/**
 * Tiny framework-free store for the page transition („Umblättern“).
 * TransitionLink calls `startTransition`; Curtain subscribes, animates,
 * pushes the route and finally calls `endTransition`.
 */
export type TransitionPhase = "idle" | "cover" | "pushed" | "reveal";

export interface TransitionState {
  phase: TransitionPhase;
  href: string | null;
  label?: string;
}

type Listener = (state: TransitionState) => void;

let state: TransitionState = { phase: "idle", href: null };
const listeners = new Set<Listener>();

function emit() {
  listeners.forEach((cb) => cb(state));
}

/** Begins a transition. Returns false when one is already running (double clicks). */
export function startTransition(href: string, label?: string): boolean {
  if (state.phase !== "idle") return false;
  state = { phase: "cover", href, label };
  emit();
  return true;
}

/** Internal: Curtain advances the phase. */
export function setTransitionPhase(phase: TransitionPhase) {
  state = { ...state, phase };
  emit();
}

export function endTransition() {
  state = { phase: "idle", href: null };
  emit();
}

export function isTransitioning(): boolean {
  return state.phase !== "idle";
}

export function getTransitionState(): TransitionState {
  return state;
}

export function subscribe(cb: Listener): () => void {
  listeners.add(cb);
  return () => {
    listeners.delete(cb);
  };
}