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+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#pragma once
+
+/** \file
+ * \ingroup fn
+ *
+ * A `LazyFunction` encapsulates a computation which has inputs, outputs and potentially side
+ * effects. Most importantly, a `LazyFunction` supports laziness in its inputs and outputs:
+ * - Only outputs that are actually used have to be computed.
+ * - Inputs can be requested lazily based on which outputs are used or what side effects the
+ * function has.
+ *
+ * A lazy-function that uses laziness may be executed more than once. The most common example is
+ * the geometry nodes switch node. Depending on a condition input, it decides which one of the
+ * other inputs is actually used. From the perspective of the switch node, its execution works as
+ * follows:
+ * 1. The switch node is first executed. It sees that the output is used. Now it requests the
+ * condition input from the caller and exits.
+ * 2. Once the caller is able to provide the condition input the switch node is executed again.
+ * This time it retrieves the condition and requests one of the other inputs. Then the node
+ * exits again, giving back control to the caller.
+ * 3. When the caller computed the second requested input the switch node executes a last time.
+ * This time it retrieves the new input and forwards it to the output.
+ *
+ * In some sense, a lazy-function can be thought of like a state machine. Every time it is
+ * executed, it advances its state until all required outputs are ready.
+ *
+ * The lazy-function interface is designed to support composition of many such functions into a new
+ * lazy-functions, all while keeping the laziness working. For example, in geometry nodes a switch
+ * node in a node group should still be able to decide whether a node in the parent group will be
+ * executed or not. This is essential to avoid doing unnecessary work.
+ *
+ * The lazy-function system consists of multiple core components:
+ * - The interface of a lazy-function itself including its calling convention.
+ * - A graph data structure that allows composing many lazy-functions by connecting their inputs
+ * and outputs.
+ * - An executor that allows multi-threaded execution or such a graph.
+ */
+
+#include "BLI_cpp_type.hh"
+#include "BLI_generic_pointer.hh"
+#include "BLI_linear_allocator.hh"
+#include "BLI_vector.hh"
+
+namespace blender::fn::lazy_function {
+
+enum class ValueUsage {
+ /**
+ * The value is definitely used and therefore has to be computed.
+ */
+ Used,
+ /**
+ * It's unknown whether this value will be used or not. Computing it is ok but the result may be
+ * discarded.
+ */
+ Maybe,
+ /**
+ * The value will definitely not be used. It can still be computed but the result will be
+ * discarded in all cases.
+ */
+ Unused,
+};
+
+class LazyFunction;
+
+/**
+ * This allows passing arbitrary data into a lazy-function during execution. For that, #UserData
+ * has to be subclassed. This mainly exists because it's more type safe than passing a `void *`
+ * with no type information attached.
+ *
+ * Some lazy-functions may expect to find a certain type of user data when executed.
+ */
+class UserData {
+ public:
+ virtual ~UserData() = default;
+};
+
+/**
+ * Passed to the lazy-function when it is executed.
+ */
+struct Context {
+ /**
+ * If the lazy-function has some state (which only makes sense when it is executed more than once
+ * to finish its job), the state is stored here. This points to memory returned from
+ * #LazyFunction::init_storage.
+ */
+ void *storage;
+ /**
+ * Custom user data that can be used in the function.
+ */
+ UserData *user_data;
+};
+
+/**
+ * Defines the calling convention for a lazy-function. During execution, a lazy-function retrieves
+ * its inputs and sets the outputs through #Params.
+ */
+class Params {
+ public:
+ /**
+ * The lazy-function this #Params has been prepared for.
+ */
+ const LazyFunction &fn_;
+
+ public:
+ Params(const LazyFunction &fn);
+
+ /**
+ * Get a pointer to an input value if the value is available already. Otherwise null is returned.
+ *
+ * The #LazyFunction must leave returned object in an initialized state, but can move from it.
+ */
+ void *try_get_input_data_ptr(int index) const;
+
+ /**
+ * Same as #try_get_input_data_ptr, but if the data is not yet available, request it. This makes
+ * sure that the data will be available in a future execution of the #LazyFunction.
