// Mgmt
// Copyright (C) James Shubin and the project contributors
// Written by James Shubin <james@shubin.ca> and the project contributors
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.
//
// Additional permission under GNU GPL version 3 section 7
//
// If you modify this program, or any covered work, by linking or combining it
// with embedded mcl code and modules (and that the embedded mcl code and
// modules which link with this program, contain a copy of their source code in
// the authoritative form) containing parts covered by the terms of any other
// license, the licensors of this program grant you additional permission to
// convey the resulting work. Furthermore, the licensors of this program grant
// the original author, James Shubin, additional permission to update this
// additional permission if he deems it necessary to achieve the goals of this
// additional permission.

package unification

import (
	"context"
	"fmt"

	"github.com/purpleidea/mgmt/lang/interfaces"
	"github.com/purpleidea/mgmt/lang/types"
	"github.com/purpleidea/mgmt/util"
)

const (
	// ErrAmbiguous means we couldn't find a solution, but we weren't
	// inconsistent.
	ErrAmbiguous = util.Error("can't unify, no equalities were consumed, we're ambiguous")

	// StrategyNameKey is the string key used when choosing a solver name.
	StrategyNameKey = "name"

	// StrategyOptimizationsKey is the string key used to tell the solver
	// about the specific optimizations you'd like to request. The format
	// can be specific to each solver.
	StrategyOptimizationsKey = "optimizations"
)

// Init contains some handles that are used to initialize every solver. Each
// individual solver can choose to omit using some of the fields.
type Init struct {
	// Strategy is a hack to tune unification performance until we have an
	// overall cleaner unification algorithm in place.
	Strategy map[string]string

	// UnifiedState stores a common representation of our unification vars.
	UnifiedState *types.UnifiedState

	Debug bool
	Logf  func(format string, v ...interface{})
}

// Data contains the input data for the solver to process.
type Data struct {
	// UnificationInvariants is an alternate data representation for Solve.
	UnificationInvariants []*interfaces.UnificationInvariant
}

// Solver is the general interface that any solver needs to implement.
type Solver interface {
	// Init initializes the solver struct before first use.
	Init(*Init) error

	// Solve performs the actual solving. It must return as soon as possible
	// if the context is closed.
	Solve(context.Context, *Data) (*InvariantSolution, error)
}

// registeredSolvers is a global map of all possible unification solvers which
// can be used. You should never touch this map directly. Use methods like
// Register instead.
var registeredSolvers = make(map[string]func() Solver) // must initialize

// Register takes a solver and its name and makes it available for use. It is
// commonly called in the init() method of the solver at program startup. There
// is no matching Unregister function.
func Register(name string, solver func() Solver) {
	if _, exists := registeredSolvers[name]; exists {
		panic(fmt.Sprintf("a solver named %s is already registered", name))
	}

	//gob.Register(solver())
	registeredSolvers[name] = solver
}

// Lookup returns a pointer to the solver's struct.
func Lookup(name string) (Solver, error) {
	solver, exists := registeredSolvers[name]
	if !exists {
		return nil, fmt.Errorf("not found")
	}
	return solver(), nil
}

// LookupDefault attempts to return a "default" solver.
func LookupDefault() (Solver, error) {
	if len(registeredSolvers) == 0 {
		return nil, fmt.Errorf("no registered solvers")
	}
	if len(registeredSolvers) == 1 {
		for _, solver := range registeredSolvers {
			return solver(), nil // return the first and only one
		}
	}

	// TODO: Should we remove this empty string feature?
	// If one was registered with no name, then use that as the default.
	if solver, exists := registeredSolvers[""]; exists { // empty name
		return solver(), nil
	}

	return nil, fmt.Errorf("no registered default solver")
}