import { useCallback, useEffect, useMemo, useReducer, useState } from "react";
import { useLocation } from "react-router-dom";
import {
  getOnboardingSnapshot,
  hideChecklist,
  initializeGuidedPath,
  resumeStep,
  showChecklist,
  skipStep,
  type OnboardingSnapshot,
} from "../services/onboardingService";
import { subscribeDataChanged } from "../services/dataEvents";
import { logError } from "../services/logService";
import { computeChecklist, type Checklist } from "../utils/onboardingChecklist";
import {
  bannerMode,
  latchCardPage,
  nextStep as findNextStep,
  type BannerMode,
} from "../utils/onboardingPath";
import { useEntitlement } from "./useEntitlement";
import {
  CHECKLIST_ITEM_KEYS,
  type ChecklistItem,
  type ChecklistItemKey,
  type OnboardingCounters,
  type OnboardingStatus,
} from "../shared/types";

// State of the getting-started path (#351, #396). One instance lives in
// `OnboardingProvider` (AppShell) and is shared by every page. The reducer, the
// loader and the data-channel follower are exported so the state machine is
// tested without renderHook (same pattern as `licenseReducer`,
// `performSubmitKey`, `revocationCheckLauncher`).

export interface OnboardingChecklistState {
  /** `error` only while no load ever succeeded; a failed re-read keeps `ready`. */
  status: OnboardingStatus;
  /** Null until the first successful load. */
  counters: OnboardingCounters | null;
  /** True when an import folder has been chosen (read with the counters). */
  importFolderSet: boolean;
  /** Hidden by the user or by rule D9. Untouched by a load error. */
  hidden: boolean;
  /** Skipped steps, in checklist order. */
  skipped: ChecklistItemKey[];
  /** Number of the last load started: the outcome of any other load is stale. */
  loadSeq: number;
  /** `loadSeq` at the last HIDE / SHOW: the snapshot of a load numbered up to it predates the action. */
  hiddenSetAt: number;
  /** Same, for SKIP / RESUME and the skipped steps. */
  skippedSetAt: number;
  /** The page that carries a step card (`latchCardPage`), null otherwise. */
  cardPage: string | null;
}

export type OnboardingChecklistAction =
  | { type: "LOAD_START"; seq: number }
  | { type: "LOAD_DONE"; seq: number; snapshot: OnboardingSnapshot }
  | { type: "LOAD_ERROR"; seq: number }
  | { type: "HIDE" }
  | { type: "SHOW" }
  | { type: "SKIP"; key: ChecklistItemKey }
  | { type: "RESUME"; key: ChecklistItemKey }
  | { type: "SET_CARD_PAGE"; cardPage: string | null };

export const initialOnboardingChecklistState: OnboardingChecklistState = {
  status: "loading",
  counters: null,
  importFolderSet: false,
  hidden: false,
  skipped: [],
  loadSeq: 0,
  hiddenSetAt: 0,
  skippedSetAt: 0,
  cardPage: null,
};

/** `keys` plus `key`, kept in checklist order and without duplicates. */
function withStep(keys: readonly ChecklistItemKey[], key: ChecklistItemKey): ChecklistItemKey[] {
  return CHECKLIST_ITEM_KEYS.filter((k) => k === key || keys.includes(k));
}

export function onboardingChecklistReducer(
  state: OnboardingChecklistState,
  action: OnboardingChecklistAction,
): OnboardingChecklistState {
  switch (action.type) {
    case "LOAD_START":
      // A re-read keeps what is on screen, `ready` included: only a path that
      // never loaded shows `loading`.
      return {
        ...state,
        loadSeq: action.seq,
        status: state.status === "ready" ? "ready" : "loading",
      };
    case "LOAD_DONE": {
      if (action.seq !== state.loadSeq) return state;
      const { snapshot } = action;
      return {
        ...state,
        status: "ready",
        counters: snapshot.counters,
        importFolderSet: snapshot.importFolderSet,
        // A local action dispatched after this load started wins over its
        // snapshot, read before the action's write (#384).
        hidden: action.seq <= state.hiddenSetAt ? state.hidden : snapshot.hidden,
        skipped: action.seq <= state.skippedSetAt ? state.skipped : snapshot.skipped,
      };
    }
    case "LOAD_ERROR":
      if (action.seq !== state.loadSeq) return state;
      // After a first success the last state stays on screen; `hidden` is never
      // touched, so a failed read cannot resurface a path the user dismissed.
      return { ...state, status: state.status === "ready" ? "ready" : "error" };
    case "HIDE":
      return { ...state, hidden: true, hiddenSetAt: state.loadSeq };
    case "SHOW":
      return { ...state, hidden: false, hiddenSetAt: state.loadSeq };
    case "SKIP":
      return {
        ...state,
        skipped: withStep(state.skipped, action.key),
        skippedSetAt: state.loadSeq,
        // "Skip" releases the card latch; the provider sets it again on the
        // next render if the new next step is on the same page.
        cardPage: null,
      };
    case "RESUME":
      return {
        ...state,
        skipped: state.skipped.filter((key) => key !== action.key),
        skippedSetAt: state.loadSeq,
      };
    case "SET_CARD_PAGE":
      return state.cardPage === action.cardPage ? state : { ...state, cardPage: action.cardPage };
  }
}

