internal/component: Move Pool into lazy package

pkg/lazy/pool.go

@@ -0,0 +1,170 @@
+package lazy
+
+import (
+	"reflect"
+	"sync"
+)
+
+// Pool represents a pool of laziliy initialized and potentially referencing Component instances.
+//
+// Component must be an interface type, that should be implemented by various pointers to structs.
+// Components may reference each other, even circularly.
+//
+// Each type of struct is considered a singleton an initialized only once.
+//
+// See [Pool.All], [ExportComponents] and [ExportComponent].
+//
+// The zero value is ready to use.
+type Pool[Component any, InitParams any] struct {
+	// Init is called on every component to be initialized.
+	Init func(Component, InitParams) Component
+
+	all Lazy[[]Component]
+}
+
+// All initializes or returns all components stored in this pool.
+//
+// The All function should return an array of calls to [Make] with the provided context.
+// Multiple calls to the this method return the same return value.
+func (p *Pool[Component, InitParams]) All(Params InitParams, All func(context *PoolContext[Component]) []Component) []Component {
+	return p.all.Get(func() []Component {
+		// create a new context
+		context := &PoolContext[Component]{
+			all: All,
+			init: func(c Component) Component {
+				if p.Init == nil {
+					return c
+				}
+				return p.Init(c, Params)
+			},
+			cache: make(map[string]Component),
+		}
+
+		// and process them all
+		all := context.all(context)
+		context.Process(all)
+		return all
+	})
+}
+
+// PoolContext is a context used during [Make].
+type PoolContext[Component any] struct {
+	init func(Component) Component                         // initializes a new component
+	all  func(context *PoolContext[Component]) []Component // initializes all components
+
+	// cache for metas
+
+	// function to return all components
+
+	metaCache sync.Map
+	cache     map[string]Component     // cached components
+	queue     []delayedInit[Component] // init queue
+}
+
+type delayedInit[Component any] struct {
+	meta  meta[Component]
+	value reflect.Value
+}
+
+// Process processes all components in the queue
+func (p *PoolContext[Component]) Process(all []Component) {
+	index := 0
+	for len(p.queue) > index {
+		p.queue[index].Run(all)
+		index++
+	}
+	p.queue = nil
+}
+
+func (di *delayedInit[Component]) Run(all []Component) {
+	di.meta.InitComponent(di.value, all)
+}
+
+// Make creates or returns a cached component of the given Context.
+//
+// Components are initialized by first
+// Then all component-like fields of fields are filled with their appropriate components.
+//
+// A component-like field has one of the following types:
+//
+// - A pointer to a struct type that implements component
+// - A slice type of an interface type that implements component
+//
+// These fields are initialized in an undefined order during initialization.
+// The init function may not rely on these existing.
+// Furthermore, the init function may not cause other components to be initialized.
+//
+// The init function may be nil, indicating that no additional initialization is required.
+func Make[Component any, ConcreteComponent any](context *PoolContext[Component], init func(component ConcreteComponent)) ConcreteComponent {
+	// get a description of the type
+	cd := getMeta[Component, ConcreteComponent](&context.metaCache)
+
+	// if an instance already exists, return it!
+	if instance, ok := context.cache[cd.Name]; ok {
+		return any(instance).(ConcreteComponent)
+	}
+
+	// create a fresh new instance and store it in the cache
+	context.cache[cd.Name] = context.init(cd.New())
+	instance := any(context.cache[cd.Name]).(ConcreteComponent)
+
+	// call the passed init function
+	if init != nil {
+		init(instance)
+	}
+
+	// and queue it up
+	if cd.NeedsInitComponent() {
+		context.queue = append(context.queue, delayedInit[Component]{
+			meta:  cd,
+			value: reflect.ValueOf(instance),
+		})
+	}
+
+	return instance
+}
+
+//
+// PUBLIC FUNCTIONS
+//
+
+// ExportComponents exports all components that are a ConcreteComponentType from the pool.
+//
+// All should be the function of the core that initializes all components.
+// All should only make calls to [InitComponent].
+func ExportComponents[Component any, InitParams any, ConcreteComponentType any](
+	p *Pool[Component, InitParams],
+	Params InitParams,
+	All func(context *PoolContext[Component]) []Component,
+) []ConcreteComponentType {
+	components := p.All(Params, All)
+
+	results := make([]ConcreteComponentType, 0, len(components))
+	for _, comp := range components {
+		if cc, ok := any(comp).(ConcreteComponentType); ok {
+			results = append(results, cc)
+		}
+	}
+	return results
+}
+
+// ExportComponent exports the first component that is a ConcreteComponent from the pool.
+//
+// All should be the function of the core that initializes all components.
+// All should only make calls to [InitComponent].
+func ExportComponent[Component any, InitParams any, ConcreteComponentType any](
+	pool *Pool[Component, InitParams],
+	Params InitParams,
+	All func(context *PoolContext[Component]) []Component,
+) ConcreteComponentType {
+	components := pool.All(Params, All)
+
+	for _, comp := range components {
+		if cc, ok := any(comp).(ConcreteComponentType); ok {
+			return cc
+		}
+	}
+
+	var component ConcreteComponentType
+	return component
+}