import type { SourceRef } from "./sessionData";
import type { MissionCatalog, MissionCatalogChapter, MissionSelection } from "./missionAdapter";
import {
  getChapterProgress,
  getMissionProgress,
  isMissionComplete,
} from "./missionProgress";
import { getDueRevisionItems } from "./revisionScheduler";

/**
 * Local Today Study Queue.
 *
 * Decides what DocDoe should suggest first when /tuition opens, using only the
 * existing local data (revision scheduler, mission progress, chapter progress)
 * plus the mission catalog from the metadata API. Pure — no React, no writes.
 * source_refs are carried through unchanged when available.
 */

export type StudyQueueType = "revision" | "continue_mission" | "next_mission" | "chapter_complete";

export type StudyQueueItem = {
  type: StudyQueueType;
  priority: number;
  title: string;
  subtitle: string;
  discipline: string;
  chapterId: string;
  chapterTitle: string;
  chapterNo: number;
  missionId: string;
  missionTitle: string;
  source_refs?: SourceRef[];
  reason: string;
  /** Selection to switch to (mission actions). */
  selection?: MissionSelection;
  /** Revision item id (revision actions). */
  revisionItemId?: string;
};

export const STUDY_QUEUE_PRIORITY = {
  overdue_revision: 100,
  due_revision: 80,
  continue_mission: 70,
  next_mission: 50,
  chapter_complete: 10,
} as const;

type MissionDescriptor = {
  discipline: string;
  chapterId: string;
  chapterTitle: string;
  chapterNo: number;
  missionId: string;
  missionTitle: string;
};

function missionShortTitle(title: string): string {
  return title.replace(/^M\d+:\s*/i, "").trim() || title;
}

function startOfDay(now: number): number {
  const date = new Date(now);
  date.setHours(0, 0, 0, 0);
  return date.getTime();
}

function chapterMetaById(
  catalog: MissionCatalog | null,
  chapterId: string,
): { discipline: string; chapterNo: number; chapterTitle: string } | null {
  if (!catalog) return null;
  for (const subject of catalog.subjects) {
    const chapter = subject.chapters.find((item) => item.chapter_id === chapterId);
    if (chapter) {
      return { discipline: chapter.discipline || "", chapterNo: chapter.chapter_no, chapterTitle: chapter.chapter_title };
    }
  }
  return null;
}

export function chapterForSelection(
  catalog: MissionCatalog | null,
  selection: MissionSelection,
): MissionCatalogChapter | null {
  if (!catalog) return null;
  const subject = catalog.subjects.find((s) => s.subject === selection.subject);
  if (!subject) return null;
  return (
    subject.chapters.find(
      (chapter) => chapter.chapter_id === selection.chapterId || (chapter.discipline === selection.discipline && chapter.chapter_no === selection.chapterNo),
    ) ?? null
  );
}

function flatMissions(catalog: MissionCatalog | null): MissionDescriptor[] {
  const out: MissionDescriptor[] = [];
  for (const subject of catalog?.subjects ?? []) {
    for (const chapter of subject.chapters) {
      for (const mission of chapter.missions) {
        out.push({
          discipline: chapter.discipline || "",
          chapterId: chapter.chapter_id,
          chapterTitle: chapter.chapter_title,
          chapterNo: chapter.chapter_no,
          missionId: mission.mission_id,
          missionTitle: mission.mission_title,
        });
      }
    }
  }
  return out;
}

function toSelection(descriptor: MissionDescriptor, subjectName: "Social Science" | "Physics" = "Social Science"): MissionSelection {
  return { board: "Kerala SSLC", classLevel: "10", subject: subjectName, discipline: descriptor.discipline, chapterNo: descriptor.chapterNo, chapterId: descriptor.chapterId, missionId: descriptor.missionId };
}

/** The mission the student currently has selected, as a descriptor. */
export function resolveCurrentMission(
  catalog: MissionCatalog | null,
  selection: MissionSelection,
): MissionDescriptor | null {
  const chapter = chapterForSelection(catalog, selection);
  if (!chapter) return null;
  const mission = chapter.missions.find((item) => item.mission_id === selection.missionId);
  return {
    discipline: chapter.discipline || "",
    chapterId: chapter.chapter_id,
    chapterTitle: chapter.chapter_title,
    chapterNo: chapter.chapter_no,
    missionId: selection.missionId,
    missionTitle: mission?.mission_title ?? selection.missionId,
  };
}

/** The saved progress for the currently selected mission (or null). */
export function resolveCurrentMissionProgress(
  studentId: string,
  catalog: MissionCatalog | null,
  selection: MissionSelection,
) {
  const current = resolveCurrentMission(catalog, selection);
  if (!current) return null;
  return getMissionProgress(studentId, current.chapterId, current.missionId);
}

export type NextMissionResolution = MissionDescriptor & { unlocked: boolean; reason: string };

