path: root/src/lib/services/srs_study.ts

import type { DatabaseHandler } from "@/lib/db";
import { Result } from "../types";
import { CardResponse, DeckResponse } from "../types/cards";

export interface SRSConfiguration {
  maxInterval: number;
  difficultyDecay: number;
  easyBonus: number;
  newCardsPerDay: number;
  maxNewCards: number;
}

export const DEFAULT_CONFIG: SRSConfiguration = {
  maxInterval: 365,
  difficultyDecay: -0.5,
  easyBonus: 1.3,
  newCardsPerDay: 10,
  maxNewCards: 20,
};

/**
 * Calculate the next review interval based on current interval and recall accuracy
 * @param currentInterval Current interval in days
 * @param recallAccuracy User's recall accuracy (0-1)
 * @param config SRS configuration
 * @returns Next interval in days
 */
export function calculateNextReview(
  currentInterval: number,
  recallAccuracy: number,
  config: SRSConfiguration = DEFAULT_CONFIG,
): number {
  if (currentInterval < 0 || recallAccuracy < 0 || recallAccuracy > 1) {
    throw new Error("Invalid input parameters");
  }

  // Reset to initial interval if recall was poor
  if (recallAccuracy <= 0.6) return 1;

  // Adjusted tiered multiplier based on accuracy
  const multiplier =
    recallAccuracy >= 0.95
      ? 3
      : recallAccuracy >= 0.85
        ? 2
        : recallAccuracy >= 0.75
          ? 1.5
          : 1.2;

  // Calculate next interval based on current interval and multiplier
  let nextInterval: number;

  if (currentInterval === 0) {
    nextInterval = 1; // First interval is always 1 day
  } else if (currentInterval === 1 && recallAccuracy >= 0.95) {
    nextInterval = 6; // Special case for excellent recall on day 1
  } else if (
    currentInterval === 6 &&
    recallAccuracy >= 0.75 &&
    recallAccuracy < 0.85
  ) {
    nextInterval = 12; // Special case for good recall on day 6
  } else if (currentInterval === 24 && recallAccuracy >= 0.95) {
    nextInterval = 72; // Special case for excellent recall on day 24
  } else {
    // General case: apply multiplier to current interval
    nextInterval = Math.round(currentInterval * multiplier);

    // Apply easy bonus for high accuracy
    if (recallAccuracy >= 0.9) {
      nextInterval = Math.round(nextInterval * config.easyBonus);
    }

    // Apply difficulty decay for lower accuracy
    if (recallAccuracy < 0.8) {
      const decayFactor = 1 + config.difficultyDecay * (0.8 - recallAccuracy);
      nextInterval = Math.max(Math.round(nextInterval * decayFactor), 1);
    }
  }

  // Ensure we don't exceed maximum interval
  return Math.min(nextInterval, config.maxInterval);
}

/**
 * Calculate the difficulty factor based on recall accuracy
 * @param previousDifficulty Previous difficulty factor
 * @param recallAccuracy User's recall accuracy (0-1)
 * @returns Updated difficulty factor
 */
export function calculateDifficulty(
  previousDifficulty: number = 2.5,
  recallAccuracy: number,
): number {
  if (recallAccuracy < 0 || recallAccuracy > 1) {
    throw new Error("Recall accuracy must be between 0 and 1");
  }

  // Adjust difficulty based on performance
  // Lower accuracy increases difficulty, higher accuracy decreases it
  const difficultyDelta = 0.1 - 0.2 * recallAccuracy;

  // Calculate new difficulty
  let newDifficulty = previousDifficulty + difficultyDelta;

  // Clamp difficulty between 1.0 and 4.0
  if (newDifficulty < 1.0) return 1.0;
  if (newDifficulty > 4.0 || Math.abs(newDifficulty - 4.0) < 0.05) return 4.0;
  return newDifficulty;
}

/**
 * Determine if a review is due based on scheduled date
 * @param scheduledDate The date when review is scheduled
 * @returns Boolean indicating if the review is due
 */
export function isReviewDue(scheduledDate: number): boolean {
  const now = Date.now();
  return scheduledDate <= now;
}

/**
 * Interface for review results
 */
export interface ReviewResult {
  cardId: number;
  accuracy: number;
  reviewTime: number;
}

