/** Cursor pagination state (previous-page stack) shared by all paged tables. */
import { useCallback, useEffect, useMemo, useState } from "react";

export interface CursorPager {
  /** Cursor to pass to the backend for the current page (null = first page). */
  cursor: string | null;
  /** 1-based page number. */
  page: number;
  canPrev: boolean;
  goNext: (nextCursor: string | null) => void;
  goPrev: () => void;
  reset: () => void;
}

export interface PagerState {
  cursor: string | null;
  /** Stack of cursors that produced the previous pages (page 1 = null). */
  stack: (string | null)[];
}

export const INITIAL_PAGER_STATE: PagerState = { cursor: null, stack: [] };

/** Advance to the page identified by `nextCursor`; unchanged when null. */
export function advancePager(state: PagerState, nextCursor: string | null): PagerState {
  if (!nextCursor) return state;
  return { cursor: nextCursor, stack: [...state.stack, state.cursor] };
}

/** Go back one page; unchanged on the first page. */
export function retreatPager(state: PagerState): PagerState {
  if (state.stack.length === 0) return state;
  const stack = [...state.stack];
  const cursor = stack.pop() ?? null;
  return { cursor, stack };
}

export function pageNumber(state: PagerState): number {
  return state.stack.length + 1;
}

/**
 * Cursor pager that resets whenever `resetKey` changes (run id, filters…).
 */
export function useCursorPager(resetKey: string): CursorPager {
  const [state, setState] = useState<PagerState>(INITIAL_PAGER_STATE);

  useEffect(() => {
    setState(INITIAL_PAGER_STATE);
  }, [resetKey]);

  const goNext = useCallback((nextCursor: string | null) => {
    setState((s) => advancePager(s, nextCursor));
  }, []);

  const goPrev = useCallback(() => {
    setState((s) => retreatPager(s));
  }, []);

  const reset = useCallback(() => setState(INITIAL_PAGER_STATE), []);

  return useMemo(
    () => ({
      cursor: state.cursor,
      page: pageNumber(state),
      canPrev: state.stack.length > 0,
      goNext,
      goPrev,
      reset,
    }),
    [state, goNext, goPrev, reset],
  );
}