package main

type Component1 struct{ value int }
type Component2 struct{ value int }
type Component3 struct{ value int }

type System struct {
	// Always use pointer to base component type (!)
	// Add `ento:"bind"` tag to automatically bind components
	Component1 *Component1 `ento:"bind"`

	// Components are bound by type, not name
	Renamed *Component2 `ento:"bind"`

	// Systems can have non-component-bound fields
	calls int
}

// Update implements ento.System
// It will be called only for entities that have both tagged components
func (s *System) Update(entity *ento.Entity) {
	// Access automatically bound components from currently passed entity
	s.Component1.value += s.Renamed.value

	// Get components from entity manually (somewhat slower than automatic binding)
	// Can be used for "optional" components
	var c3 *Component3
	entity.Get(&c3)

	// If entity does not have the component, the pointer value will be set to nil
	if c3 != nil {
		s.Component1.value += c3.value
	}

	s.calls++
}

func (s *System) reportCalls() {
	println(s.calls)
	s.calls = 0
}

func main() {
	// Create the world and register components
	world := ento.NewWorldBuilder().
		// Use "zero-values" as values are ignored when registering
		WithSparseComponents(Component1{}, Component2{}, Component3{}).
		// Pre-allocate space for 256 entities (world can grow beyond that automatically)
		Build(256)

	// Add systems
	system := &System{}
	world.AddSystems(system)

	// Create entities (they are added to the world immediately)
	world.NewEntity(Component1{1}, Component2{2}, Component3{3})

	// Use Set to add or update their components
	entity := world.NewEntity()
	entity.Set(Component1{0}) // Will not be handled by System

	// Update the world
	world.Update()
	system.reportCalls() // Prints: 1

	// Update the entity (add Component2)
	entity.Set(Component2{5})

	world.Update()
	system.reportCalls() // Prints: 2

	// Update the entity (remove Component2)
	entity.Rem(Component2{})

	world.Update()
	system.reportCalls() // Prints: 1
}

MIT License

Copyright (c) 2021 Wojciech Franczyk

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.