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, 158 insertions, 0 deletions

diff --git a/source/blender/functions/FN_generic_span.hh b/source/blender/functions/FN_generic_span.hh
new file mode 100644
index 00000000000..31b67dd3d70
--- /dev/null
+++ b/source/blender/functions/FN_generic_span.hh
@@ -0,0 +1,158 @@
+/*
+ * 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 fn
+ */
+
+#include "BLI_span.hh"
+
+#include "FN_cpp_type.hh"
+
+namespace blender::fn {
+
+/**
+ * A generic span. It behaves just like a blender::Span<T>, but the type is only known at run-time.
+ */
+class GSpan {
+ private:
+  const CPPType *type_;
+  const void *data_;
+  int64_t size_;
+
+ public:
+  GSpan(const CPPType &type, const void *buffer, int64_t size)
+      : type_(&type), data_(buffer), size_(size)
+  {
+    BLI_assert(size >= 0);
+    BLI_assert(buffer != nullptr || size == 0);
+    BLI_assert(type.pointer_has_valid_alignment(buffer));
+  }
+
+  GSpan(const CPPType &type) : GSpan(type, nullptr, 0)
+  {
+  }
+
+  template<typename T>
+  GSpan(Span<T> array)
+      : GSpan(CPPType::get<T>(), static_cast<const void *>(array.data()), array.size())
+  {
+  }
+
+  const CPPType &type() const
+  {
+    return *type_;
+  }
+
+  bool is_empty() const
+  {
+    return size_ == 0;
+  }
+
+  int64_t size() const
+  {
+    return size_;
+  }
+
+  const void *data() const
+  {
+    return data_;
+  }
+
+  const void *operator[](int64_t index) const
+  {
+    BLI_assert(index < size_);
+    return POINTER_OFFSET(data_, type_->size() * index);
+  }
+
+  template<typename T> Span<T> typed() const
+  {
+    BLI_assert(type_->is<T>());
+    return Span<T>(static_cast<const T *>(data_), size_);
+  }
+};
+
+/**
+ * A generic mutable span. It behaves just like a blender::MutableSpan<T>, but the type is only
+ * known at run-time.
+ */
+class GMutableSpan {
+ private:
+  const CPPType *type_;
+  void *data_;
+  int64_t size_;
+
+ public:
+  GMutableSpan(const CPPType &type, void *buffer, int64_t size)
+      : type_(&type), data_(buffer), size_(size)
+  {
+    BLI_assert(size >= 0);
+    BLI_assert(buffer != nullptr || size == 0);
+    BLI_assert(type.pointer_has_valid_alignment(buffer));
+  }
+
+  GMutableSpan(const CPPType &type) : GMutableSpan(type, nullptr, 0)
+  {
+  }
+
+  template<typename T>
+  GMutableSpan(MutableSpan<T> array)
+      : GMutableSpan(CPPType::get<T>(), static_cast<void *>(array.begin()), array.size())
+  {
+  }
+
+  operator GSpan() const
+  {
+    return GSpan(*type_, data_, size_);
+  }
+
+  const CPPType &type() const
+  {
+    return *type_;
+  }
+
+  bool is_empty() const
+  {
+    return size_ == 0;
+  }
+
+  int64_t size() const
+  {
+    return size_;
+  }
+
+  void *data() const
+  {
+    return data_;
+  }
+
+  void *operator[](int64_t index) const
+  {
+    BLI_assert(index >= 0);
+    BLI_assert(index < size_);
+    return POINTER_OFFSET(data_, type_->size() * index);
+  }
+
+  template<typename T> MutableSpan<T> typed() const
+  {
+    BLI_assert(type_->is<T>());
+    return MutableSpan<T>(static_cast<T *>(data_), size_);
+  }
+};
+
+}  // namespace blender::fn