/**
 * Navigation safety (§41): keep recent page fingerprints / urls / actions and detect A→B→A→B patterns.
 */
export class LoopDetector {
  private fingerprints: string[] = [];
  private actions: string[] = [];
  constructor(private readonly window = 12) {}

  record(pageFingerprint: string, actionKey: string): void {
    this.fingerprints.push(pageFingerprint);
    this.actions.push(actionKey);
    if (this.fingerprints.length > this.window) this.fingerprints.shift();
    if (this.actions.length > this.window) this.actions.shift();
  }

  /** True when the last four page states alternate (A B A B) or the same page repeats 4×. */
  detect(): { loop: boolean; pattern?: string } {
    const f = this.fingerprints;
    if (f.length >= 4) {
      const [a, b, c, d] = f.slice(-4);
      if (a === c && b === d && a !== b) return { loop: true, pattern: "ABAB" };
      if (a === b && b === c && c === d) return { loop: true, pattern: "AAAA" };
    }
    const acts = this.actions.slice(-6);
    if (acts.length === 6 && new Set(acts).size === 1 && !acts[0]!.startsWith("SCROLL_DOWN")) return { loop: true, pattern: "same_action×6" };
    return { loop: false };
  }

  recentActionTypes(): string[] {
    return this.actions.map((a) => a.split(":")[0]!);
  }
}