System ocen (mini)

{
  "grades": [
    {
      "id": 1,
      "subject": "Matematyka",
      "grade": 4,
      "weight": 2,
      "description": "Sprawdzian - równania",
      "created_at": "2026-02-10"
    },
    {
      "id": 2,
      "subject": "Matematyka",
      "grade": 5,
      "weight": 1,
      "description": "Kartkówka",
      "created_at": "2026-02-12"
    },
    {
      "id": 3,
      "subject": "Polski",
      "grade": 3,
      "weight": 3,
      "description": "Wypracowanie",
      "created_at": "2026-02-13"
    }
  ]
}

$grade = filter_var($_POST['grade'], FILTER_VALIDATE_INT);

if ($grade === false || $grade < 1 || $grade > 6) {
    $errors[] = "Ocena musi być liczbą od 1 do 6";
}

$weight = filter_var($_POST['weight'] ?? 1, FILTER_VALIDATE_INT);
if ($weight < 1 || $weight > 3) {
    $weight = 1; // domyślna waga
}

function calculateAverage(array $grades): float {
    if (empty($grades)) {
        return 0;
    }

    $sum = array_sum(array_column($grades, 'grade'));
    return round($sum / count($grades), 2);
}

function calculateWeightedAverage(array $grades): float {
    if (empty($grades)) {
        return 0;
    }

    $weightedSum = 0;
    $totalWeight = 0;

    foreach ($grades as $g) {
        $weight = $g['weight'] ?? 1;
        $weightedSum += $g['grade'] * $weight;
        $totalWeight += $weight;
    }

    return $totalWeight > 0 ? round($weightedSum / $totalWeight, 2) : 0;
}

function groupBySubject(array $grades): array {
    $grouped = [];

    foreach ($grades as $g) {
        $subject = $g['subject'];
        if (!isset($grouped[$subject])) {
            $grouped[$subject] = [];
        }
        $grouped[$subject][] = $g;
    }

    return $grouped;
}

// Użycie
$bySubject = groupBySubject($grades);
foreach ($bySubject as $subject => $subjectGrades) {
    $avg = calculateWeightedAverage($subjectGrades);
    echo "$subject: średnia ważona $avg";
}

function predictNeededGrade(array $grades, float $targetAvg, int $newWeight = 1): ?int {
    $currentWeightedSum = 0;
    $currentTotalWeight = 0;

    foreach ($grades as $g) {
        $weight = $g['weight'] ?? 1;
        $currentWeightedSum += $g['grade'] * $weight;
        $currentTotalWeight += $weight;
    }

    // Szukamy X: (currentSum + X * newWeight) / (currentWeight + newWeight) = targetAvg
    $neededGrade = ($targetAvg * ($currentTotalWeight + $newWeight) - $currentWeightedSum) / $newWeight;

    if ($neededGrade < 1) return 1;
    if ($neededGrade > 6) return null; // niemożliwe
    return (int) ceil($neededGrade);
}