Expose strings_column_view in Cython and use it for char buffer size

Rather than reinventing the wheel, use the libcudf functionality.

Author: wence-

cpp/include/cudf/strings/strings_column_view.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2024, NVIDIA CORPORATION.
+ * Copyright (c) 2019-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -43,6 +43,8 @@ class strings_column_view : private column_view {
    * @param strings_column The column view to wrap.
    */
   strings_column_view(column_view strings_column);
+  // So we can use this from cython.
+  strings_column_view()                           = default;
   strings_column_view(strings_column_view&&)      = default;  ///< Move constructor
   strings_column_view(strings_column_view const&) = default;  ///< Copy constructor
   ~strings_column_view() override                 = default;

python/pylibcudf/pylibcudf/column.pyx

@@ -8,7 +8,7 @@ from cpython.pycapsule cimport (
 )
 
 from libc.stddef cimport size_t
-from libc.stdint cimport uintptr_t, int32_t, int64_t
+from libc.stdint cimport uintptr_t
 
 from libcpp.limits cimport numeric_limits
 from libcpp.memory cimport make_unique, unique_ptr
@@ -27,7 +27,8 @@ from pylibcudf.libcudf.interop cimport (
     to_arrow_schema_raw,
 )
 from pylibcudf.libcudf.null_mask cimport bitmask_allocation_size_bytes
-from pylibcudf.libcudf.scalar.scalar cimport scalar, numeric_scalar
+from pylibcudf.libcudf.scalar.scalar cimport scalar
+from pylibcudf.libcudf.strings.strings_column_view cimport strings_column_view
 from pylibcudf.libcudf.types cimport size_type, size_of as cpp_size_of, bitmask_type
 from pylibcudf.libcudf.utilities.traits cimport is_fixed_width
 from pylibcudf.libcudf.copying cimport get_element
@@ -88,26 +89,13 @@ cdef class OwnerWithCAI:
         obj = OwnerWithCAI()
         obj.owner = owner
         cdef size_t size
-        cdef column_view offsets_column
-        cdef unique_ptr[scalar] last_offset
         if cv.type().id() == type_id.EMPTY:
             size = cv.size()
         elif is_fixed_width(cv.type()):
             size = cv.size() * cpp_size_of(cv.type())
         elif cv.type().id() == type_id.STRING:
-            # The size of the character array in the parent is the offsets size
-            num_children = cv.num_children()
-            size = 0
-            # A strings column with no children is created for empty/all null
-            if num_children:
-                offsets_column = cv.child(0)
-                last_offset = get_element(offsets_column, offsets_column.size() - 1)
-                if offsets_column.type().id() == type_id.INT32:
-                    size = (<numeric_scalar[int32_t] *> last_offset.get()).value()
-                elif offsets_column.type().id() == type_id.INT64:
-                    size = (<numeric_scalar[int64_t] *>last_offset.get()).value()
-                else:
-                    raise RuntimeError("Invalid strings column offset dtype")
+            # TODO: stream-ordered
+            size = strings_column_view(cv).chars_size(_get_stream().view())
         else:
             # All other types store data in the children, so the parent size is 0
             size = 0

python/pylibcudf/pylibcudf/libcudf/strings/strings_column_view.pxd

@@ -0,0 +1,15 @@
+# Copyright (c) 2025, NVIDIA CORPORATION.
+
+from libc.stdint cimport int64_t
+from libcpp cimport bool
+from libcpp.vector cimport vector
+from pylibcudf.exception_handler cimport libcudf_exception_handler
+from pylibcudf.libcudf.types cimport bitmask_type, data_type, size_type
+from pylibcudf.libcudf.column.column_view cimport column_view
+
+from rmm.librmm.cuda_stream_view cimport cuda_stream_view
+
+cdef extern from "cudf/strings/strings_column_view.hpp" namespace "cudf" nogil:
+    cdef cppclass strings_column_view:
+        strings_column_view(column_view) except +libcudf_exception_handler
+        int64_t chars_size(cuda_stream_view) except +libcudf_exception_handler