fix: [9018]Fixed RunArray slice offsets(row, cast, eq) (#9213)

# Which issue does this PR close?

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- Closes #9018 .

# Rationale for this change

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in the issue then this section is not needed.
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The existing implementation had issue for row or cast ops for RunArray.
The equality implementation also did not support logical index-based
comparisons.
# What changes are included in this PR?

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is sometimes worth providing a summary of the individual changes in this
PR.
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1. Fixed RunArray row and cast handling
2. Added logical index based equality check for RunArray.
# Are these changes tested?

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Yes
# Are there any user-facing changes?

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No.

Author: manishkr

arrow-array/src/array/run_array.rs

@@ -141,6 +141,9 @@ impl<R: RunEndIndexType> RunArray<R> {
     ///
     /// [`values`]: Self::values
     pub fn values_slice(&self) -> ArrayRef {
+        if self.is_empty() {
+            return self.values.slice(0, 0);
+        }
         let start = self.get_start_physical_index();
         let end = self.get_end_physical_index();
         self.values.slice(start, end - start + 1)
@@ -653,6 +656,7 @@ mod tests {
     use super::*;
     use crate::builder::PrimitiveRunBuilder;
     use crate::cast::AsArray;
+    use crate::new_empty_array;
     use crate::types::{Int8Type, UInt32Type};
     use crate::{Int16Array, Int32Array, StringArray};
 
@@ -750,6 +754,26 @@ mod tests {
         assert_eq!(run_ends.values(), &run_ends_values);
     }
 
+    #[test]
+    fn test_run_array_empty() {
+        let runs = new_empty_array(&DataType::Int16);
+        let runs = runs.as_primitive::<Int16Type>();
+        let values = new_empty_array(&DataType::Int64);
+        let array = RunArray::try_new(runs, &values).unwrap();
+
+        fn assertions(array: &RunArray<Int16Type>) {
+            assert!(array.is_empty());
+            assert_eq!(array.get_start_physical_index(), 0);
+            assert_eq!(array.get_end_physical_index(), 0);
+            assert!(array.get_physical_indices::<i16>(&[]).unwrap().is_empty());
+            assert!(array.run_ends().is_empty());
+            assert_eq!(array.run_ends().sliced_values().count(), 0);
+        }
+
+        assertions(&array);
+        assertions(&array.slice(0, 0));
+    }
+
     #[test]
     fn test_run_array_fmt_debug() {
         let mut builder = PrimitiveRunBuilder::<Int16Type, UInt32Type>::with_capacity(3);
@@ -1184,4 +1208,91 @@ mod tests {
         let values_slice2 = values_slice2.as_primitive::<Int32Type>();
         assert_eq!(values_slice2.values(), &[1]);
     }
+
+    #[test]
+    fn test_run_array_values_slice_empty() {
+        let run_ends = Int32Array::from(vec![2, 5, 10]);
+        let values = StringArray::from(vec!["a", "b", "c"]);
+        let array = RunArray::<Int32Type>::try_new(&run_ends, &values).unwrap();
+
+        let slice = array.slice(0, 0);
+        assert_eq!(slice.len(), 0);
+
+        let values_slice = slice.values_slice();
+        assert_eq!(values_slice.len(), 0);
+        assert_eq!(values_slice.data_type(), &DataType::Utf8);
+    }
+
+    #[test]
+    fn test_run_array_eq_empty() {
+        let run_ends = Int32Array::from(vec![2, 5, 10]);
+        let values = StringArray::from(vec!["a", "b", "c"]);
+        let array = RunArray::<Int32Type>::try_new(&run_ends, &values).unwrap();
+
+        let slice1 = array.slice(0, 0);
+        let slice2 = array.slice(1, 0);
+        let slice3 = array.slice(10, 0);
+
+        assert_eq!(slice1, slice2);
+        assert_eq!(slice2, slice3);
+
+        let empty_array = new_empty_array(array.data_type());
+        let empty_array = crate::cast::as_run_array::<Int32Type>(empty_array.as_ref());
+
+        assert_eq!(&slice1, empty_array);
+    }
+
+    #[test]
+    fn test_run_array_eq_diff_physical_same_logical() {
+        let run_ends1 = Int32Array::from(vec![1, 3, 6]);
+        let values1 = StringArray::from(vec!["a", "b", "c"]);
+        let array1 = RunArray::<Int32Type>::try_new(&run_ends1, &values1).unwrap();
+
+        let run_ends2 = Int32Array::from(vec![1, 2, 3, 4, 5, 6]);
+        let values2 = StringArray::from(vec!["a", "b", "b", "c", "c", "c"]);
+        let array2 = RunArray::<Int32Type>::try_new(&run_ends2, &values2).unwrap();
+
+        assert_eq!(array1, array2);
+    }
+
+    #[test]
+    fn test_run_array_eq_sliced() {
+        let run_ends1 = Int32Array::from(vec![2, 5, 10]);
+        let values1 = StringArray::from(vec!["a", "b", "c"]);
+        let array1 = RunArray::<Int32Type>::try_new(&run_ends1, &values1).unwrap();
+        // Logical: a, a, b, b, b, c, c, c, c, c
+
+        let slice1 = array1.slice(1, 6);
+        // Logical: a, b, b, b, c, c
+
+        let run_ends2 = Int32Array::from(vec![1, 4, 6]);
+        let values2 = StringArray::from(vec!["a", "b", "c"]);
+        let array2 = RunArray::<Int32Type>::try_new(&run_ends2, &values2).unwrap();
+        // Logical: a, b, b, b, c, c
+
+        assert_eq!(slice1, array2);
+
+        let slice2 = array1.slice(2, 3);
+        // Logical: b, b, b
+        let run_ends3 = Int32Array::from(vec![3]);
+        let values3 = StringArray::from(vec!["b"]);
+        let array3 = RunArray::<Int32Type>::try_new(&run_ends3, &values3).unwrap();
+        assert_eq!(slice2, array3);
+    }
+
+    #[test]
+    fn test_run_array_eq_sliced_different_offsets() {
+        let run_ends1 = Int32Array::from(vec![2, 5, 10]);
+        let values1 = StringArray::from(vec!["a", "b", "c"]);
+        let array1 = RunArray::<Int32Type>::try_new(&run_ends1, &values1).unwrap();
+        let array2 = array1.clone();
+        assert_eq!(array1, array2);
+
+        let slice1 = array1.slice(1, 4); // a, b, b, b
+        let slice2 = array1.slice(1, 4);
+        assert_eq!(slice1, slice2);
+
+        let slice3 = array1.slice(0, 4); // a, a, b, b
+        assert_ne!(slice1, slice3);
+    }
 }