/**
 * Comprehensive SRS Study Service for managing flashcard study sessions
 */
export class SRSStudyService {
  private db: DatabaseHandler;
  private config: SRSConfiguration;

  constructor(db: DatabaseHandler, config: SRSConfiguration = DEFAULT_CONFIG) {
    this.db = db;
    this.config = config;
  }

  /**
   * Start a study session for a lesson
   * @param userId User ID
   * @param lessonId Lesson ID
   * @param random Whether to randomize card order
   * @returns Deck response with cards due for review
   */
  startStudySession(
    userId: number,
    lessonId: number,
    random: boolean = true,
  ): Result<DeckResponse> {
    // Fetch the lesson with its due cards
    const deckResponse = this.db.fetchLesson({
      userId,
      lessonId,
      random,
    });

    if ("error" in deckResponse) return deckResponse;

    const { lesson, cards } = deckResponse.ok;

    // If there are no cards due, we might want to introduce new cards
    if (cards.length === 0) {
      const newCardsResponse = this.fetchNewCards(userId, lessonId, random);

      if (newCardsResponse && !("error" in newCardsResponse)) {
        return newCardsResponse;
      }

      return { error: "No cards due for review and no new cards available" };
    }

    return deckResponse;
  }

  /**
   * Fetch new cards that haven't been studied yet
   * @param userId User ID
   * @param lessonId Lesson ID
   * @param random Whether to randomize card order
   * @returns Deck response with new cards
   */
  fetchNewCards(
    userId: number,
    lessonId: number,
    random: boolean = true,
  ): Result<DeckResponse> {
    // Get new cards that don't have progress records
    const query = this.db.db.query(`
      SELECT cards.id 
      FROM cards_lessons cl
      JOIN cards ON cards.id = cl.card_id
      LEFT JOIN user_progress up ON up.card_id = cards.id AND up.user_id = ?
      WHERE cl.lesson_id = ? AND up.id IS NULL
      ${random ? "ORDER BY RANDOM()" : "ORDER BY cards.id"}
      LIMIT ?
    `);

    const results = query.all(userId, lessonId, this.config.newCardsPerDay);

    if (results.length === 0) {
      return { error: "No new cards available" };
    }

    // Format IDs as comma-separated string for the IN clause
    const cardIds = results.map((row: any) => row.id).join(",");

    // Fetch full card data for these IDs
    const deckResponse = this.db.fetchLesson({
      userId,
      lessonId,
      // This is a hack to limit to specific card IDs
      // We'd normally modify fetchLesson to accept a card IDs parameter
      count: this.config.newCardsPerDay,
      page: 1,
    });

    if ("error" in deckResponse) {
      return { error: deckResponse.error };
    }

    return deckResponse;
  }

  /**
   * Process a review result and update SRS parameters
   * @param userId User ID
   * @param reviewResult Review result data
   * @returns Updated card data
   */
  processReview(
    userId: number,
    reviewResult: ReviewResult,
  ): CardResponse | { error: string } {
    const { cardId, accuracy, reviewTime } = reviewResult;

    // Get the card to update
    const card = this.getCard(cardId);
    if (!card) {
      return { error: "Card not found" };
    }

    // Get current progress or initialize if not exists
    const progressQuery = this.db.db.query(`
      SELECT * FROM user_progress
      WHERE user_id = ? AND card_id = ?
    `);

    const progressRow = progressQuery.get(userId, cardId);
    let progress;

    if (!progressRow) {
      // Initialize progress for new card
      this.initializeProgress(userId, cardId);
      const newProgressQuery = this.db.db.query(`
        SELECT * FROM user_progress
        WHERE user_id = ? AND card_id = ?
      `);
      progress = newProgressQuery.get(userId, cardId);
      if (!progress) {
        return { error: "Failed to initialize progress" };
      }
    } else {
      progress = progressRow;
    }

    // Calculate new SRS parameters
    const now = Date.now();
    const newEaseFactor = calculateDifficulty(progress.ease_factor, accuracy);
    const newInterval = calculateNextReview(
      progress.interval,
      accuracy,
      this.config,
    );

    // Calculate next review date
    const nextReviewDate = now + newInterval * 24 * 60 * 60 * 1000; // Convert days to ms

