package main

type Message interface{}

type Syncer interface {
    Sync() error
}

type EntityId interface{}

type Entity interface {
    Id() EntityId
    Consume(Message) (Syncer, bool)
}

type Downstream chan Message

type StopChan chan struct{}

type EntityFactory func(message Message) Entity

type EntityRunner struct {
    entity     Entity
    downstream Downstream
}

type Core struct {
    sources  map[Downstream]StopChan
    mux      Downstream
    entities map[EntityId]EntityRunner

    upstreamHandlerCount int

    stopChan StopChan
}

func NewCore(upstreamCount int) *Core {
    return &Core{
        sources:              make(map[Downstream]StopChan),
        mux:                  make(Downstream),
        entities:             make(map[EntityId]EntityRunner),
        upstreamHandlerCount: upstreamCount,
        stopChan:             make(StopChan),
    }
}

func (c *Core) WithSources(srcList ...Downstream) *Core {
    for _, src := range srcList {
        stopChan := make(StopChan)
        c.sources[src] = stopChan
        go func(st StopChan, source Downstream) {
            for {
                select {
                case msg := <-source:
                    c.mux <- msg
                case <-st:
                    return
                }
            }
        }(stopChan, src)
    }
    return c
}

func (c *Core) RemoveSource(src Downstream) *Core {
    if s, ok := c.sources[src]; ok {
        close(s)
        delete(c.sources, src)
    }
    return c
}

func (core *Core) Stop() {
    close(core.stopChan)
}
func (core *Core) Run(efactory EntityFactory) {
    go func() {
        upstream := make(chan Syncer)

        for i := 0; i < core.upstreamHandlerCount; i++ {
            go func() {
                for {
                    select {
                    case upstreamMsg := <-upstream:
                        upstreamMsg.Sync()
                    case <-core.stopChan:
                        return
                    }
                }
            }()
        }

        for {
            select {
            case msg := <-core.mux:
                entity := efactory(msg)
                id := entity.Id()
                if entityRunner, ok := core.entities[id]; ok {
                    entityRunner.downstream <- msg
                } else {
                    entityDownstream := make(Downstream)
                    core.entities[id] = EntityRunner{entity, entityDownstream}
                    go func() {
                        for {
                            select {
                            case entityMsg := <-entityDownstream:
                                if res, ok := entity.Consume(entityMsg); ok {
                                    upstream <- res
                                }
                            case <-core.stopChan:
                                return
                            }
                        }
                    }()
                    entityDownstream <- msg
                }
            case <-core.stopChan:
                core.entities = make(map[EntityId]EntityRunner)
                return
            }
        }
    }()
}