Function to access list offsets (#87)

arro3-compute/python/arro3/compute/_compute.pyi

@@ -46,6 +46,28 @@ def list_flatten(
         _description_
     """
 
+@overload
+def list_offsets(input: ArrowArrayExportable, *, physical: bool = True) -> Array: ...
+@overload
+def list_offsets(
+    input: ArrowStreamExportable, *, physical: bool = True
+) -> ArrayReader: ...
+def list_offsets(
+    input: ArrowArrayExportable | ArrowStreamExportable, *, physical: bool = True
+) -> Array | ArrayReader:
+    """Access the offsets of this ListArray or LargeListArray
+
+    Args:
+        input: _description_
+        physical: If True, return the physical (unsliced) offsets of the provided list array. Slicing offsets (False) is not yet implemented.
+
+    Raises:
+        Exception if not a list-typed array.
+
+    Returns:
+        _description_
+    """
+
 def struct_field(
     values: ArrowArrayExportable,
     /,

arro3-compute/src/lib.rs

@@ -3,6 +3,7 @@ use pyo3::prelude::*;
 mod cast;
 mod concat;
 mod list_flatten;
+mod list_offsets;
 mod struct_field;
 mod take;
 
@@ -22,6 +23,7 @@ fn _compute(_py: Python, m: &Bound<PyModule>) -> PyResult<()> {
     m.add_wrapped(wrap_pyfunction!(take::take))?;
 
     m.add_wrapped(wrap_pyfunction!(list_flatten::list_flatten))?;
+    m.add_wrapped(wrap_pyfunction!(list_offsets::list_offsets))?;
     m.add_wrapped(wrap_pyfunction!(struct_field::struct_field))?;
 
     Ok(())

arro3-compute/src/list_offsets.rs

@@ -0,0 +1,67 @@
+use std::sync::Arc;
+
+use arrow::array::AsArray;
+use arrow_array::{ArrayRef, Int32Array, Int64Array};
+use arrow_schema::{ArrowError, DataType, Field};
+use pyo3::prelude::*;
+use pyo3_arrow::error::PyArrowResult;
+use pyo3_arrow::ffi::ArrayIterator;
+use pyo3_arrow::input::AnyArray;
+use pyo3_arrow::{PyArray, PyArrayReader};
+
+#[pyfunction]
+#[pyo3(signature = (input, *, physical=true))]
+pub fn list_offsets(py: Python, input: AnyArray, physical: bool) -> PyArrowResult<PyObject> {
+    match input {
+        AnyArray::Array(array) => {
+            let (array, _field) = array.into_inner();
+            let offsets = _list_offsets(array, physical)?;
+            Ok(PyArray::from_array_ref(offsets).to_arro3(py)?)
+        }
+        AnyArray::Stream(stream) => {
+            let reader = stream.into_reader()?;
+            let out_field = match reader.field().data_type() {
+                DataType::List(_) => Field::new("", DataType::Int32, false),
+                DataType::LargeList(_) => Field::new("", DataType::Int64, false),
+                _ => {
+                    return Err(
+                        ArrowError::SchemaError("Expected list-typed Array".to_string()).into(),
+                    );
+                }
+            };
+
+            let iter = reader.into_iter().map(move |array| {
+                let out = _list_offsets(array?, physical)?;
+                Ok(out)
+            });
+            Ok(
+                PyArrayReader::new(Box::new(ArrayIterator::new(iter, out_field.into())))
+                    .to_arro3(py)?,
+            )
+        }
+    }
+}
+
+fn _list_offsets(array: ArrayRef, physical: bool) -> Result<ArrayRef, ArrowError> {
+    if !physical {
+        return Err(ArrowError::ComputeError(
+            "Logical list offset slicing not yet implemented".to_string(),
+        ));
+    }
+
+    match array.data_type() {
+        DataType::List(_) => {
+            let arr = array.as_list::<i32>();
+            let offsets = arr.offsets();
+            Ok(Arc::new(Int32Array::from(offsets.to_vec())))
+        }
+        DataType::LargeList(_) => {
+            let arr = array.as_list::<i64>();
+            let offsets = arr.offsets();
+            Ok(Arc::new(Int64Array::from(offsets.to_vec())))
+        }
+        _ => Err(ArrowError::SchemaError(
+            "Expected list-typed Array".to_string(),
+        )),
+    }
+}