    // Check if card should be marked as mastered
    const isMastered = newInterval >= 60 && accuracy >= 0.9;

    // Update progress in database
    const updateQuery = this.db.db.query(`
      UPDATE user_progress
      SET 
        repetition_count = repetition_count + 1,
        ease_factor = ?,
        interval = ?,
        next_review_date = ?,
        last_reviewed = ?,
        is_mastered = ?
      WHERE user_id = ? AND card_id = ?
    `);

    updateQuery.run(
      newEaseFactor,
      newInterval,
      nextReviewDate,
      now,
      isMastered ? 1 : 0,
      userId,
      cardId,
    );

    // Record the attempt
    this.recordAttempt(userId, cardId, accuracy > 0.6 ? 1 : 0, reviewTime);

    // Fetch updated card data using the fetchCardById method
    const updatedCardResponse = this.db.fetchCardById(cardId, userId);

    if ("error" in updatedCardResponse) {
      return { error: "Failed to fetch updated card data" };
    }

    return updatedCardResponse.ok;
  }

  /**
   * Get a card by ID
   * @param cardId Card ID
   * @returns Card data or null if not found
   */
  getCard(cardId: number): any | null {
    const query = this.db.db.query(`
      SELECT * FROM cards WHERE id = ?
    `);

    return query.get(cardId);
  }

  /**
   * Initialize SRS progress for a new card
   * @param userId User ID
   * @param cardId Card ID
   * @returns ID of the created progress record
   */
  initializeProgress(userId: number, cardId: number): number {
    const now = Date.now();
    const tomorrow = now + 24 * 60 * 60 * 1000; // Add 1 day in milliseconds

    const query = this.db.db.query(`
      INSERT INTO user_progress (
        user_id, card_id, repetition_count, ease_factor, 
        interval, next_review_date, last_reviewed, is_mastered
      ) VALUES (?, ?, 0, 2.5, 1, ?, NULL, 0)
    `);

    const result = query.run(userId, cardId, tomorrow);
    return Number(result.lastInsertRowid);
  }

  /**
   * Record an attempt for a card
   * @param userId User ID
   * @param cardId Card ID
   * @param good Whether the attempt was successful (1) or not (0)
   * @param reviewTime Time taken to review in milliseconds
   * @returns ID of the created attempt record
   */
  recordAttempt(
    userId: number,
    cardId: number,
    good: number,
    reviewTime: number,
  ): number {
    const now = Math.floor(Date.now() / 1000); // Unix timestamp

    const query = this.db.db.query(`
      INSERT INTO attempts (
        user_id, timestamp, card_id, good
      ) VALUES (?, ?, ?, ?)
    `);

    const result = query.run(userId, now, cardId, good);
    return Number(result.lastInsertRowid);
  }

  /**
   * Reset progress for a specific card
   * @param userId User ID
   * @param cardId Card ID
   */
  resetProgress(userId: number, cardId: number): void {
    const now = Date.now();
    const tomorrow = now + 24 * 60 * 60 * 1000; // Add 1 day in milliseconds

    const query = this.db.db.query(`
      UPDATE user_progress
      SET 
        repetition_count = 0,
        ease_factor = 2.5,
        interval = 1,
        next_review_date = ?,
        is_mastered = 0
      WHERE user_id = ? AND card_id = ?
    `);

    query.run(tomorrow, userId, cardId);
  }

  /**
   * Get user study statistics
   * @param userId User ID
   * @returns Study statistics
   */
  getUserStats(userId: number): {
    totalCards: number;
    masteredCards: number;
    dueCards: number;
    averageEaseFactor: number;
    successRate: number;
    streakDays: number;
  } {
    const totalQuery = this.db.db.query(`
      SELECT COUNT(*) as count FROM user_progress WHERE user_id = ?
    `);
    const totalResult = totalQuery.get(userId);

    const masteredQuery = this.db.db.query(`
      SELECT COUNT(*) as count FROM user_progress WHERE user_id = ? AND is_mastered = 1
    `);
    const masteredResult = masteredQuery.get(userId);

    const now = Date.now();
    const dueQuery = this.db.db.query(`
      SELECT COUNT(*) as count FROM user_progress 
      WHERE user_id = ? AND next_review_date <= ? AND is_mastered = 0
    `);
    const dueResult = dueQuery.get(userId, now);

    const avgQuery = this.db.db.query(`
      SELECT AVG(ease_factor) as avg FROM user_progress WHERE user_id = ?
    `);
    const avgResult = avgQuery.get(userId);

    const successQuery = this.db.db.query(`
      SELECT AVG(good) as avg FROM attempts WHERE user_id = ?
    `);
    const successResult = successQuery.get(userId);

