# A running solver for a given problem, that can be configured and controlled.
#
#
# # Basic run and results.
#
# 1. Instantiate it with the method `solve` from `BacktrackProblem`.
# 2. Apply the method `run`, that will search and return a solution.
# 3. Retrieve information from the solution.
#
# ~~~~nitish
# var p: BacktrackProblem = new MyProblem
# var solver = p.solve
# var res = solver.run
# if res != null then
#   print "Found solution in {res.depth} actions: {res.plan.join(", ")}"
#   print "The state of the solution is: {solver.state}"
# end
# ~~~~
#
#
# # Step-by-step runs and multiple runs
#
# The `run_steps` method (see also `steps`, and `steps_limit`) can be used to run only a maximum number of steps.
# Thus, this method can be used as a *co-routine* and be run periodically for a small amount of time.
#
# `run` and `run_steps` return the next solution.
# A subsequent call to `run` returns the following solution and so on.
#
# When there is no more solutions available, `null` is returned and `is_running` become false.
#
# Between run, the state of the current search can be read.
#
#
# # Search-trees
#
# Internally, solvers use a zipper on the virtual search-tree where nodes are elements in the apply/backtrack graph.
# See the class `BacktrackNode` for details
#
# The `run` and `node` methods return a `BacktrackNode` that can be used to retrieve a lot of useful information,
# like the full `path` or the `plan`.
# If only the solved state is required, the `state` method from the solver gives it.
class BacktrackSolver[S: Object, A]
	# The problem currently solved
	var problem: BacktrackProblem[S,A]

	# The current state.
	# Do not modify it directly: the solver will do that by its own use of
	# `problem.apply_action` and `problem.backtrack`.
	var state: S

	# The current `node` in the backtrack-zipper.
	var node: nullable BacktrackNode[A] = null

	# Is the solver still running?
	# A running solver has not yet exhausted all the possible solutions.
	var is_running = true

	# Initialize `node`
	private fun start: BacktrackNode[A]
	do
		assert node == null
		var node = new BacktrackNode[A](null, null, 0, 0)
		self.node = node
		return node
	end


	# The total steps executed since the beginning.
	var steps = 0

	# Limit in the number of steps for a `run`.
	#
	# One can modify this value then `run` or just call `run_steps`.
	#
	# Use 0 for no limit.
	# Default: 0
	var steps_limit = 0 is writable

	# Update `steps_limit` then just run some additional steps.
	# Return the `node` corresponding to the found solution, or `null` if no solution is found.
	fun run_steps(steps: Int): nullable BacktrackNode[A]
	do
		steps_limit = self.steps + steps
		return run
	end

	# Run the solver and return the next solution found (if any).
	# Return null is one of these is true:
	# * `steps_limit` is reached
	# * no more reachable solution, in this case `is_running` become false.
	fun run: nullable BacktrackNode[A]
	do
		var node = self.node
		# Not yet started, of finished?
		if node == null then
			if steps > 0 then return null
			node = start
			var res = problem.is_goal(state)
			if res then return node
		end

		loop
			if steps_limit > 0 and steps > steps_limit then break
			steps += 1

			var totry = node.totry

			# It is the first visit in this state?
			if totry == null then
				var actions = problem.actions(state, node)
				if actions != null and not actions.is_empty then
					totry = actions.to_a
					node.totry = totry
				end
			end

			#print state
			#print node

			# No remaining actions?
			if totry == null or totry.is_empty then
				#print "Backtrack"
				var a = node.action
				if a == null then
					#print "no more action"
					is_running = false
					self.node = null
					return null
				end

				problem.backtrack(state, a)
				node = node.parent
				assert node != null
				continue
			end

			var a = totry.pop
			problem.apply_action(state, a)
			#print "Play {a or else ""}"
			node = new BacktrackNode[A](node, a, node.depth+1, steps)

			var res = problem.is_goal(state)
			if res then
				self.node = node
				return node
			end
		end
		self.node = node
		return null
	end

	redef fun to_s do return "{node or else "#0"}"
end