import type { RouterConfig, SessionBoundaryAction, SessionStartReason } from "./types.js";

export const SESSION_START_REASONS = ["startup", "resume", "new", "reload", "fork"] as const;
export const DEFAULT_RESTORE_ON: SessionStartReason[] = ["startup", "resume"];
export const DEFAULT_RESELECT_ON: SessionStartReason[] = ["new", "reload", "fork"];

/**
 * Resolves whether a session start should restore the saved selection or choose a fresh model.
 *
 * The default policy preserves continuity for process startup and explicit resume, while session
 * creation, extension reload, and fork boundaries intentionally get a fresh weighted selection.
 * Optional config arrays can override those buckets; omitted or unknown buckets fall back to the
 * default policy for forward-compatible behavior.
 */
export function resolveSessionBoundaryAction(
  reason: SessionStartReason,
  config?: Pick<RouterConfig, "sessionBoundary">,
): SessionBoundaryAction {
  const restoreOn = config?.sessionBoundary?.restoreOn ?? DEFAULT_RESTORE_ON;
  const reselectOn = config?.sessionBoundary?.reselectOn ?? DEFAULT_RESELECT_ON;

  if (restoreOn.includes(reason)) return "restore";
  if (reselectOn.includes(reason)) return "reselect";

  return DEFAULT_RESTORE_ON.includes(reason) ? "restore" : "reselect";
}

/** Returns true when a string is one of pi's supported session_start reason values. */
export function isSessionStartReason(value: string): value is SessionStartReason {
  return (SESSION_START_REASONS as readonly string[]).includes(value);
}