/**
 * The next mission the student should do. If the current mission is in progress
 * it returns the mission that comes after it ("up next"); otherwise it returns
 * the first not-completed mission at or after the current one (the current
 * mission itself when it is fresh, or the next chapter's first mission when the
 * current chapter is complete).
 */
export function resolveNextUnlockedMission(
  studentId: string,
  catalog: MissionCatalog | null,
  selection: MissionSelection,
): NextMissionResolution | null {
  const flat = flatMissions(catalog);
  if (flat.length === 0) return null;

  const current = resolveCurrentMission(catalog, selection);
  const currentIndex = current
    ? flat.findIndex((item) => item.chapterId === current.chapterId && item.missionId === current.missionId)
    : -1;
  const start = currentIndex >= 0 ? currentIndex : 0;

  const currentProgress = current ? getMissionProgress(studentId, current.chapterId, current.missionId) : null;
  const currentInProgress = Boolean(
    currentProgress && currentProgress.missionStarted && !isMissionComplete(currentProgress),
  );
  const from = currentInProgress ? start + 1 : start;

  for (let index = from; index < flat.length; index += 1) {
    const descriptor = flat[index];
    const chapterProgress = getChapterProgress(studentId, descriptor.chapterId);
    if (chapterProgress.completedMissionIds.includes(descriptor.missionId)) continue;

    const unlocked = descriptor.missionId === selection.missionId || chapterProgress.unlockedMissionIds.includes(descriptor.missionId) || isFirstMissionOfChapter(catalog, descriptor);
    const sameChapter = current && descriptor.chapterId === current.chapterId;
    const reason = currentInProgress
      ? "Up next after this mission"
      : sameChapter
        ? "Your current mission is ready"
        : "Next chapter unlocked";

    return { ...descriptor, unlocked, reason };
  }
  return null;
}

function isFirstMissionOfChapter(catalog: MissionCatalog | null, descriptor: MissionDescriptor): boolean {
  if (!catalog) return false;
  for (const subject of catalog.subjects) {
    const chapter = subject.chapters.find((item) => item.chapter_id === descriptor.chapterId);
    if (chapter) {
      return chapter.missions[0]?.mission_id === descriptor.missionId;
    }
  }
  return false;
}

function isChapterComplete(studentId: string, chapter: MissionCatalogChapter): boolean {
  if (chapter.missions.length === 0) return false;
  const chapterProgress = getChapterProgress(studentId, chapter.chapter_id);
  const completed = new Set(chapterProgress.completedMissionIds);
  return chapter.missions.every((mission) => completed.has(mission.mission_id));
}

/** Build the ordered Today study queue. */
export function buildTodayStudyQueue(
  studentId: string,
  catalog: MissionCatalog | null,
  selection: MissionSelection,
  now: number = Date.now(),
): StudyQueueItem[] {
  const items: StudyQueueItem[] = [];
  const dayStart = startOfDay(now);

  // 1. Due revision items (overdue first).
  for (const revision of getDueRevisionItems(studentId, now)) {
    const overdue = new Date(revision.dueAt).getTime() < dayStart;
    const meta = chapterMetaById(catalog, revision.chapterId);
    items.push({
      type: "revision",
      priority: overdue ? STUDY_QUEUE_PRIORITY.overdue_revision : STUDY_QUEUE_PRIORITY.due_revision,
      title: `Revise: ${revision.weakAreaTag}`,
      subtitle: revision.chapterTitle,
      discipline: meta?.discipline ?? "",
      chapterId: revision.chapterId,
      chapterTitle: revision.chapterTitle,
      chapterNo: meta?.chapterNo ?? 0,
      missionId: revision.missionId,
      missionTitle: "",
      source_refs: revision.source_refs,
      reason: overdue ? "Overdue revision" : "Revision due today",
      revisionItemId: revision.id,
    });
  }

  // 2. Unfinished current mission.
  const current = resolveCurrentMission(catalog, selection);
  if (current) {
    const progress = getMissionProgress(studentId, current.chapterId, current.missionId);
    if (progress && progress.missionStarted && !isMissionComplete(progress)) {
      items.push({
        type: "continue_mission",
        priority: STUDY_QUEUE_PRIORITY.continue_mission,
        title: `Continue: ${missionShortTitle(current.missionTitle)}`,
        subtitle: current.chapterTitle,
        discipline: current.discipline,
        chapterId: current.chapterId,
        chapterTitle: current.chapterTitle,
        chapterNo: current.chapterNo,
        missionId: current.missionId,
        missionTitle: current.missionTitle,
        source_refs: progress.source_refs,
        reason: `Resume from the ${progress.lastStage} stage`,
        selection: toSelection(current, selection.subject),
      });
    }
  }

  // 3. Next unlocked / up-next mission.
  const next = resolveNextUnlockedMission(studentId, catalog, selection);
  if (next) {
    items.push({
      type: "next_mission",
      priority: STUDY_QUEUE_PRIORITY.next_mission,
      title: `Start: ${missionShortTitle(next.missionTitle)}`,
      subtitle: next.chapterTitle,
      discipline: next.discipline,
      chapterId: next.chapterId,
      chapterTitle: next.chapterTitle,
      chapterNo: next.chapterNo,
      missionId: next.missionId,
      missionTitle: next.missionTitle,
      reason: next.reason,
      selection: toSelection(next, selection.subject),
    });
  }

  // 4. Completed chapter (info).
  const currentChapter = chapterForSelection(catalog, selection);
  if (currentChapter && isChapterComplete(studentId, currentChapter)) {
    items.push({
      type: "chapter_complete",
      priority: STUDY_QUEUE_PRIORITY.chapter_complete,
      title: `Chapter complete: ${currentChapter.chapter_title}`,
      subtitle: `${currentChapter.missions.length}/${currentChapter.missions.length} missions`,
      discipline: currentChapter.discipline || "",
      chapterId: currentChapter.chapter_id,
      chapterTitle: currentChapter.chapter_title,
      chapterNo: currentChapter.chapter_no,
      missionId: "",
      missionTitle: "",
      reason: "All missions in this chapter are completed",
    });
  }

  return items.sort((a, b) => b.priority - a.priority);
}

/** The single highest-priority action (or null when nothing is queued). */
export function getTopStudyAction(
  studentId: string,
  catalog: MissionCatalog | null,
  selection: MissionSelection,
  now: number = Date.now(),
): StudyQueueItem | null {
  return buildTodayStudyQueue(studentId, catalog, selection, now)[0] ?? null;
}