Implement GetOptimalObjectiveValue function to evaluate objective at solution point (#26)

* Initial plan

* Implement GetOptimalObjectiveValue function with comprehensive tests

Co-authored-by: kwesiRutledge <[email protected]>

* Change FindValueOfExpression signature to return symbolic.Expression (#27)

* Initial plan

* Change FindValueOfExpression signature to return symbolic.Expression

Co-authored-by: kwesiRutledge <[email protected]>

* Add helper function to reduce code duplication in tests

Co-authored-by: kwesiRutledge <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: kwesiRutledge <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: kwesiRutledge <[email protected]>

Author: Copilot

solution/solution.go

@@ -44,8 +44,8 @@ func ExtractValueOfVariable(s Solution, v symbolic.Variable) (float64, error) {
 
 // FindValueOfExpression evaluates a symbolic expression using the values from a solution.
 // It substitutes all variables in the expression with their values from the solution
-// and returns the resulting scalar value.
-func FindValueOfExpression(s Solution, expr symbolic.Expression) (float64, error) {
+// and returns the resulting symbolic expression (typically a constant).
+func FindValueOfExpression(s Solution, expr symbolic.Expression) (symbolic.Expression, error) {
 	// Get all variables in the expression
 	vars := expr.Variables()
 
@@ -54,7 +54,7 @@ func FindValueOfExpression(s Solution, expr symbolic.Expression) (float64, error
 	for _, v := range vars {
 		val, err := ExtractValueOfVariable(s, v)
 		if err != nil {
-			return 0.0, fmt.Errorf(
+			return nil, fmt.Errorf(
 				"failed to extract value for variable %v: %w",
 				v.ID,
 				err,
@@ -66,6 +66,31 @@ func FindValueOfExpression(s Solution, expr symbolic.Expression) (float64, error
 	// Substitute all variables with their values
 	resultExpr := expr.SubstituteAccordingTo(subMap)
 
+	return resultExpr, nil
+}
+
+// GetOptimalObjectiveValue evaluates the objective function of an optimization problem
+// at the solution point. It uses the FindValueOfExpression function to compute the value
+// of the objective expression using the variable values from the solution.
+func GetOptimalObjectiveValue(sol Solution) (float64, error) {
+	// Get the problem from the solution
+	prob := sol.GetProblem()
+	if prob == nil {
+		return 0.0, fmt.Errorf("solution does not have an associated problem")
+	}
+
+	// Get the objective expression from the problem
+	objectiveExpr := prob.Objective.Expression
+	if objectiveExpr == nil {
+		return 0.0, fmt.Errorf("problem does not have a defined objective")
+	}
+
+	// Use FindValueOfExpression to evaluate the objective at the solution point
+	resultExpr, err := FindValueOfExpression(sol, objectiveExpr)
+	if err != nil {
+		return 0.0, fmt.Errorf("failed to evaluate objective expression: %w", err)
+	}
+
 	// Type assert to K (constant) to extract the float64 value
 	resultK, ok := resultExpr.(symbolic.K)
 	if !ok {

testing/solution/solution_test.go

@@ -17,6 +17,15 @@ Description:
 	(This seems like it is highly representative of the Gurobi solver; is there a reason to make it this way?)
 */
 
+// Helper function to convert a symbolic.Expression to float64
+func exprToFloat64(t *testing.T, expr symbolic.Expression) float64 {
+	resultK, ok := expr.(symbolic.K)
+	if !ok {
+		t.Fatalf("Expected result to be a constant, got type %T", expr)
+	}
+	return float64(resultK)
+}
+
 func TestSolution_ToMessage1(t *testing.T) {
 	// Constants
 	tempSol := solution.DummySolution{
@@ -161,11 +170,13 @@ func TestSolution_FindValueOfExpression1(t *testing.T) {
 	expr := v1.Multiply(symbolic.K(2.0)).Plus(v2.Multiply(symbolic.K(3.0)))
 
