Replace multi-variable connections with functions (#518)

include/cse/algorithm/algorithm.hpp

@@ -2,8 +2,8 @@
 #define CSECORE_ALGORITHM_ALGORITHM_HPP
 
 #include <cse/algorithm/simulator.hpp>
-#include <cse/connection.hpp>
 #include <cse/execution.hpp>
+#include <cse/function/function.hpp>
 #include <cse/model.hpp>
 #include <cse/observer/observer.hpp>
 
@@ -64,25 +64,65 @@ class algorithm
     virtual void remove_simulator(simulator_index index) = 0;
 
     /**
-     * Adds a connection to the co-simulation.
+     *  Adds a function to the co-simulation.
      *
-     * After this, the algorithm is responsible for acquiring the values of
-     * the connection's source variables, and distributing the connection's
-     * destination variable values at communication points.
+     *  \param index
+     *      A numerical index that will be used to identify the function
+     *      in other function calls.
+     *  \param fun
+     *      A pointer to an object that is used to access the function.
+     *      Note that the algorithm does not have resource ownership of
+     *      the object it points to (i.e., should not try to delete it).
+     */
+    virtual void add_function(function_index index, function* fun) = 0;
+
+    /**
+     *  Connects a simulator output variable to a simulator input variable.
+     *
+     *  After this, the algorithm is responsible for acquiring the value of
+     *  the output variable and assigning it to the input variable at
+     *  communication points.
      *
-     * It is assumed that the variables contained by the connection are valid
-     * and that there are no existing connections to any of the connection's
-     * destination variables.
+     *  \param output
+     *      A reference to the output variable.
+     *  \param input
+     *      A reference to the input variable.
      */
-    virtual void add_connection(std::shared_ptr<connection> conn) = 0;
+    virtual void connect_variables(variable_id output, variable_id input) = 0;
 
     /**
-     * Removes a connection from the co-simulation.
+     *  Connects a simulator output variable to a function input variable.
      *
-     * It is assumed that the connection has previously been added to the
-     * co-simulation with `add_connection()`.
+     *  After this, the algorithm is responsible for acquiring the value of
+     *  the output variable and assigning it to the input variable before
+     *  the function is calculated.
+     *
+     *  \param output
+     *      A reference to the output variable.
+     *  \param input
+     *      A reference to the input variable.
+     */
+    virtual void connect_variables(variable_id output, function_io_id input) = 0;
+
+    /**
+     *  Connects a function output variable to a simulator input variable.
+     *
+     *  After this, the algorithm is responsible for acquiring the value of
+     *  the output variable and assigning it to the input variable after
+     *  the function is calculated.
+     *
+     *  \param output
+     *      A reference to the output variable.
+     *  \param input
+     *      A reference to the input variable.
      */
-    virtual void remove_connection(std::shared_ptr<connection> conn) = 0;
+    virtual void connect_variables(function_io_id output, variable_id input) = 0;
+
+    /// Breaks any previously established connection to input variable `input`.
+    virtual void disconnect_variable(variable_id input) = 0;
+
+    /// Breaks any previously established connection to input variable `input`.
+    virtual void disconnect_variable(function_io_id input) = 0;
 
     /**
      *  Performs initial setup.

include/cse/algorithm/fixed_step_algorithm.hpp

@@ -35,8 +35,12 @@ class fixed_step_algorithm : public algorithm
     // `algorithm` methods
     void add_simulator(simulator_index i, simulator* s, duration stepSizeHint) override;
     void remove_simulator(simulator_index i) override;
-    void add_connection(std::shared_ptr<connection> c) override;
-    void remove_connection(std::shared_ptr<connection> c) override;
+    void add_function(function_index i, function* f) override;
+    void connect_variables(variable_id output, variable_id input) override;
+    void connect_variables(variable_id output, function_io_id input) override;
+    void connect_variables(function_io_id output, variable_id input) override;
+    void disconnect_variable(variable_id input) override;
+    void disconnect_variable(function_io_id input) override;
     void setup(time_point startTime, std::optional<time_point> stopTime) override;
     void initialize() override;
     std::pair<duration, std::unordered_set<simulator_index>> do_step(time_point currentT) override;