check for valid .GLD file

src/routes/(angemeldet)/upload/+page.svelte

@@ -47,6 +47,11 @@
 			success = false;
 		}
 
+		if (!(await check_valid_glb_file())) {
+			formErrors = { file: 'Keine gültige .GLD-Datei', ...formErrors };
+			success = false;
+		}
+
 		return success;
 	}
 
@@ -97,6 +102,47 @@
 		name = '';
 		files = null;
 	}
+
+	// `val` is hex string
+	function swap_endian(val) {
+		// from https://www.geeksforgeeks.org/bit-manipulation-swap-endianness-of-a-number/
+
+		let leftmost_byte = (val & eval(0x000000FF)) >> 0;
+		let left_middle_byte = (val & eval(0x0000FF00)) >> 8;
+		let right_middle_byte = (val & eval(0x00FF0000)) >> 16;
+		let rightmost_byte = (val & eval(0xFF000000)) >> 24;
+
+		leftmost_byte <<= 24;
+		left_middle_byte <<= 16;
+		right_middle_byte <<= 8;
+		rightmost_byte <<= 0;
+
+		let res = (leftmost_byte | left_middle_byte | right_middle_byte | rightmost_byte)
+
+		return res
+	}
+
+
+	async function check_valid_glb_file() {
+		// GLD Header, magic value 0x46546C67, identifies data as binary glTF, 4 bytes
+		// little endian!
+		const GLD_MAGIC = 0x46546C67;
+
+		// big endian!
+		let file = files[0];
+		let file_header = file.slice(0, 4)
+		let header_bytes = await file_header.bytes()
+		let file_header_hex = '0x' + header_bytes.toHex().toString();
+
+
+		if (GLD_MAGIC == swap_endian(file_header_hex)) {
+			return true;
+		} else {
+			return false;
+		}
+
+	}
+
 </script>
 
 <div class="mx-auto max-w-2xl">