french-vocab/src/utils/srs.ts

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

export function calculateNextReview(
  progress: StudyProgress,
  rating: DifficultyRating
): StudyProgress {
  const newProgress = { ...progress };
  
  switch (rating) {
    case 'again':
      newProgress.interval = 1;
      newProgress.repetitions = 0;
      newProgress.easeFactor = Math.max(1.3, progress.easeFactor - 0.2);
      break;
    case 'hard':
      newProgress.interval = Math.round(progress.interval * 1.2);
      newProgress.repetitions += 1;
      newProgress.easeFactor = Math.max(1.3, progress.easeFactor - 0.15);
      break;
    case 'good':
      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':
      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;
  }
  
  const now = new Date();
  newProgress.nextReviewDate = new Date(now.getTime() + newProgress.interval * 24 * 60 * 60 * 1000);
  newProgress.lastStudiedDate = now;
  
  return newProgress;
}