 	// Algorithm
-	result, err := solution.FindValueOfExpression(&tempSol, expr)
+	resultExpr, err := solution.FindValueOfExpression(&tempSol, expr)
 	if err != nil {
 		t.Errorf("FindValueOfExpression returned an error: %v", err)
 	}
 
+	result := exprToFloat64(t, resultExpr)
+
 	expected := 13.0
 	if result != expected {
 		t.Errorf(
@@ -194,11 +205,13 @@ func TestSolution_FindValueOfExpression2(t *testing.T) {
 	expr := symbolic.K(42.0)
 
 	// Algorithm
-	result, err := solution.FindValueOfExpression(&tempSol, expr)
+	resultExpr, err := solution.FindValueOfExpression(&tempSol, expr)
 	if err != nil {
 		t.Errorf("FindValueOfExpression returned an error: %v", err)
 	}
 
+	result := exprToFloat64(t, resultExpr)
+
 	expected := 42.0
 	if result != expected {
 		t.Errorf(
@@ -231,11 +244,13 @@ func TestSolution_FindValueOfExpression3(t *testing.T) {
 	expr := v1.Plus(symbolic.K(10.0))
 
 	// Algorithm
-	result, err := solution.FindValueOfExpression(&tempSol, expr)
+	resultExpr, err := solution.FindValueOfExpression(&tempSol, expr)
 	if err != nil {
 		t.Errorf("FindValueOfExpression returned an error: %v", err)
 	}
 
+	result := exprToFloat64(t, resultExpr)
+
 	expected := 15.5
 	if result != expected {
 		t.Errorf(
@@ -304,11 +319,13 @@ func TestSolution_FindValueOfExpression5(t *testing.T) {
 	expr := v1.Plus(v2).Multiply(v3).Plus(symbolic.K(5.0))
 
 	// Algorithm
-	result, err := solution.FindValueOfExpression(&tempSol, expr)
+	resultExpr, err := solution.FindValueOfExpression(&tempSol, expr)
 	if err != nil {
 		t.Errorf("FindValueOfExpression returned an error: %v", err)
 	}
 
