// ─── Chapter progress — saved on this device ──────────────────────────────────
// Lightweight, on-device progress for curriculum chapters. Powers the
// progress-aware behaviour of the intelligent teach engine and /teach UI.
//
// SSR-safe: every accessor guards `window`. Corruption-safe: reads never throw,
// they fall back to an empty store. Follows the app's existing local-store
// conventions (docdoe_* key, docdoe:*-updated event).

import { getSubject, getChapter } from "./curriculum/index";
import type { ChapterMeta } from "./curriculum/types";

// ─── Public types ───────────────────────────────────────────────────────────

export type ChapterStatus =
  | "not_started"
  | "started"
  | "completed"
  | "revised"
  | "weak";

export type ConfidenceLevel = "low" | "medium" | "high";

export type ChapterProgressEntry = {
  key: string; // `${level}::${subjectSlug}::${chapterSlug}`
  level: string;
  subjectSlug: string;
  chapterSlug: string;
  status: ChapterStatus;
  studyCount: number;
  quizScore: number | null; // most recent quiz %, 0–100
  quizAttempts: number;
  weakTopics: string[];
  confidence: ConfidenceLevel | null;
  startedAt: string | null; // ISO
  lastStudied: string | null; // ISO
  completedAt: string | null; // ISO
};

export type RecommendationType = "revision" | "next" | "easy_win";

export type Recommendation = {
  level: string;
  subjectSlug: string;
  subjectName: string;
  chapter: ChapterMeta;
  reason: string;
  type: RecommendationType;
};

// ─── Storage plumbing ─────────────────────────────────────────────────────────

const STORAGE_KEY = "docdoe_chapter_progress";
export const PROGRESS_EVENT = "docdoe:chapter-progress-updated";

type ProgressStore = Record<string, ChapterProgressEntry>;

function canUseStorage(): boolean {
  return typeof window !== "undefined" && Boolean(window.localStorage);
}

function keyOf(level: string, subjectSlug: string, chapterSlug: string): string {
  return `${level}::${subjectSlug}::${chapterSlug}`;
}

function nowIso(): string {
  return new Date().toISOString();
}

function readStore(): ProgressStore {
  if (!canUseStorage()) return {};
  try {
    const raw = window.localStorage.getItem(STORAGE_KEY);
    if (!raw) return {};
    const parsed = JSON.parse(raw);
    if (!parsed || typeof parsed !== "object" || Array.isArray(parsed)) return {};
    return parsed as ProgressStore;
  } catch {
    return {};
  }
}

function writeStore(store: ProgressStore): void {
  if (!canUseStorage()) return;
  try {
    window.localStorage.setItem(STORAGE_KEY, JSON.stringify(store));
    window.dispatchEvent(new Event(PROGRESS_EVENT));
  } catch {
    // storage full / blocked — progress is best-effort, never crash the UI
  }
}

function emptyEntry(level: string, subjectSlug: string, chapterSlug: string): ChapterProgressEntry {
  return {
    key: keyOf(level, subjectSlug, chapterSlug),
    level,
    subjectSlug,
    chapterSlug,
    status: "not_started",
    studyCount: 0,
    quizScore: null,
    quizAttempts: 0,
    weakTopics: [],
    confidence: null,
    startedAt: null,
    lastStudied: null,
    completedAt: null,
  };
}

function mutate(
  level: string,
  subjectSlug: string,
  chapterSlug: string,
  apply: (entry: ChapterProgressEntry) => void,
): ChapterProgressEntry {
  const store = readStore();
  const key = keyOf(level, subjectSlug, chapterSlug);
  const entry = store[key] ?? emptyEntry(level, subjectSlug, chapterSlug);
  apply(entry);
  store[key] = entry;
  writeStore(store);
  return entry;
}