+ */
+ void *try_get_input_data_ptr_or_request(int index);
+
+ /**
+ * Get a pointer to where the output value should be stored.
+ * The value at the pointer is in an uninitialized state at first.
+ * The #LazyFunction is responsible for initializing the value.
+ * After the output has been initialized to its final value, #output_set has to be called.
+ */
+ void *get_output_data_ptr(int index);
+
+ /**
+ * Call this after the output value is initialized. After this is called, the value must not be
+ * touched anymore. It may be moved or destructed immediately.
+ */
+ void output_set(int index);
+
+ /**
+ * Allows the #LazyFunction to check whether an output was computed already without keeping
+ * track of it itself.
+ */
+ bool output_was_set(int index) const;
+
+ /**
+ * Can be used to detect which outputs have to be computed.
+ */
+ ValueUsage get_output_usage(int index) const;
+
+ /**
+ * Tell the caller of the #LazyFunction that a specific input will definitely not be used.
+ * Only an input that was not #ValueUsage::Used can become unused.
+ */
+ void set_input_unused(int index);
+
+ /**
+ * Typed utility methods that wrap the methods above.
+ */
+ template<typename T> T extract_input(int index);
+ template<typename T> const T &get_input(int index);
+ template<typename T> T *try_get_input_data_ptr_or_request(int index);
+ template<typename T> void set_output(int index, T &&value);
+
+ /**
+ * Utility to initialize all outputs that haven't been set yet.
+ */
+ void set_default_remaining_outputs();
+
+ private:
+ /**
+ * Methods that need to be implemented by subclasses. Those are separate from the non-virtual
+ * methods above to make it easy to insert additional debugging logic on top of the
+ * implementations.
+ */
+ virtual void *try_get_input_data_ptr_impl(int index) const = 0;
+ virtual void *try_get_input_data_ptr_or_request_impl(int index) = 0;
+ virtual void *get_output_data_ptr_impl(int index) = 0;
+ virtual void output_set_impl(int index) = 0;
+ virtual bool output_was_set_impl(int index) const = 0;
+ virtual ValueUsage get_output_usage_impl(int index) const = 0;
+ virtual void set_input_unused_impl(int index) = 0;
+};
+
+/**
+ * Describes an input of a #LazyFunction.
+ */
+struct Input {
+ /**
+ * Name used for debugging purposes. The string has to be static or has to be owned by something
+ * else.
+ */
+ const char *debug_name;
+ /**
+ * Data type of this input.
+ */
+ const CPPType *type;
+ /**
+ * Can be used to indicate a caller or this function if this input is used statically before
+ * executing it the first time. This is technically not needed but can improve efficiency because
+ * a round-trip through the `execute` method can be avoided.
+ *
+ * When this is #ValueUsage::Used, the caller has to ensure that the input is definitely
+ * available when the #execute method is first called. The #execute method does not have to check
+ * whether the value is actually available.
+ */
+ ValueUsage usage;
+
+ Input(const char *debug_name, const CPPType &type, const ValueUsage usage = ValueUsage::Used)
+ : debug_name(debug_name), type(&type), usage(usage)
+ {
+ }
+};
+
+struct Output {
+ /**
+ * Name used for debugging purposes. The string has to be static or has to be owned by something
+ * else.
+ */
+ const char *debug_name;
+ /**
+ * Data type of this output.
+ */
+ const CPPType *type = nullptr;
+
+ Output(const char *debug_name, const CPPType &type) : debug_name(debug_name), type(&type)
+ {
+ }
+};
+
+/**
+ * A function that can compute outputs and request inputs lazily. For more details see the comment
+ * at the top of the file.
+ */
+class LazyFunction {
+ protected:
+ const char *debug_name_ = "<unknown>";
+ Vector<Input> inputs_;
+ Vector<Output> outputs_;
+
+ public:
+ virtual ~LazyFunction() = default;
+
+ /**
+ * Get a name of the function or an input or output. This is mainly used for debugging.
