package westworld

// Entity is what every game object in the world must be able to do.
// The C++ code used a pure virtual function (Update()=0) to force
// every subclass to write its own Update. Go has no subclasses, so
// instead we say: "anything with an Update() and an ID() method
// counts as an Entity." A type only satisfies this interface once
// it has its own Update method - just like the C++ version couldn't
// be used until Update was implemented.
type Entity interface {
	Update()
	ID() int
}

// nextValidID plays the role of the C++ "static int m_iNextValidID".
// In C++, "static" on a class member means "one shared copy for the
// whole class, not one per object." Go doesn't have that keyword for
// structs, so we just use a normal package-level variable - it's
// shared by every BaseGameEntity for the same reason: there's only
// one of it.
var nextValidID int

// BaseGameEntity is the Go version of the C++ class BaseGameEntity.
// Go doesn't have "classes" or "inheritance" - instead, other types
// (like a Miner or a Wife later in the book) will embed this struct
// inside themselves to borrow its fields and methods, and then add
// their own Update method on top.
type BaseGameEntity struct {
	id int // was "m_ID" - the entity's own unique number
}

// NewBaseGameEntity replaces the C++ constructor:
//
//	BaseGameEntity(int id) { SetID(id); }
//
// Go structs don't run any code automatically when you create them,
// so we write a plain function that builds one correctly. Any code
// that wants a BaseGameEntity should call this instead of building
// the struct by hand.
func NewBaseGameEntity(id int) *BaseGameEntity {
	e := &BaseGameEntity{}
	e.setID(id)
	return e
}

// setID is the Go version of the private C++ method SetID. It does
// exactly what the comment in the book says: check the requested ID
// isn't lower than the next free one, store it, then move the
// "next free ID" counter past it.
//
// The C++ code used assert() for the check, which only runs in
// debug builds and does nothing in a release build. Go has no
// assert, so we use panic - it always runs, and it stops the
// program immediately if this ever happens, which is exactly what
// a broken invariant like this deserves.
func (e *BaseGameEntity) setID(val int) {
	if val < nextValidID {
		panic("BaseGameEntity.setID: invalid ID")
	}
	e.id = val
	nextValidID = e.id + 1
}

// ID returns the entity's unique number. This is the Go version of
// the C++ "int ID() const { return m_ID; }" - a small read-only
// accessor for the private field.
func (e *BaseGameEntity) ID() int {
	return e.id
}