fix: v2 writer used the wrong value to calculate how many rows were a…

…vailable in array (#2704)

rust/lance-encoding/src/encodings/logical/primitive.rs

@@ -454,7 +454,7 @@ impl PrimitiveFieldEncoder {
                 let mut offset = 0;
                 let part_size = bit_util::ceil(array.len(), num_parts);
                 for _ in 0..num_parts {
-                    let avail = array.len() - part_size;
+                    let avail = array.len() - offset;
                     let chunk_size = avail.min(part_size);
                     let part = array.slice(offset, chunk_size);
                     let task = self.create_encode_task(vec![part])?;

rust/lance-encoding/src/encodings/physical/binary.rs

@@ -604,7 +604,7 @@ pub mod tests {
     }
 
     #[test_log::test(tokio::test)]
-    async fn test_few_rows_bigger_than_max_page_size() {
+    async fn test_bigger_than_max_page_size() {
         // Create an array with one single 32MiB string
         let big_string = String::from_iter((0..(32 * 1024 * 1024)).map(|_| '0'));
         let string_array = StringArray::from(vec![
@@ -624,6 +624,19 @@ pub mod tests {
             HashMap::new(),
         )
         .await;
+
+        // This is a regression testing the case where a page with X rows is split into Y parts
+        // where the number of parts is not evenly divisible by the number of rows.  In this
+        // case we are splitting 90 rows into 4 parts.
+        let big_string = String::from_iter((0..(1000 * 1000)).map(|_| '0'));
+        let string_array = StringArray::from_iter_values((0..90).map(|_| big_string.clone()));
+
+        check_round_trip_encoding_of_data(
+            vec![Arc::new(string_array)],
+            &TestCases::default(),
+            HashMap::new(),
+        )
+        .await;
     }
 
     #[test_log::test(tokio::test)]