Functions: refactor virtual array data structures

When a function is executed for many elements (e.g. per point) it is often the case
that some parameters are different for every element and other parameters are
the same (there are some more less common cases). To simplify writing such
functions one can use a "virtual array". This is a data structure that has a value
for every index, but might not be stored as an actual array internally. Instead, it
might be just a single value or is computed on the fly. There are various tradeoffs
involved when using this data structure which are mentioned in `BLI_virtual_array.hh`.
It is called "virtual", because it uses inheritance and virtual methods.

Furthermore, there is a new virtual vector array data structure, which is an array
of vectors. Both these types have corresponding generic variants, which can be used
when the data type is not known at compile time. This is typically the case when
building a somewhat generic execution system. The function system used these virtual
data structures before, but now they are more versatile.

I've done this refactor in preparation for the attribute processor and other features of
geometry nodes. I moved the typed virtual arrays to blenlib, so that they can be used
independent of the function system.

One open question for me is whether all the generic data structures (and `CPPType`)
should be moved to blenlib as well. They are well isolated and don't really contain
any business logic. That can be done later if necessary.

1 files changed, 95 insertions, 0 deletions

diff --git a/source/blender/blenlib/BLI_virtual_vector_array.hh b/source/blender/blenlib/BLI_virtual_vector_array.hh
new file mode 100644
index 00000000000..ab5afd2d80a
--- /dev/null
+++ b/source/blender/blenlib/BLI_virtual_vector_array.hh
@@ -0,0 +1,95 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#pragma once
+
+/** \file
+ * \ingroup bli
+ *
+ * A virtual vector array gives access to an array of vectors. The individual vectors in the array
+ * can have different sizes.
+ *
+ * The tradeoffs here are similar to virtual arrays.
+ */
+
+#include "BLI_virtual_array.hh"
+
+namespace blender {
+
+/* A readonly virtual array of vectors. */
+template<typename T> class VVectorArray {
+ protected:
+  int64_t size_;
+
+ public:
+  VVectorArray(const int64_t size) : size_(size)
+  {
+    BLI_assert(size >= 0);
+  }
+
+  virtual ~VVectorArray() = default;
+
+  /* Returns the number of vectors in the vector array. */
+  int64_t size() const
+  {
+    return size_;
+  }
+
+  /* Returns true when there is no vector in the vector array. */
+  bool is_empty() const
+  {
+    return size_ == 0;
+  }
+
+  /* Returns the size of the vector at the given index. */
+  int64_t get_vector_size(const int64_t index) const
+  {
+    BLI_assert(index >= 0);
+    BLI_assert(index < size_);
+    return this->get_vector_size_impl(index);
+  }
+
+  /* Returns an element from one of the vectors. */
+  T get_vector_element(const int64_t index, const int64_t index_in_vector) const
+  {
+    BLI_assert(index >= 0);
+    BLI_assert(index < size_);
+    BLI_assert(index_in_vector >= 0);
+    BLI_assert(index_in_vector < this->get_vector_size(index));
+    return this->get_vector_element_impl(index, index_in_vector);
+  }
+
+  /* Returns true when the same vector is used at every index. */
+  bool is_single_vector() const
+  {
+    if (size_ == 1) {
+      return true;
+    }
+    return this->is_single_vector_impl();
+  }
+
+ protected:
+  virtual int64_t get_vector_size_impl(const int64_t index) const = 0;
+
+  virtual T get_vector_element_impl(const int64_t index, const int64_t index_in_vetor) const = 0;
+
+  virtual bool is_single_vector_impl() const
+  {
+    return false;
+  }
+};
+
+}  // namespace blender