// ─── Reads ────────────────────────────────────────────────────────────────────

export function getChapterEntry(
  level: string,
  subjectSlug: string,
  chapterSlug: string,
): ChapterProgressEntry | null {
  const store = readStore();
  return store[keyOf(level, subjectSlug, chapterSlug)] ?? null;
}

export function getProgress(): ChapterProgressEntry[] {
  return Object.values(readStore());
}

export function statusLabel(status: ChapterStatus): string {
  switch (status) {
    case "not_started": return "Not started";
    case "started": return "In progress";
    case "completed": return "Completed";
    case "revised": return "Revised";
    case "weak": return "Needs revision";
  }
}

// ─── Writes ───────────────────────────────────────────────────────────────────

export function markChapterStarted(
  level: string,
  subjectSlug: string,
  chapterSlug: string,
): ChapterProgressEntry {
  return mutate(level, subjectSlug, chapterSlug, (e) => {
    e.studyCount += 1;
    e.lastStudied = nowIso();
    if (!e.startedAt) e.startedAt = nowIso();
    // Don't downgrade a completed/revised chapter just because it was reopened.
    if (e.status === "not_started") e.status = "started";
  });
}

export function markChapterCompleted(
  level: string,
  subjectSlug: string,
  chapterSlug: string,
): ChapterProgressEntry {
  return mutate(level, subjectSlug, chapterSlug, (e) => {
    e.status = "completed";
    e.completedAt = nowIso();
    e.lastStudied = nowIso();
    if (!e.startedAt) e.startedAt = nowIso();
  });
}

export function markChapterRevised(
  level: string,
  subjectSlug: string,
  chapterSlug: string,
): ChapterProgressEntry {
  return mutate(level, subjectSlug, chapterSlug, (e) => {
    e.status = "revised";
    e.lastStudied = nowIso();
    if (!e.startedAt) e.startedAt = nowIso();
  });
}

export function markChapterWeak(
  level: string,
  subjectSlug: string,
  chapterSlug: string,
  weakTopics?: string[],
): ChapterProgressEntry {
  return mutate(level, subjectSlug, chapterSlug, (e) => {
    e.status = "weak";
    e.lastStudied = nowIso();
    if (!e.startedAt) e.startedAt = nowIso();
    if (weakTopics && weakTopics.length > 0) {
      const merged = new Set([...e.weakTopics, ...weakTopics]);
      e.weakTopics = Array.from(merged).slice(0, 8);
    }
  });
}

export function saveQuizScore(
  level: string,
  subjectSlug: string,
  chapterSlug: string,
  score: number,
): ChapterProgressEntry {
  const clamped = Math.max(0, Math.min(100, Math.round(score)));
  return mutate(level, subjectSlug, chapterSlug, (e) => {
    e.quizScore = clamped;
    e.quizAttempts += 1;
    e.lastStudied = nowIso();
    if (!e.startedAt) e.startedAt = nowIso();
    if (clamped < 50) {
      e.status = "weak";
    } else if (e.status === "not_started" || e.status === "started") {
      // A solid score on an unfinished chapter counts as a pass-through revise.
      e.status = clamped >= 80 ? "revised" : "started";
    }
    e.confidence = clamped >= 80 ? "high" : clamped >= 50 ? "medium" : "low";
  });
}

export function resetChapter(
  level: string,
  subjectSlug: string,
  chapterSlug: string,
): void {
  const store = readStore();
  delete store[keyOf(level, subjectSlug, chapterSlug)];
  writeStore(store);
}

// ─── Subscriptions ──────────────────────────────────────────────────────────

/** Subscribe to progress changes on this device. Returns an unsubscribe fn. */
export function subscribeToProgress(listener: () => void): () => void {
  if (typeof window === "undefined") return () => {};
  const handler = () => listener();
  window.addEventListener(PROGRESS_EVENT, handler);
  window.addEventListener("storage", handler);
  return () => {
    window.removeEventListener(PROGRESS_EVENT, handler);
    window.removeEventListener("storage", handler);
  };
}

// ─── Recommendations ──────────────────────────────────────────────────────────

function isUnfinished(status: ChapterStatus): boolean {
  return status === "not_started" || status === "started";
}

