            .map(|idx| PartitionedFile::new(format!("{idx}"), idx))
+            .collect()];
 
-        assert_eq!(output.len(), 2);
+        repartition_by_size(file_groups, 16);
     }
 }