+	result := exprToFloat64(t, resultExpr)
+
 	expected := 14.0
 	if result != expected {
 		t.Errorf(
@@ -377,3 +394,173 @@ func TestSolution_GetProblem2(t *testing.T) {
 		t.Errorf("Expected GetProblem to return nil when no problem is set")
 	}
 }
+
+/*
+TestSolution_GetOptimalObjectiveValue1
+Description:
+
+	This function tests whether we can compute the objective value at the solution point
+	for a simple linear objective.
+*/
+func TestSolution_GetOptimalObjectiveValue1(t *testing.T) {
+	// Constants
+	p := problem.NewProblem("TestProblem")
+	v1 := p.AddVariable()
+	v2 := p.AddVariable()
+
+	// Set objective: 2*v1 + 3*v2
+	objectiveExpr := v1.Multiply(symbolic.K(2.0)).Plus(v2.Multiply(symbolic.K(3.0)))
+	err := p.SetObjective(objectiveExpr, problem.SenseMinimize)
+	if err != nil {
+		t.Errorf("Failed to set objective: %v", err)
+	}
+
+	// Create solution with v1=2.0, v2=3.0
+	// Expected objective value: 2*2.0 + 3*3.0 = 4.0 + 9.0 = 13.0
+	tempSol := solution.DummySolution{
+		Values: map[uint64]float64{
+			v1.ID: 2.0,
+			v2.ID: 3.0,
+		},
+		Objective: 13.0,
+		Status:    solution_status.OPTIMAL,
+		Problem:   p,
+	}
+
+	// Algorithm
+	objectiveValue, err := solution.GetOptimalObjectiveValue(&tempSol)
+	if err != nil {
+		t.Errorf("GetOptimalObjectiveValue returned an error: %v", err)
+	}
+
+	expected := 13.0
+	if objectiveValue != expected {
+		t.Errorf(
+			"Expected objective value to be %v; received %v",
+			expected,
+			objectiveValue,
+		)
+	}
+}
+
+/*
+TestSolution_GetOptimalObjectiveValue2
+Description:
+
+	This function tests whether GetOptimalObjectiveValue returns an error
+	when the solution has no associated problem.
+*/
+func TestSolution_GetOptimalObjectiveValue2(t *testing.T) {
+	// Constants
+	v1 := symbolic.NewVariable()
+
+	tempSol := solution.DummySolution{
+		Values: map[uint64]float64{
+			v1.ID: 2.0,
+		},
+		Objective: 2.3,
+		Status:    solution_status.OPTIMAL,
+		Problem:   nil,
+	}
+
+	// Algorithm
+	_, err := solution.GetOptimalObjectiveValue(&tempSol)
+	if err == nil {
+		t.Errorf("Expected GetOptimalObjectiveValue to return an error for nil problem, but got nil")
+	}
+}
+
+/*
+TestSolution_GetOptimalObjectiveValue3
+Description:
+
+	This function tests whether we can compute the objective value
+	for a constant objective function.
+*/
+func TestSolution_GetOptimalObjectiveValue3(t *testing.T) {
+	// Constants
+	p := problem.NewProblem("TestProblem")
+	v1 := p.AddVariable()
+
+	// Set constant objective: 42.0
+	objectiveExpr := symbolic.K(42.0)
+	err := p.SetObjective(objectiveExpr, problem.SenseMaximize)
+	if err != nil {
+		t.Errorf("Failed to set objective: %v", err)
+	}
+
+	// Create solution
+	tempSol := solution.DummySolution{
+		Values: map[uint64]float64{
+			v1.ID: 1.0,
+		},
+		Objective: 42.0,
+		Status:    solution_status.OPTIMAL,
+		Problem:   p,
+	}
+
+	// Algorithm
+	objectiveValue, err := solution.GetOptimalObjectiveValue(&tempSol)
+	if err != nil {
+		t.Errorf("GetOptimalObjectiveValue returned an error: %v", err)
+	}
+
+	expected := 42.0
+	if objectiveValue != expected {
+		t.Errorf(
+			"Expected objective value to be %v; received %v",
+			expected,
+			objectiveValue,
+		)
+	}
+}
+
+/*
+TestSolution_GetOptimalObjectiveValue4
+Description:
+
+	This function tests whether we can compute the objective value
+	for a more complex objective with multiple variables and operations.
+*/
+func TestSolution_GetOptimalObjectiveValue4(t *testing.T) {
+	// Constants
+	p := problem.NewProblem("TestProblem")
+	v1 := p.AddVariable()
+	v2 := p.AddVariable()
+	v3 := p.AddVariable()
+
+	// Set objective: (v1 + v2) * v3 + 5
+	objectiveExpr := v1.Plus(v2).Multiply(v3).Plus(symbolic.K(5.0))
+	err := p.SetObjective(objectiveExpr, problem.SenseMinimize)
+	if err != nil {
+		t.Errorf("Failed to set objective: %v", err)
+	}
+
+	// Create solution with v1=1.0, v2=2.0, v3=3.0
+	// Expected objective: (1.0 + 2.0) * 3.0 + 5 = 3.0 * 3.0 + 5 = 14.0
+	tempSol := solution.DummySolution{
+		Values: map[uint64]float64{
+			v1.ID: 1.0,
+			v2.ID: 2.0,
+			v3.ID: 3.0,
+		},
+		Objective: 14.0,
+		Status:    solution_status.OPTIMAL,
+		Problem:   p,
+	}
+
+	// Algorithm
+	objectiveValue, err := solution.GetOptimalObjectiveValue(&tempSol)
+	if err != nil {
+		t.Errorf("GetOptimalObjectiveValue returned an error: %v", err)
+	}
+
+	expected := 14.0
+	if objectiveValue != expected {
+		t.Errorf(
+			"Expected objective value to be %v; received %v",
+			expected,
+			objectiveValue,
+		)
+	}
+}