arrow-buffer/src/buffer/run.rs

@@ -199,9 +199,16 @@ where
     pub fn sliced_values(&self) -> impl Iterator<Item = E> + '_ {
         let offset = self.logical_offset;
         let len = self.logical_length;
-        let start = self.get_start_physical_index();
-        let end = self.get_end_physical_index();
-        self.run_ends[start..=end].iter().map(move |&val| {
+        // Doing this roundabout way since the iterator type we return must be
+        // the same (i.e. cannot use std::iter::empty())
+        let physical_slice = if self.is_empty() {
+            &self.run_ends[0..0]
+        } else {
+            let start = self.get_start_physical_index();
+            let end = self.get_end_physical_index();
+            &self.run_ends[start..=end]
+        };
+        physical_slice.iter().map(move |&val| {
             let val = val.as_usize().saturating_sub(offset).min(len);
             E::from_usize(val).unwrap()
         })

arrow-cast/src/cast/mod.rs

@@ -12590,4 +12590,32 @@ mod tests {
             assert_eq!(casted.as_ref(), &expected);
         }
     }
+
+    #[test]
+    fn test_cast_between_sliced_run_end_encoded() {
+        let run_ends = Int16Array::from(vec![2, 5, 8]);
+        let values = StringArray::from(vec!["a", "b", "c"]);
+
+        let ree_array = RunArray::<Int16Type>::try_new(&run_ends, &values).unwrap();
+        let ree_array = ree_array.slice(1, 2);
+        let array_ref = Arc::new(ree_array) as ArrayRef;
+
+        let target_type = DataType::RunEndEncoded(
+            Arc::new(Field::new("run_ends", DataType::Int64, false)),
+            Arc::new(Field::new("values", DataType::Utf8, true)),
+        );
+        let cast_options = CastOptions {
+            safe: false,
+            format_options: FormatOptions::default(),
+        };
+
+        let result = cast_with_options(&array_ref, &target_type, &cast_options).unwrap();
+        let run_array = result.as_run::<Int64Type>();
+        let run_array = run_array.downcast::<StringArray>().unwrap();
+
+        let expected = vec!["a", "b"];
+        let actual = run_array.into_iter().flatten().collect::<Vec<_>>();
+
+        assert_eq!(expected, actual);
+    }
 }

arrow-cast/src/cast/run_array.rs

@@ -32,17 +32,18 @@ pub(crate) fn run_end_encoded_cast<K: RunEndIndexType>(
                 .downcast_ref::<RunArray<K>>()
                 .ok_or_else(|| ArrowError::CastError("Expected RunArray".to_string()))?;
 
-            let values = run_array.values();
-
             match to_type {
                 // Stay as RunEndEncoded, cast only the values
                 DataType::RunEndEncoded(target_index_field, target_value_field) => {
-                    let cast_values =
-                        cast_with_options(values, target_value_field.data_type(), cast_options)?;
+                    let values = run_array.values_slice();
+                    let cast_values = cast_with_options(
+                        values.as_ref(),
+                        target_value_field.data_type(),
+                        cast_options,
+                    )?;
 
-                    let run_ends_array = PrimitiveArray::<K>::from_iter_values(
-                        run_array.run_ends().values().iter().copied(),
-                    );
+                    let run_ends_array =
+                        PrimitiveArray::<K>::from_iter_values(run_array.run_ends().sliced_values());
                     let cast_run_ends = cast_with_options(
                         &run_ends_array,
                         target_index_field.data_type(),
@@ -72,6 +73,7 @@ pub(crate) fn run_end_encoded_cast<K: RunEndIndexType>(
 
                 // Expand to logical form
                 _ => {
+                    let values = run_array.values();
                     let len = run_array.len();
                     let offset = run_array.offset();
                     let run_ends = run_array.run_ends().values();