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Mechanical Properties and Deformation Mechanism
<?php
include "../db.php";

/* ---------------- BASIC VALIDATION ---------------- */
if (!isset($_POST['chapter_id'], $_POST['upload_category'])) {
    die("Invalid request");
}

$chapter_id = (int)$_POST['chapter_id'];
$category   = $_POST['upload_category']; // pdf | image | text
$file_type  = "note";

$description = mysqli_real_escape_string($conn, $_POST['description'] ?? '');
$linksRaw    = $_POST['links'] ?? '';

if ($chapter_id <= 0 || empty($category)) {
    die("Missing required fields");
}

/* ---------------- LINKS ---------------- */
$linksArray = array_filter(array_map('trim', explode("\n", $linksRaw)));
$linksJson  = json_encode($linksArray);

/* ---------------- DIRECTORIES ---------------- */
$pdfDir   = "../uploads/pdf/";
$imageDir = "../uploads/images/";
$textDir  = "../uploads/text/";

if (!is_dir($pdfDir))   mkdir($pdfDir, 0777, true);
if (!is_dir($imageDir)) mkdir($imageDir, 0777, true);
if (!is_dir($textDir))  mkdir($textDir, 0777, true);

/* =================================================
   PDF UPLOAD
================================================= */
if ($category === "pdf") {

    if (empty($_FILES['pdf_file']['name'])) {
        die("PDF file missing");
    }

    $safeName = time() . "_" . preg_replace("/[^A-Za-z0-9._-]/", "_", $_FILES['pdf_file']['name']);
    move_uploaded_file($_FILES['pdf_file']['tmp_name'], $pdfDir . $safeName);

    $path = "uploads/pdf/" . $safeName;

    mysqli_query($conn, "
        INSERT INTO uploads
        (chapter_id, file_name, file_path, file_type, description, links)
        VALUES
        ('$chapter_id', '$safeName', '$path', '$file_type', '$description', '$linksJson')
    ");
}

/* =================================================
   IMAGE UPLOAD
================================================= */
elseif ($category === "image") {

    if (empty($_FILES['image_file']['name'])) {
        die("Image file missing");
    }

    $safeName = time() . "_" . preg_replace("/[^A-Za-z0-9._-]/", "_", $_FILES['image_file']['name']);
    move_uploaded_file($_FILES['image_file']['tmp_name'], $imageDir . $safeName);

    $path = "uploads/images/" . $safeName;

    mysqli_query($conn, "
        INSERT INTO uploads
        (chapter_id, file_name, file_path, file_type, description, links)
        VALUES
        ('$chapter_id', '$safeName', '$path', '$file_type', '$description', '$linksJson')
    ");
}

/* =================================================
   TEXT UPLOAD (EDITOR: TEXT + IMAGES)
================================================= */
elseif ($category === "text") {

    $content = $_POST['content'] ?? '';

    if (trim($content) === "") {
        die("No content received");
    }

    /* Save editor content as .txt (HTML inside, same as components) */
    $baseName = "note_" . time();
    $textFile = $baseName . ".txt";
    $textPath = $textDir . $textFile;

    file_put_contents($textPath, $content);

    /* Save pasted image if exists */
    if (!empty($_POST['pasted_image'])) {

        $data = explode(',', $_POST['pasted_image']);
        if (count($data) === 2) {
            $imgData = base64_decode($data[1]);
            file_put_contents($textDir . $baseName . ".png", $imgData);
        }
    }

    $dbPath = "uploads/text/" . $textFile;

    mysqli_query($conn, "
        INSERT INTO uploads
        (chapter_id, file_name, file_path, file_type, description, links)
        VALUES
        ('$chapter_id', '$textFile', '$dbPath', '$file_type', '$description', '$linksJson')
    ");
}

/* ---------------- DONE ---------------- */
header("Location: upload_notes.php?success=1");
exit;