+static unsigned int size_to_syncsafe(unsigned int size)
+{
+ return (((size) & (0x7f << 0)) >> 0) +
+ (((size) & (0x7f << 8)) >> 1) +
+ (((size) & (0x7f << 16)) >> 2) +
+ (((size) & (0x7f << 24)) >> 3);
+}
+
+/* No real verification, only check that the tag consists of
+ * a combination of capital alpha-numerical characters */
+static int is_tag(const char *buf, unsigned int len)
+{
+ if (!len)
+ return 0;
+
+ while (len--)
+ if ((buf[len] < 'A' ||
+ buf[len] > 'Z') &&
+ (buf[len] < '0' ||
+ buf[len] > '9'))
+ return 0;
+
+ return 1;
+}
+
+/**
+ * Return 1 if the tag of length len at the given offset is valid, 0 if not, -1 on error
+ */
+static int check_tag(AVIOContext *s, int offset, unsigned int len)
+{
+ char tag[4];
+
+ if (len > 4 ||
+ avio_seek(s, offset, SEEK_SET) < 0 ||
+ avio_read(s, tag, len) < len)
+ return -1;
+ else if (!AV_RB32(tag) || is_tag(tag, len))
+ return 1;
+
+ return 0;
+}
+