fix: Support slicing RunEndBool arrays to 0 elements (#1511)

Author: robert3005

encodings/runend-bool/src/compute/mod.rs

@@ -69,16 +69,52 @@ impl TakeFn<RunEndBoolArray> for RunEndBoolEncoding {
 
 impl SliceFn<RunEndBoolArray> for RunEndBoolEncoding {
     fn slice(&self, array: &RunEndBoolArray, start: usize, stop: usize) -> VortexResult<ArrayData> {
-        let slice_begin = array.find_physical_index(start)?;
-        let slice_end = array.find_physical_index(stop)?;
+        let new_length = stop - start;
+
+        let (slice_begin, slice_end) = if new_length == 0 {
+            let ends_len = array.ends().len();
+            (ends_len, ends_len)
+        } else {
+            let physical_begin = array.find_physical_index(start)?;
+            let physical_end = array.find_physical_index(stop)?;
+            (physical_begin, physical_end + 1)
+        };
 
         Ok(RunEndBoolArray::with_offset_and_size(
-            slice(array.ends(), slice_begin, slice_end + 1)?,
+            slice(array.ends(), slice_begin, slice_end)?,
             value_at_index(slice_begin, array.start()),
-            array.validity().slice(slice_begin, slice_end + 1)?,
-            stop - start,
-            start + array.offset(),
+            array.validity().slice(start, stop)?,
+            new_length,
+            if new_length == 0 {
+                0
+            } else {
+                start + array.offset()
+            },
         )?
         .into_array())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use vortex_array::compute::slice;
+    use vortex_array::validity::Validity;
+    use vortex_array::{ArrayLen, IntoArrayData};
+
+    use crate::RunEndBoolArray;
+
+    #[test]
+    fn slice_at_end() {
+        let re_array =
+            RunEndBoolArray::try_new(vec![7_u64, 10].into_array(), false, Validity::NonNullable)
+                .unwrap();
+
+        assert_eq!(re_array.len(), 10);
+
+        let sliced_array = slice(&re_array, re_array.len(), re_array.len()).unwrap();
+        assert!(sliced_array.is_empty());
+
+        let re_slice = RunEndBoolArray::try_from(sliced_array).unwrap();
+        assert!(re_slice.ends().is_empty());
+    }
+}

encodings/runend/src/array.rs

@@ -68,7 +68,7 @@ impl RunEndArray {
             );
         }
 
-        if offset != 0 && !ends.is_empty() {
+        if offset != 0 {
             let first_run_end: usize = scalar_at(&ends, 0)?.as_ref().try_into()?;
             if first_run_end <= offset {
                 vortex_bail!("First run end {first_run_end} must be bigger than offset {offset}");

encodings/runend/src/compute/mod.rs

@@ -132,7 +132,11 @@ impl SliceFn<RunEndArray> for RunEndEncoding {
             slice(array.ends(), slice_begin, slice_end)?,
             slice(array.values(), slice_begin, slice_end)?,
             array.validity().slice(start, stop)?,
-            start + array.offset(),
+            if new_length == 0 {
+                0
+            } else {
+                start + array.offset()
+            },
             new_length,
         )?
         .into_array())