/*
Copyright 2021 Josh Deprez

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package engine

import "encoding/gob"

// Ensure Anim satisfies Animer.
var _ interface {
	Cell() int
	Reset()
	Updater
} = &Anim{}

func init() {
	gob.Register(&Anim{})
}

// AnimDef defines an animation, as a sequence of steps and other information.
type AnimDef struct {
	Steps   []AnimStep
	OneShot bool
}

// NewAnim spawns a new anim using this def, or nil if d is nil.
func (d *AnimDef) NewAnim() *Anim {
	if d == nil {
		return nil
	}
	return &Anim{Def: d}
}

// AnimStep describes a step in an animation.
type AnimStep struct {
	Cell     int // show this cell
	Duration int // for this long, in ticks
}

// Anim is the current state of an animation being played (think of it as an
// instance of an AnimDef). nil *Anim can be used, but always returns 0 for the
// current frame.
type Anim struct {
	Def   *AnimDef
	Index int // current step index
	Ticks int // ticks spent at this step
}

// Cell returns the cell index for the current step.
func (a *Anim) Cell() int {
	if a == nil {
		return 0
	}
	return a.Def.Steps[a.Index].Cell
}

// Reset resets both Index and Ticks to 0.
func (a *Anim) Reset() {
	if a == nil {
		return
	}
	a.Index, a.Ticks = 0, 0
}

// Update increments the tick count and advances the frame if necessary.
func (a *Anim) Update() error {
	if a == nil {
		return nil
	}
	a.Ticks++
	if a.Def.OneShot && a.Index == len(a.Def.Steps)-1 {
		// on the last frame of a one shot so remain on final frame
		return nil
	}
	if a.Ticks >= a.Def.Steps[a.Index].Duration {
		a.Ticks = 0
		a.Index++
	}
	if !a.Def.OneShot && a.Index >= len(a.Def.Steps) {
		a.Index = 0
	}
	return nil
}