+ * These are virtual functions because the names are often not used outside of debugging
+ * workflows. This way the names are only generated when they are actually needed.
+ */
+ virtual std::string name() const;
+ virtual std::string input_name(int index) const;
+ virtual std::string output_name(int index) const;
+
+ /**
+ * Allocates storage for this function. The storage will be passed to every call to #execute.
+ * If the function does not keep track of any state, this does not have to be implemented.
+ */
+ virtual void *init_storage(LinearAllocator<> &allocator) const;
+
+ /**
+ * Destruct the storage created in #init_storage.
+ */
+ virtual void destruct_storage(void *storage) const;
+
+ /**
+ * Inputs of the function.
+ */
+ Span<Input> inputs() const;
+ /**
+ * Outputs of the function.
+ */
+ Span<Output> outputs() const;
+
+ /**
+ * During execution the function retrieves inputs and sets outputs in #params. For some
+ * functions, this method is called more than once. After execution, the function either has
+ * computed all required outputs or is waiting for more inputs.
+ */
+ void execute(Params &params, const Context &context) const;
+
+ /**
+ * Utility to check that the guarantee by #Input::usage is followed.
+ */
+ bool always_used_inputs_available(const Params &params) const;
+
+ private:
+ /**
+ * Needs to be implemented by subclasses. This is separate from #execute so that additional
+ * debugging logic can be implemented in #execute.
+ */
+ virtual void execute_impl(Params &params, const Context &context) const = 0;
+};
+
+/* -------------------------------------------------------------------- */
+/** \name #LazyFunction Inline Methods
+ * \{ */
+
+inline Span<Input> LazyFunction::inputs() const
+{
+ return inputs_;
+}
+
+inline Span<Output> LazyFunction::outputs() const
+{
+ return outputs_;
+}
+
+inline void LazyFunction::execute(Params &params, const Context &context) const
+{
+ BLI_assert(this->always_used_inputs_available(params));
+ this->execute_impl(params, context);
+}
+
+/** \} */
+
+/* -------------------------------------------------------------------- */
+/** \name #Params Inline Methods
+ * \{ */
+
+inline Params::Params(const LazyFunction &fn) : fn_(fn)
+{
+}
+
+inline void *Params::try_get_input_data_ptr(const int index) const
+{
+ return this->try_get_input_data_ptr_impl(index);
+}
+
+inline void *Params::try_get_input_data_ptr_or_request(const int index)
+{
+ return this->try_get_input_data_ptr_or_request_impl(index);
+}
+
+inline void *Params::get_output_data_ptr(const int index)
+{
+ return this->get_output_data_ptr_impl(index);
+}
+
+inline void Params::output_set(const int index)
+{
+ this->output_set_impl(index);
+}
+
+inline bool Params::output_was_set(const int index) const
+{
+ return this->output_was_set_impl(index);
+}
+
+inline ValueUsage Params::get_output_usage(const int index) const
+{
+ return this->get_output_usage_impl(index);
+}
+
+inline void Params::set_input_unused(const int index)
+{
+ this->set_input_unused_impl(index);
+}
+
+template<typename T> inline T Params::extract_input(const int index)
+{
+ void *data = this->try_get_input_data_ptr(index);
+ BLI_assert(data != nullptr);
+ T return_value = std::move(*static_cast<T *>(data));
+ return return_value;
+}
+
+template<typename T> inline const T &Params::get_input(const int index)
+{
+ const void *data = this->try_get_input_data_ptr(index);
+ BLI_assert(data != nullptr);
+ return *static_cast<const T *>(data);
+}
+
+template<typename T> inline T *Params::try_get_input_data_ptr_or_request(const int index)
+{
+ return static_cast<T *>(this->try_get_input_data_ptr_or_request(index));
+}
+
+template<typename T> inline void Params::set_output(const int index, T &&value)
+{
+ using DecayT = std::decay_t<T>;
+ void *data = this->get_output_data_ptr(index);
+ new (data) DecayT(std::forward<T>(value));
+ this->output_set(index);
+}
+
+/** \} */
+
+} // namespace blender::fn::lazy_function