add missing file

Signed-off-by: BubbleCal <bubble-cal@outlook.com>

Author: BubbleCal

rust/lance-index/src/vector/flat/transform.rs

@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The Lance Authors
+
+use arrow::array::AsArray;
+use arrow_array::RecordBatch;
+use arrow_schema::Field;
+use lance_arrow::RecordBatchExt;
+use lance_core::Error;
+use snafu::{location, Location};
+use tracing::instrument;
+
+use crate::vector::transform::Transformer;
+
+use super::storage::FLAT_COLUMN;
+
+#[derive(Debug)]
+pub struct FlatTransformer {
+    input_column: String,
+}
+
+impl FlatTransformer {
+    pub fn new(input_column: impl AsRef<str>) -> Self {
+        Self {
+            input_column: input_column.as_ref().to_owned(),
+        }
+    }
+}
+
+impl Transformer for FlatTransformer {
+    #[instrument(name = "FlatTransformer::transform", level = "debug", skip_all)]
+    fn transform(&self, batch: &RecordBatch) -> crate::Result<RecordBatch> {
+        let input_arr = batch
+            .column_by_name(&self.input_column)
+            .ok_or(Error::Index {
+                message: format!(
+                    "FlatTransform: column {} not found in batch",
+                    self.input_column
+                ),
+                location: location!(),
+            })?;
+        let field = Field::new(
+            FLAT_COLUMN,
+            input_arr.data_type().clone(),
+            input_arr.is_nullable(),
+        );
+        // rename the column to FLAT_COLUMN
+        let batch = batch
+            .drop_column(&self.input_column)?
+            .try_with_column(field, input_arr.clone())?;
+        Ok(batch)
+    }
+}