/**
 * Suggest what to study next, blending three signals:
 *   - revision: chapters marked weak or with a low last quiz
 *   - next:     the next unfinished chapter in the preferred subject's sequence
 *   - easy_win: exam-relevant, easy chapters not yet completed
 *
 * When a subject is given, lead with the next chapter in that subject; otherwise
 * lead with revision. Deduped by chapter, capped at `n`.
 */
export function getRecommendedNextChapters(
  preferLevel?: string | null,
  preferSubject?: string | null,
  n = 3,
): Recommendation[] {
  const store = readStore();
  const entries = Object.values(store);

  const revisionRecs: Recommendation[] = [];
  for (const e of entries) {
    const isWeak = e.status === "weak";
    const lowQuiz = e.quizScore !== null && e.quizScore < 50;
    if (!isWeak && !lowQuiz) continue;
    const subject = getSubject(e.level, e.subjectSlug);
    const chapter = getChapter(e.level, e.subjectSlug, e.chapterSlug);
    if (!subject || !chapter) continue;
    const reason = isWeak
      ? "You marked this weak — revise it before moving on"
      : `Last quiz was ${e.quizScore}% — a revision will help`;
    revisionRecs.push({
      level: e.level,
      subjectSlug: e.subjectSlug,
      subjectName: subject.name,
      chapter,
      reason,
      type: "revision",
    });
  }

  const nextRecs: Recommendation[] = [];
  if (preferLevel && preferSubject) {
    const subject = getSubject(preferLevel, preferSubject);
    if (subject) {
      for (const chapter of subject.chapters) {
        const entry = store[keyOf(preferLevel, preferSubject, chapter.slug)];
        const status: ChapterStatus = entry?.status ?? "not_started";
        if (!isUnfinished(status)) continue;
        nextRecs.push({
          level: preferLevel,
          subjectSlug: preferSubject,
          subjectName: subject.name,
          chapter,
          reason:
            status === "started"
              ? `Continue your ${subject.name} sequence`
              : `Next up in ${subject.name}`,
          type: "next",
        });
        break; // only the single next chapter in sequence
      }
    }
  }

  const easyWinRecs: Recommendation[] = [];
  {
    const level = preferLevel ?? null;
    const subjectSlug = preferSubject ?? null;
    const subjects = subjectsToScan(level, subjectSlug);
    for (const ctx of subjects) {
      for (const chapter of ctx.subject.chapters) {
        const entry = store[keyOf(ctx.level, ctx.subjectSlug, chapter.slug)];
        const status: ChapterStatus = entry?.status ?? "not_started";
        if (status === "completed" || status === "revised") continue;
        const highYield = chapter.pyqFrequency === "very_high" || chapter.pyqFrequency === "high";
        if (highYield && chapter.difficulty === "easy") {
          easyWinRecs.push({
            level: ctx.level,
            subjectSlug: ctx.subjectSlug,
            subjectName: ctx.subject.name,
            chapter,
            reason: "Quick to learn — exam-relevant",
            type: "easy_win",
          });
        }
      }
    }
  }

  const ordered = preferSubject
    ? [...nextRecs, ...revisionRecs, ...easyWinRecs]
    : [...revisionRecs, ...nextRecs, ...easyWinRecs];

  const seen = new Set<string>();
  const deduped: Recommendation[] = [];
  for (const rec of ordered) {
    const id = keyOf(rec.level, rec.subjectSlug, rec.chapter.slug);
    if (seen.has(id)) continue;
    seen.add(id);
    deduped.push(rec);
    if (deduped.length >= n) break;
  }
  return deduped;
}

type SubjectScanContext = { level: string; subjectSlug: string; subject: { name: string; chapters: ChapterMeta[] } };

function subjectsToScan(level: string | null, subjectSlug: string | null): SubjectScanContext[] {
  if (level && subjectSlug) {
    const subject = getSubject(level, subjectSlug);
    return subject ? [{ level, subjectSlug, subject }] : [];
  }
  return [];
}