<?php
/**
 * SM-2 Spaced Repetition Algorithm
 * Ratings: easy=5, medium=3, hard=2, wrong=0
 */
function sm2_calculate($rating_name, $current_ease, $current_interval, $current_reps) {
    $rating_map = ['wrong' => 0, 'hard' => 2, 'medium' => 3, 'easy' => 5];
    $q = $rating_map[$rating_name] ?? 3;

    $ease = $current_ease;
    $interval = $current_interval;
    $reps = $current_reps;

    if ($q < 3) {
        $reps = 0;
        $interval = 1;
    } else {
        if ($reps === 0) {
            $interval = 1;
        } elseif ($reps === 1) {
            $interval = 3;
        } else {
            $interval = (int) round($interval * $ease);
        }
        $reps++;
    }

    $ease = $ease + 0.1 - (5 - $q) * (0.08 + (5 - $q) * 0.02);
    if ($ease < 1.3) $ease = 1.3;

    $next_review = date('Y-m-d', strtotime("+{$interval} days"));

    return [
        'ease_factor' => round($ease, 2),
        'interval_days' => $interval,
        'repetitions' => $reps,
        'next_review' => $next_review
    ];
}