Overview

This PR adds a new FindValueOfExpression function to the solution package that evaluates symbolic expressions using variable values from a solution object. This addresses the need to compute the value of arbitrary expressions after solving an optimization problem.

Motivation

When working with optimization problems, users often need to evaluate expressions (e.g., objective functions, constraint expressions, or derived quantities) using the optimal variable values found by the solver. Previously, there was no convenient way to do this programmatically using the Solution interface.

Changes

New Function

func FindValueOfExpression(s Solution, expr symbolic.Expression) (float64, error)

Parameters:

• s Solution: A solution object containing variable values
• expr symbolic.Expression: A symbolic expression to evaluate

Returns:

• float64: The evaluated value of the expression
• error: An error if any variable in the expression is missing from the solution

Implementation

The function:

1. Extracts all variables from the symbolic expression
2. Retrieves each variable's value from the solution using the existing ExtractValueOfVariable helper
3. Creates a substitution map to replace variables with constants
4. Substitutes all variables with their values from the solution
5. Returns the resulting constant value

Usage Example

// Create variables and a solution
x := symbolic.NewVariable()
y := symbolic.NewVariable()

sol := solution.DummySolution{
    Values: map[uint64]float64{
        x.ID: 2.0,
        y.ID: 3.0,
    },
    Status: solution_status.OPTIMAL,
}

// Evaluate expression: 2*x + 3*y = 2*2 + 3*3 = 13
expr := x.Multiply(symbolic.K(2.0)).Plus(y.Multiply(symbolic.K(3.0)))
value, err := solution.FindValueOfExpression(&sol, expr)
// value = 13.0

Testing

Added 5 comprehensive test cases covering:

• Linear expressions with multiple variables
• Constant expressions (no variables)
• Single-variable expressions
• Error handling for missing variables
• Complex expressions with nested operations

All tests pass successfully.

Design Decisions

• Uses the existing Solution interface methods, maintaining compatibility with any implementation
• Leverages the ExtractValueOfVariable helper for consistency with existing code
• Provides clear error messages when variables are missing or unexpected types are encountered
• Follows the repository's error handling patterns and code style

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