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/*
* 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 array is a data structure that behaves similar to an array, but its elements are
* accessed through virtual methods. This improves the decoupling of a function from its callers,
* because it does not have to know exactly how the data is laid out in memory, or if it is stored
* in memory at all. It could just as well be computed on the fly.
*
* Taking a virtual array as parameter instead of a more specific non-virtual type has some
* tradeoffs. Access to individual elements of the individual elements is higher due to function
* call overhead. On the other hand, potential callers don't have to convert the data into the
* specific format required for the function. This can be a costly conversion if only few of the
* elements are accessed in the end.
*
* Functions taking a virtual array as input can still optimize for different data layouts. For
* example, they can check if the array is stored as an array internally or if it is the same
* element for all indices. Whether it is worth to optimize for different data layouts in a
* function has to be decided on a case by case basis. One should always do some benchmarking to
* see of the increased compile time and binary size is worth it.
*/
#include "BLI_span.hh"
namespace blender {
/* An immutable virtual array. */
template<typename T> class VArray {
protected:
int64_t size_;
public:
VArray(const int64_t size) : size_(size)
{
BLI_assert(size_ >= 0);
}
virtual ~VArray() = default;
T get(const int64_t index) const
{
BLI_assert(index >= 0);
BLI_assert(index < size_);
return this->get_impl(index);
}
int64_t size() const
{
return size_;
}
bool is_empty() const
{
return size_ == 0;
}
/* Returns true when the virtual array is stored as a span internally. */
bool is_span() const
{
if (size_ == 0) {
return true;
}
return this->is_span_impl();
}
/* Returns the internally used span of the virtual array. This invokes undefined behavior is the
* virtual array is not stored as a span internally. */
Span<T> get_span() const
{
BLI_assert(this->is_span());
if (size_ == 0) {
return {};
}
return this->get_span_impl();
}
/* Returns true when the virtual array returns the same value for every index. */
bool is_single() const
{
if (size_ == 1) {
return true;
}
return this->is_single_impl();
}
/* Returns the value that is returned for every index. This invokes undefined behavior if the
* virtual array would not return the same value for every index. */
T get_single() const
{
BLI_assert(this->is_single());
if (size_ == 1) {
return this->get(0);
}
return this->get_single_impl();
}
T operator[](const int64_t index) const
{
return this->get(index);
}
protected:
virtual T get_impl(const int64_t index) const = 0;
virtual bool is_span_impl() const
{
return false;
}
virtual Span<T> get_span_impl() const
{
BLI_assert(false);
return {};
}
virtual bool is_single_impl() const
{
return false;
}
virtual T get_single_impl() const
{
/* Provide a default implementation, so that subclasses don't have to provide it. This method
* should never be called because `is_single_impl` returns false by default. */
BLI_assert(false);
return T();
}
};
/**
* A virtual array implementation for a span. This class is final so that it can be devirtualized
* by the compiler in some cases (e.g. when #devirtualize_varray is used).
*/
template<typename T> class VArrayForSpan final : public VArray<T> {
private:
const T *data_;
public:
VArrayForSpan(const Span<T> data) : VArray<T>(data.size()), data_(data.data())
{
}
protected:
T get_impl(const int64_t index) const override
{
return data_[index];
}
bool is_span_impl() const override
{
return true;
}
Span<T> get_span_impl() const override
{
return Span<T>(data_, this->size_);
}
};
/**
* A virtual array implementation that returns the same value for every index. This class is final
* so that it can be devirtualized by the compiler in some cases (e.g. when #devirtualize_varray is
* used).
*/
template<typename T> class VArrayForSingle final : public VArray<T> {
private:
T value_;
public:
VArrayForSingle(T value, const int64_t size) : VArray<T>(size), value_(std::move(value))
{
}
protected:
T get_impl(const int64_t UNUSED(index)) const override
{
return value_;
}
bool is_span_impl() const override
{
return this->size_ == 1;
}
Span<T> get_span_impl() const override
{
return Span<T>(&value_, 1);
}
bool is_single_impl() const override
{
return true;
}
T get_single_impl() const override
{
return value_;
}
};
/**
* Generate multiple versions of the given function optimized for different virtual arrays.
* One has to be careful with nesting multiple devirtualizations, because that results in an
* exponential number of function instantiations (increasing compile time and binary size).
*
* Generally, this function should only be used when the virtual method call overhead to get an
* element from a virtual array is significant.
*/
template<typename T, typename Func>
inline void devirtualize_varray(const VArray<T> &varray, const Func &func, bool enable = true)
{
/* Support disabling the devirtualization to simplify benchmarking. */
if (enable) {
if (varray.is_single()) {
/* `VArrayForSingle` can be used for devirtualization, because it is declared `final`. */
const VArrayForSingle<T> varray_single{varray.get_single(), varray.size()};
func(varray_single);
return;
}
if (varray.is_span()) {
/* `VArrayForSpan` can be used for devirtualization, because it is declared `final`. */
const VArrayForSpan<T> varray_span{varray.get_span()};
func(varray_span);
return;
}
}
func(varray);
}
/**
* Same as `devirtualize_varray`, but devirtualizes two virtual arrays at the same time.
* This is better than nesting two calls to `devirtualize_varray`, because it instantiates fewer
* cases.
*/
template<typename T1, typename T2, typename Func>
inline void devirtualize_varray2(const VArray<T1> &varray1,
const VArray<T2> &varray2,
const Func &func,
bool enable = true)
{
/* Support disabling the devirtualization to simplify benchmarking. */
if (enable) {
const bool is_span1 = varray1.is_span();
const bool is_span2 = varray2.is_span();
const bool is_single1 = varray1.is_single();
const bool is_single2 = varray2.is_single();
if (is_span1 && is_span2) {
const VArrayForSpan<T1> varray1_span{varray1.get_span()};
const VArrayForSpan<T2> varray2_span{varray2.get_span()};
func(varray1_span, varray2_span);
return;
}
if (is_span1 && is_single2) {
const VArrayForSpan<T1> varray1_span{varray1.get_span()};
const VArrayForSingle<T2> varray2_single{varray2.get_single(), varray2.size()};
func(varray1_span, varray2_single);
return;
}
if (is_single1 && is_span2) {
const VArrayForSingle<T1> varray1_single{varray1.get_single(), varray1.size()};
const VArrayForSpan<T2> varray2_span{varray2.get_span()};
func(varray1_single, varray2_span);
return;
}
if (is_single1 && is_single2) {
const VArrayForSingle<T1> varray1_single{varray1.get_single(), varray1.size()};
const VArrayForSingle<T2> varray2_single{varray2.get_single(), varray2.size()};
func(varray1_single, varray2_single);
return;
}
}
/* This fallback is used even when one of the inputs could be optimized. It's probably not worth
* it to optimize just one of the inputs, because then the compiler still has to call into
* unknown code, which inhibits many compiler optimizations. */
func(varray1, varray2);
}
} // namespace blender
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