// Mgmt
// Copyright (C) 2013-2021+ 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 <http://www.gnu.org/licenses/>.

package structs

import (
	"fmt"

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

// FunctionFunc is a function that passes through the function body it receives.
type FunctionFunc struct {
	Type *types.Type // this is the type of the function that we hold
	Edge string      // name of the edge used (typically "body")
	Func interfaces.Func
	Fn   *types.FuncValue

	init   *interfaces.Init
	last   types.Value // last value received to use for diff
	result types.Value // last calculated output

	closeChan chan struct{}
}

// fn returns the function that wraps the Func interface if that API is used.
func (obj *FunctionFunc) fn() (*types.FuncValue, error) {
	fn := func(args []types.Value) (types.Value, error) {
		// FIXME: can we run a recursive engine
		// to support running non-pure functions?
		if !obj.Func.Info().Pure {
			return nil, fmt.Errorf("only pure functions can be used by value")
		}

		// XXX: this might not be needed anymore...
		return funcs.PureFuncExec(obj.Func, args)
	}

	result := types.NewFunc(obj.Type) // new func
	if err := result.Set(fn); err != nil {
		return nil, errwrap.Wrapf(err, "can't build func from built-in")
	}

	return result, nil
}

// Validate makes sure we've built our struct properly.
func (obj *FunctionFunc) Validate() error {
	if obj.Type == nil {
		return fmt.Errorf("must specify a type")
	}
	if obj.Type.Kind != types.KindFunc {
		return fmt.Errorf("can't use type `%s`", obj.Type.String())
	}
	if obj.Edge == "" && obj.Func == nil && obj.Fn == nil {
		return fmt.Errorf("must specify an Edge, Func, or Fn")
	}

	if obj.Fn != nil && obj.Fn.Type() != obj.Type {
		return fmt.Errorf("type of Fn did not match input Type")
	}

	return nil
}

// Info returns some static info about itself.
func (obj *FunctionFunc) Info() *interfaces.Info {
	var typ *types.Type
	if obj.Type != nil { // don't panic if called speculatively
		typ = &types.Type{
			Kind: types.KindFunc, // function type
			Map:  make(map[string]*types.Type),
			Ord:  []string{},
			Out:  obj.Type, // after the function is called it's this...
		}

		// type of body is what we'd get by running the function (what's inside)
		if obj.Edge != "" {
			typ.Map[obj.Edge] = obj.Type.Out
			typ.Ord = append(typ.Ord, obj.Edge)
		}
	}

	pure := true // assume true
	if obj.Func != nil {
		pure = obj.Func.Info().Pure
	}

	return &interfaces.Info{
		Pure: pure,  // TODO: can we guarantee this?
		Memo: false, // TODO: ???
		Sig:  typ,
		Err:  obj.Validate(),
	}
}

// Init runs some startup code for this composite function.
func (obj *FunctionFunc) Init(init *interfaces.Init) error {
	obj.init = init
	obj.closeChan = make(chan struct{})
	return nil
}

// Stream takes an input struct in the format as described in the Func and Graph
// methods of the Expr, and returns the actual expected value as a stream based
// on the changing inputs to that value.
func (obj *FunctionFunc) Stream() error {
	defer close(obj.init.Output) // the sender closes
	for {
		select {
		case input, ok := <-obj.init.Input:
			if !ok {
				if obj.Edge != "" { // then it's not a built-in
					return nil // can't output any more
				}

				var result *types.FuncValue

				if obj.Fn != nil {
					result = obj.Fn
				} else {
					var err error
					result, err = obj.fn()
					if err != nil {
						return err
					}
				}

				// if we never had input args, send the function
				select {
				case obj.init.Output <- result: // send
					// pass
				case <-obj.closeChan:
					return nil
				}

				return nil
			}
			//if err := input.Type().Cmp(obj.Info().Sig.Input); err != nil {
			//	return errwrap.Wrapf(err, "wrong function input")
			//}
			if obj.last != nil && input.Cmp(obj.last) == nil {
				continue // value didn't change, skip it
			}
			obj.last = input // store for next

			var result types.Value

			st := input.(*types.StructValue)     // must be!
			value, exists := st.Lookup(obj.Edge) // single argName
			if !exists {
				return fmt.Errorf("missing expected input argument `%s`", obj.Edge)
			}

			result = obj.Type.New() // new func
			fn := func([]types.Value) (types.Value, error) {
				return value, nil
			}
			if err := result.(*types.FuncValue).Set(fn); err != nil {
				return errwrap.Wrapf(err, "can't build func with body")
			}

			// skip sending an update...
			if obj.result != nil && result.Cmp(obj.result) == nil {
				continue // result didn't change
			}
			obj.result = result // store new result

		case <-obj.closeChan:
			return nil
		}

		select {
		case obj.init.Output <- obj.result: // send
			// pass
		case <-obj.closeChan:
			return nil
		}
	}
}

// Close runs some shutdown code for this function and turns off the stream.
func (obj *FunctionFunc) Close() error {
	close(obj.closeChan)
	return nil
}
