[GitHub] [arrow] bkietz commented on a change in pull request #10445: ARROW-9140: [R] Zero-copy Arrow to R where possible

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bkietz commented on a change in pull request #10445:
URL: https://github.com/apache/arrow/pull/10445#discussion_r654608083



##########
File path: r/src/altrep.cpp
##########
@@ -0,0 +1,163 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <cpp11/altrep.hpp>
+
+#include "./arrow_types.h"
+
+#if defined(HAS_ALTREP)
+
+#include <R_ext/Altrep.h>
+#include <arrow/array.h>
+
+namespace arrow {
+namespace r {
+
+template <int sexp_type>
+struct ArrayNoNull {
+  using data_type = typename std::conditional<sexp_type == INTSXP, int, double>::type;
+
+  // altrep object around an Array with no nulls
+  // data1: an external pointer to a shared pointer to the Array
+  // data2: not used
+
+  static SEXP Make(R_altrep_class_t class_t, const std::shared_ptr<Array>& array) {
+    // we don't need the whole r6 object, just an external pointer
+    // that retain the array
+    cpp11::external_pointer<std::shared_ptr<Array>> xp(new std::shared_ptr<Array>(array));
+
+    SEXP res = R_new_altrep(class_t, xp, R_NilValue);
+    MARK_NOT_MUTABLE(res);
+
+    return res;
+  }
+
+  static Rboolean Inspect(SEXP x, int pre, int deep, int pvec,
+                          void (*inspect_subtree)(SEXP, int, int, int)) {
+    const auto& array = Get(x);
+    Rprintf("std::shared_ptr<arrow::Array, %s, NONULL> (len=%d, ptr=%p)\n",
+            array->type()->ToString().c_str(), array->length(), array.get());
+    return TRUE;
+  }
+
+  static const std::shared_ptr<Array>& Get(SEXP vec) {
+    return *cpp11::external_pointer<std::shared_ptr<Array>>(R_altrep_data1(vec));
+  }
+
+  static R_xlen_t Length(SEXP vec) { return Get(vec)->length(); }
+
+  static const void* Dataptr_or_null(SEXP vec) {
+    return Get(vec)->data()->template GetValues<data_type>(1);
+  }
+
+  static SEXP Duplicate(SEXP vec, Rboolean) {
+    const auto& array = Get(vec);
+    auto size = array->length();
+
+    SEXP copy = PROTECT(Rf_allocVector(sexp_type, array->length()));
+
+    memcpy(DATAPTR(copy), Dataptr_or_null(vec), size * sizeof(data_type));
+
+    UNPROTECT(1);
+    return copy;
+  }
+
+  static void* Dataptr(SEXP vec, Rboolean writeable) {
+    return const_cast<void*>(Dataptr_or_null(vec));
+  }
+
+  // by definition, there are no NA
+  static int No_NA(SEXP vec) { return 1; }
+
+  static void Init(R_altrep_class_t class_t, DllInfo* dll) {
+    // altrep
+    R_set_altrep_Length_method(class_t, ArrayNoNull::Length);
+    R_set_altrep_Inspect_method(class_t, ArrayNoNull::Inspect);
+    R_set_altrep_Duplicate_method(class_t, ArrayNoNull::Duplicate);
+
+    // altvec
+    R_set_altvec_Dataptr_method(class_t, ArrayNoNull::Dataptr);
+    R_set_altvec_Dataptr_or_null_method(class_t, ArrayNoNull::Dataptr_or_null);
+  }
+};
+
+struct DoubleArrayNoNull {
+  static R_altrep_class_t class_t;
+
+  static void Init(DllInfo* dll) {
+    class_t = R_make_altreal_class("array_nonull_dbl_vector", "arrow", dll);
+    ArrayNoNull<REALSXP>::Init(class_t, dll);
+    R_set_altreal_No_NA_method(class_t, ArrayNoNull<REALSXP>::No_NA);
+  }
+
+  static SEXP Make(const std::shared_ptr<Array>& array) {
+    return ArrayNoNull<REALSXP>::Make(class_t, array);
+  }
+};
+
+struct Int32ArrayNoNull {
+  static R_altrep_class_t class_t;
+
+  static void Init(DllInfo* dll) {
+    class_t = R_make_altinteger_class("array_nonull_int_vector", "arrow", dll);
+    ArrayNoNull<INTSXP>::Init(class_t, dll);
+    R_set_altinteger_No_NA_method(class_t, ArrayNoNull<INTSXP>::No_NA);
+  }
+
+  static SEXP Make(const std::shared_ptr<Array>& array) {
+    return ArrayNoNull<INTSXP>::Make(class_t, array);
+  }
+};
+
+R_altrep_class_t Int32ArrayNoNull::class_t;
+R_altrep_class_t DoubleArrayNoNull::class_t;
+
+void Init_Altrep_classes(DllInfo* dll) {
+  DoubleArrayNoNull::Init(dll);
+  Int32ArrayNoNull::Init(dll);
+}
+
+SEXP MakeDoubleArrayNoNull(const std::shared_ptr<Array>& array) {
+  return DoubleArrayNoNull::Make(array);
+}
+
+SEXP MakeInt32ArrayNoNull(const std::shared_ptr<Array>& array) {
+  return Int32ArrayNoNull::Make(array);

Review comment:
       Edge case worthy of mention in a comment somewhere: IIUC this doesn't check for instances of `NA_integer_` in `array` and these will be considered null by R even though `is_altrep_int_nonull(as.vector(array))` (and we report `no_NA = 1`)

##########
File path: r/src/altrep.cpp
##########
@@ -0,0 +1,163 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <cpp11/altrep.hpp>
+
+#include "./arrow_types.h"
+
+#if defined(HAS_ALTREP)
+
+#include <R_ext/Altrep.h>
+#include <arrow/array.h>
+
+namespace arrow {
+namespace r {
+
+template <int sexp_type>
+struct ArrayNoNull {
+  using data_type = typename std::conditional<sexp_type == INTSXP, int, double>::type;
+
+  // altrep object around an Array with no nulls
+  // data1: an external pointer to a shared pointer to the Array
+  // data2: not used
+
+  static SEXP Make(R_altrep_class_t class_t, const std::shared_ptr<Array>& array) {
+    // we don't need the whole r6 object, just an external pointer
+    // that retain the array
+    cpp11::external_pointer<std::shared_ptr<Array>> xp(new std::shared_ptr<Array>(array));
+
+    SEXP res = R_new_altrep(class_t, xp, R_NilValue);
+    MARK_NOT_MUTABLE(res);
+
+    return res;
+  }
+
+  static Rboolean Inspect(SEXP x, int pre, int deep, int pvec,
+                          void (*inspect_subtree)(SEXP, int, int, int)) {
+    const auto& array = Get(x);
+    Rprintf("std::shared_ptr<arrow::Array, %s, NONULL> (len=%d, ptr=%p)\n",
+            array->type()->ToString().c_str(), array->length(), array.get());

Review comment:
       would it be useful to include the external pointer in this?

##########
File path: r/src/array_to_vector.cpp
##########
@@ -144,6 +144,24 @@ Status IngestSome(const std::shared_ptr<arrow::Array>& array, R_xlen_t n,
 // Allocate + Ingest
 SEXP ArrayVector__as_vector(R_xlen_t n, const std::shared_ptr<DataType>& type,
                             const ArrayVector& arrays) {
+  // special case when there is only one array
+#if defined(HAS_ALTREP)

Review comment:
       Nit:
   ```suggestion
   #if defined(HAS_ALTREP)
     // special case when there is only one array
   ```




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