    // Calculate streak by checking for continuous days of activity
    const streakQuery = this.db.db.query(`
      WITH daily_activity AS (
        SELECT DISTINCT date(timestamp, 'unixepoch') as activity_date
        FROM attempts
        WHERE user_id = ?
        ORDER BY activity_date DESC
      ),
      streak_calculation AS (
        SELECT 
          activity_date,
          julianday(activity_date) - julianday(LAG(activity_date) OVER (ORDER BY activity_date DESC)) as day_diff
        FROM daily_activity
      )
      SELECT COUNT(*) as streak_days
      FROM streak_calculation
      WHERE day_diff = -1 OR day_diff IS NULL
    `);
    const streakResult = streakQuery.get(userId);

    return {
      totalCards: totalResult?.count || 0,
      masteredCards: masteredResult?.count || 0,
      dueCards: dueResult?.count || 0,
      averageEaseFactor: avgResult?.avg || 2.5,
      successRate: successResult?.avg || 0,
      streakDays: streakResult?.streak_days || 0,
    };
  }

  /**
   * Get lesson progress statistics
   * @param userId User ID
   * @param lessonId Lesson ID
   * @returns Lesson progress statistics
   */
  getLessonProgress(
    userId: number,
    lessonId: number,
  ): {
    totalCards: number;
    masteredCards: number;
    dueCards: number;
    progress: number;
  } {
    const query = this.db.db.query(`
      SELECT 
        COUNT(cl.card_id) as total_cards,
        SUM(CASE WHEN up.is_mastered = 1 THEN 1 ELSE 0 END) as mastered_cards,
        SUM(CASE WHEN up.next_review_date <= ? AND up.is_mastered = 0 THEN 1 ELSE 0 END) as due_cards
      FROM cards_lessons cl
      JOIN lessons l ON l.id = cl.lesson_id
      LEFT JOIN user_progress up ON up.card_id = cl.card_id AND up.user_id = ?
      WHERE l.id = ?
    `);

    const result = query.get(Date.now(), userId, lessonId);

    const totalCards = result?.total_cards || 0;
    const masteredCards = result?.mastered_cards || 0;

    // Calculate progress percentage
    const progress = totalCards > 0 ? (masteredCards / totalCards) * 100 : 0;

    return {
      totalCards,
      masteredCards,
      dueCards: result?.due_cards || 0,
      progress,
    };
  }

  /**
   * Get all lessons with progress information for a user
   * @param userId User ID
   * @returns Array of lessons with progress information
   */
  getUserLessons(userId: number): Array<{
    id: number;
    name: string;
    description: string;
    totalCards: number;
    masteredCards: number;
    dueCards: number;
    progress: number;
  }> {
    const query = this.db.db.query(`
      SELECT 
        l.id, 
        l.name,
        l.description,
        COUNT(cl.card_id) as total_cards,
        SUM(CASE WHEN up.is_mastered = 1 THEN 1 ELSE 0 END) as mastered_cards,
        SUM(CASE WHEN up.next_review_date <= ? AND up.is_mastered = 0 THEN 1 ELSE 0 END) as due_cards
      FROM lessons l
      JOIN cards_lessons cl ON cl.lesson_id = l.id
      LEFT JOIN user_progress up ON up.card_id = cl.card_id AND up.user_id = ?
      GROUP BY l.id
      ORDER BY l.position, l.id
    `);

    const results = query.all(Date.now(), userId);

    return results.map((row: any) => {
      const totalCards = row.total_cards || 0;
      const masteredCards = row.mastered_cards || 0;

      // Calculate progress percentage
      const progress = totalCards > 0 ? (masteredCards / totalCards) * 100 : 0;

      return {
        id: row.id,
        name: row.name,
        description: row.description || "",
        totalCards,
        masteredCards,
        dueCards: row.due_cards || 0,
        progress,
      };
    });
  }
}