tsp/internal/solver/problem/problem.go

// SPDX-License-Identifier: Apache-2.0

package problem

type Problem[State comparable] interface {
	// Discards the given state, allowing any resources associated with it to be released.
	Discard(state State)

	// Returns true if the first state is more likely to be a better solution than the second state,
	// assuming the best case scenario. This is used to determine the best state to expand next.
	//
	// If this were to compare the distance traveled so far, this would be equivalent to Dijkstra's algorithm.
	//
	// If you added a heuristic to estimate the remaining distance, this would be equivalent to A*.
	//
	// For the traveling salesman problem, you might use traveled distance plus half of the remaining greedy tour length as a lower bound (optimistic) estimate.
	OptimisticLess(a State, b State) bool

	// Returns true if the first state is more likely to be a better solution than the second state,
	// assuming the worst case scenario. This is used to determine if the worst state to discard when at capacity.
	//
	// This can be equivalent to OptimisticLess in many cases.
	//
	// For the traveling salesman problem, you might use traveled distance plus the remaining greedy tour length as an upper bound (pessimistic) estimate.
	//
	// You generally want to penalize states with a lot of uncertainty about their actual cost, especially
	// for something like the traveling salesman problem where finding the optimal solution is impractical.

	// We could maybe handle finding the optimal TSP solution for 100 points, but definitely not 1000.
	// For that, you'd likely want to cluster your points (maybe using k-means) into a manageable number of groups,
	// and recursively solve the problem on each group.
	PessimisticLess(a State, b State) bool
}

type ProblemStateUpdates[State comparable] interface {
	Problem[State]

	// If this function returns true, then the solver needs to keep track of
	// known states so that it can update their relative order when they're resubmitted.
	//
	// If a problem doesn't implement this method, then true is assumed by default.
	StateUpdates() bool
}

func RequiresStateUpdates[P Problem[State], State comparable](p Problem[State]) bool {
	if p, ok := p.(ProblemStateUpdates[State]); ok {
		return p.StateUpdates()
	}

	// return true as a safe default.
	return true
}