function errorMessage(e: unknown): string {
  return e instanceof Error ? e.message : String(e);
}

export interface OnboardingLoaderDeps {
  initializeGuidedPath: () => Promise<{ hidden: boolean }>;
  getOnboardingSnapshot: () => Promise<OnboardingSnapshot>;
  dispatch: (action: OnboardingChecklistAction) => void;
  logError: (...args: unknown[]) => void;
}

/**
 * The provider's load function (#396), kept outside React so it is tested
 * without a renderer:
 * - every load is numbered, and the reducer drops the outcome of any load but
 *   the last one started (a stale answer never overwrites a newer one);
 * - the first load waits for rule D9 (`initializeGuidedPath`), run once per
 *   loader: the promise is shared, so the double mount of StrictMode does not
 *   run it twice. A failed initialization is logged and the load goes on, the
 *   path showing as stored; the flag stays unset, so the next opening retries;
 * - a failed load is logged only when it starts a failure streak: repeated
 *   re-reads must not push a real cause out of the last log lines that a
 *   feedback report carries.
 */
export function createOnboardingLoader(deps: OnboardingLoaderDeps): () => Promise<void> {
  let seq = 0;
  let initialization: Promise<void> | null = null;
  let failing = false;

  const initializeOnce = (): Promise<void> => {
    if (initialization === null) {
      initialization = deps.initializeGuidedPath().then(
        () => undefined,
        (e: unknown) => {
          deps.logError(
            "Onboarding: failed to apply the first-run rule, the path shows as stored",
            errorMessage(e),
          );
        },
      );
    }
    return initialization;
  };

  return async () => {
    const mine = ++seq;
    deps.dispatch({ type: "LOAD_START", seq: mine });
    await initializeOnce();
    try {
      const snapshot = await deps.getOnboardingSnapshot();
      if (mine === seq) failing = false;
      deps.dispatch({ type: "LOAD_DONE", seq: mine, snapshot });
    } catch (e) {
      if (mine === seq && !failing) {
        failing = true;
        deps.logError("Onboarding: failed to load the getting-started path", errorMessage(e));
      }
      deps.dispatch({ type: "LOAD_ERROR", seq: mine });
    }
  };
}

/** Wait after a data event before re-reading: a burst of writes costs one read (ADR 0022). */
export const DATA_CHANGE_DELAY_MS = 150;

/**
 * Follows the data channel: `run` is called once `delayMs` passed without a
 * new event. Returns the cleanup, which unsubscribes and drops a pending run.
 */
export function followDataChanges(
  run: () => void,
  delayMs: number = DATA_CHANGE_DELAY_MS,
): () => void {
  let timer: ReturnType<typeof setTimeout> | null = null;
  const unsubscribe = subscribeDataChanged(() => {
    if (timer !== null) clearTimeout(timer);
    timer = setTimeout(() => {
      timer = null;
      run();
    }, delayMs);
  });
  return () => {
    unsubscribe();
    if (timer !== null) {
      clearTimeout(timer);
      timer = null;
    }
  };
}

export interface OnboardingViewInput {
  state: OnboardingChecklistState;
  checklist: Checklist | null;
  pathname: string;
  budgetReady: boolean;
  /** Pages that carry a card: `CARD_PAGES` when omitted, tests pass their own. */
  cardPages?: ReadonlySet<string>;
}

export interface OnboardingView {
  nextStep: ChecklistItem | null;
  /** The card latch for this render; the provider stores it when it changed. */
  cardPage: string | null;
  mode: BannerMode;
}

/**
 * What one render of the provider shows, from its state: the next step, the
 * card latch and the mode. Pure, so the provider's render logic is tested
 * without React.
 */
export function deriveOnboardingView({
  state,
  checklist,
  pathname,
  budgetReady,
  cardPages,
}: OnboardingViewInput): OnboardingView {
  const nextStep = checklist ? findNextStep(checklist) : null;
  const cardPage = latchCardPage(state.cardPage, {
    pathname,
    nextStep,
    hidden: state.hidden,
    cardPages,
  });
  const mode = bannerMode({
    status: state.status,
    checklist,
    hidden: state.hidden,
    pathname,
    isEmptyDashboard: pathname === "/" && state.counters?.transactions === 0,
    budgetReady,
    cardPage,
  });
  return { nextStep, cardPage, mode };
}

