Implement patty_ax25_frame_decode_xid()

Implement patty_ax25_frame_decode_xid() to decode XID frame data,
adhering to the basic structure of ISO 8885 as closely as possible

include/patty/ax25/frame.h

@@ -82,6 +82,37 @@ typedef struct _patty_ax25_frame {
     size_t infolen;
 } patty_ax25_frame;
 
+#pragma pack(push)
+#pragma pack(1)
+
+typedef struct _patty_ax25_frame_xid_element {
+    uint8_t format,
+            group;
+
+    uint16_t len;
+} patty_ax25_frame_xid_element;
+
+typedef struct _patty_ax25_frame_xid_param {
+    uint8_t id, len;
+} patty_ax25_frame_xid_param;
+
+enum patty_ax25_frame_xid_param_type {
+    PATTY_AX25_FRAME_XID_PARAM_CLASS     = 2,
+    PATTY_AX25_FRAME_XID_PARAM_HDLC      = 3,
+    PATTY_AX25_FRAME_XID_PARAM_INFO_TX   = 5,
+    PATTY_AX25_FRAME_XID_PARAM_INFO_RX   = 6,
+    PATTY_AX25_FRAME_XID_PARAM_WINDOW_TX = 7,
+    PATTY_AX25_FRAME_XID_PARAM_WINDOW_RX = 8,
+    PATTY_AX25_FRAME_XID_PARAM_ACK       = 9
+};
+
+#pragma pack(pop)
+
+typedef int (*patty_ax25_frame_xid_callback)(enum patty_ax25_frame_xid_param_type type,
+                                             size_t len,
+                                             uint8_t *data,
+                                             void *ctx);
+
 ssize_t patty_ax25_frame_decode_address(patty_ax25_frame *frame,
                                         void *buf,
                                         size_t len);
@@ -92,6 +123,11 @@ ssize_t patty_ax25_frame_decode_control(patty_ax25_frame *frame,
                                         size_t offset,
                                         size_t len);
 
+ssize_t patty_ax25_frame_decode_xid(patty_ax25_frame_xid_callback callback,
+                                    void *data,
+                                    size_t offset,
+                                    size_t len,
+                                    void *ctx);
 
 ssize_t patty_ax25_frame_encode(patty_ax25_frame *frame,
                                 enum patty_ax25_frame_format format,

src/frame.c

@@ -271,6 +271,59 @@ error:
     return -1;
 }
 
+ssize_t patty_ax25_frame_decode_xid(patty_ax25_frame_xid_callback callback,
+                                    void *data,
+                                    size_t offset,
+                                    size_t len,
+                                    void *ctx) {
+    size_t start = offset;
+
+    patty_ax25_frame_xid_element *elem = ((patty_ax25_frame_xid_element *)
+        (uint8_t *)data + offset);
+
+    if (elem->format != 0x82 || elem->group != 0x80) {
+        errno = EIO;
+
+        goto error;
+    }
+
+    if (be16toh(elem->len) != len - offset) {
+        errno = EIO;
+
+        goto error;
+    }
+
+    offset += sizeof(*elem);
+
+    while (offset < start + len) {
+        patty_ax25_frame_xid_param *param = (patty_ax25_frame_xid_param *)
+            ((uint8_t *)data + offset);
+
+        if (callback(param->id,
+                     (size_t)param->len,
+                     (uint8_t *)(param + 1),
+                     ctx) < 0) {
+            goto error_callback;
+        }
+
+        offset += sizeof(*param) + param->len;
+    }
+
+    if (offset != start + len) {
+        errno = EIO;
+
+        goto error;
+    }
+
+    errno = 0;
+
+    return offset - start;
+
+error_callback:
+error:
+    return -1;
+}
+
 static ssize_t encode_address(void *buf,
                               patty_ax25_addr *dest,
                               patty_ax25_addr *src,