french-vocab/src/utils/srs.ts

import type { StudyProgress, DifficultyRating } from '../types/vocabulary';

interface SRSConfig {
  initialEase: number;
  minEase: number;
  maxInterval: number;
  learningStepInterval: number;
}

const DEFAULT_CONFIG: SRSConfig = {
  initialEase: 2.5,
  minEase: 1.3,
  maxInterval: 365, // 最大间隔 365 天
  learningStepInterval: 10, // 学习步骤间隔（分钟）
};

/**
 * 改进的 SM-2 算法实现
 * 基于 Anki 和 SuperMemo 的研究
 */
export function calculateNextReview(
  progress: StudyProgress,
  rating: DifficultyRating,
  config: SRSConfig = DEFAULT_CONFIG
): StudyProgress {
  const newProgress = { ...progress };
  const { minEase, maxInterval } = config;
  
  // 如果是新单词（repetitions === 0），使用学习步骤
  if (progress.repetitions === 0 && rating === 'again') {
    // 标记为"再次"，保持在第一步
    newProgress.interval = 0;
    newProgress.easeFactor = Math.max(minEase, progress.easeFactor - 0.2);
    newProgress.nextReviewDate = new Date(Date.now() + DEFAULT_CONFIG.learningStepInterval * 60 * 1000);
    newProgress.lastStudiedDate = new Date();
    return newProgress;
  }

  switch (rating) {
    case 'again':
      // 完全重置，但保留一些学习历史
      newProgress.interval = 1;
      newProgress.repetitions = 0;
      newProgress.easeFactor = Math.max(minEase, progress.easeFactor - 0.2);
      break;
      
    case 'hard':
      // 困难：间隔增长较慢
      newProgress.interval = Math.max(1, Math.round(progress.interval * 1.2));
      newProgress.repetitions += 1;
      newProgress.easeFactor = Math.max(minEase, progress.easeFactor - 0.15);
      break;
      
    case 'good':
      // 良好：标准 SM-2 算法
      if (progress.repetitions === 0) {
        newProgress.interval = 1;
      } else if (progress.repetitions === 1) {
        newProgress.interval = 6;
      } else {
        newProgress.interval = Math.round(progress.interval * progress.easeFactor);
      }
      newProgress.repetitions += 1;
      break;
      
    case 'easy':
      // 简单：间隔增长更快
      if (progress.repetitions === 0) {
        newProgress.interval = 4;
      } else {
        newProgress.interval = Math.round(progress.interval * progress.easeFactor * 1.3);
      }
      newProgress.repetitions += 1;
      newProgress.easeFactor += 0.15;
      break;
  }
  
  // 限制最大间隔
  newProgress.interval = Math.min(newProgress.interval, maxInterval);
  
  // 确保 easeFactor 在合理范围内
  newProgress.easeFactor = Math.max(minEase, Math.min(newProgress.easeFactor, 3.0));
  
  // 计算下次复习日期
  const now = new Date();
  newProgress.nextReviewDate = new Date(now.getTime() + newProgress.interval * 24 * 60 * 60 * 1000);
  newProgress.lastStudiedDate = now;
  
  return newProgress;
}

/**
 * 计算单词的掌握程度 (0-100%)
 */
export function calculateMastery(progress: StudyProgress): number {
  if (progress.repetitions === 0) return 0;
  
  const baseScore = Math.min(progress.repetitions * 20, 80);
  const intervalBonus = Math.min(progress.interval / 30 * 20, 20);
  
  return Math.min(100, Math.round(baseScore + intervalBonus));
}

/**
 * 获取复习优先级分数（越高越优先）
 */
export function getReviewPriority(progress: StudyProgress): number {
  const now = new Date();
  const nextReview = new Date(progress.nextReviewDate);
  const overdue = now.getTime() - nextReview.getTime();
  
  // 逾期的单词优先级更高
  if (overdue > 0) {
    return 1000 + Math.min(overdue / (1000 * 60 * 60), 1000); // 每小时增加 1 点优先级
  }
  
  // 即将到期的单词
  const timeUntilDue = nextReview.getTime() - now.getTime();
  const hoursUntilDue = timeUntilDue / (1000 * 60 * 60);
  
  if (hoursUntilDue < 24) {
    return 500 + (24 - hoursUntilDue) * 20;
  }
  
  return hoursUntilDue;
}