/** What the provider shares through `useOnboarding()`. */
export interface OnboardingValue {
  status: OnboardingStatus;
  checklist: Checklist | null;
  /** Raw counters; the dashboard reads `transactions` to decide on the empty state (#352). */
  counters: OnboardingCounters | null;
  hidden: boolean;
  budgetReady: boolean;
  /** First unlocked step neither done nor skipped; its link is `nextStep.to`. */
  nextStep: ChecklistItem | null;
  /** What the current page shows of the path (`bannerMode`). */
  mode: BannerMode;
  /** The page that carries a step card, null otherwise. */
  cardPage: string | null;
  hide: () => Promise<void>;
  show: () => Promise<void>;
  skip: (key: ChecklistItemKey) => Promise<void>;
  resume: (key: ChecklistItemKey) => Promise<void>;
  refresh: () => Promise<void>;
}

/**
 * Getting-started path: counters read from the profile database, hidden and
 * skipped preferences, the Budget entitlement that decides whether the sixth
 * row is locked, and what each page shows of it. Called once, by
 * `OnboardingProvider`; pages read it through `useOnboarding()`.
 *
 * Re-read at mount, at every route change and 150 ms after a burst of data
 * events (ADR 0022). The provider is the only writer of the onboarding
 * preferences: its actions update the state themselves, without a re-read.
 */
export function useOnboardingChecklist(): OnboardingValue {
  const [state, dispatch] = useReducer(
    onboardingChecklistReducer,
    initialOnboardingChecklistState,
  );
  const { allowed: budgetAllowed, ready: budgetReady } = useEntitlement("budget");
  const { pathname } = useLocation();

  // One loader per provider instance: a profile switch remounts AppShell, so
  // the new profile gets its own numbering and its own D9 initialization.
  const [refresh] = useState(() =>
    createOnboardingLoader({ initializeGuidedPath, getOnboardingSnapshot, dispatch, logError }),
  );

  // Read at mount, then again at every route change.
  useEffect(() => {
    void refresh();
  }, [refresh, pathname]);

  // Re-read after the writes that move a counter, once a burst is over.
  useEffect(() => followDataChanges(() => void refresh()), [refresh]);

  const hide = useCallback(async () => {
    try {
      await hideChecklist();
      dispatch({ type: "HIDE" });
    } catch (e) {
      logError("Onboarding: failed to hide checklist", errorMessage(e));
    }
  }, []);

  const show = useCallback(async () => {
    try {
      await showChecklist();
      dispatch({ type: "SHOW" });
    } catch (e) {
      logError("Onboarding: failed to show checklist", errorMessage(e));
    }
  }, []);

  const skip = useCallback(async (key: ChecklistItemKey) => {
    try {
      await skipStep(key);
      dispatch({ type: "SKIP", key });
    } catch (e) {
      logError("Onboarding: failed to skip a step", errorMessage(e));
    }
  }, []);

  const resume = useCallback(async (key: ChecklistItemKey) => {
    try {
      await resumeStep(key);
      dispatch({ type: "RESUME", key });
    } catch (e) {
      logError("Onboarding: failed to resume a step", errorMessage(e));
    }
  }, []);

  const checklist: Checklist | null = useMemo(
    () =>
      state.counters
        ? computeChecklist(state.counters, {
            importFolderSet: state.importFolderSet,
            budgetAllowed,
            skipped: state.skipped,
          })
        : null,
    [state.counters, state.importFolderSet, state.skipped, budgetAllowed],
  );

  const { nextStep, cardPage, mode } = deriveOnboardingView({
    state,
    checklist,
    pathname,
    budgetReady,
  });
  // The card latch is derived during render, then stored: an effect would
  // leave one frame showing the banner on a page that carries a card. The
  // update is for this component's own reducer, which React applies before
  // committing; `latchCardPage` is idempotent, so it settles in one pass.
  if (cardPage !== state.cardPage) {
    dispatch({ type: "SET_CARD_PAGE", cardPage });
  }

  return useMemo(
    () => ({
      status: state.status,
      checklist,
      counters: state.counters,
      hidden: state.hidden,
      budgetReady,
      nextStep,
      mode,
      cardPage,
      hide,
      show,
      skip,
      resume,
      refresh,
    }),
    [
      state.status,
      checklist,
      state.counters,
      state.hidden,
      budgetReady,
      nextStep,
      mode,
      cardPage,
      hide,
      show,
      skip,
      resume,
      refresh,
    ],
  );
}