net: mqtt: improve decoder buffer handling

Improve buffer handling logic to use local variables extensively.

This change reduces the number of pointer dereferences, which leads
to more efficient runtime and helps reduce the code size.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>

subsys/net/lib/mqtt/mqtt_decoder.c

@@ -29,13 +29,17 @@ LOG_MODULE_REGISTER(net_mqtt_dec, CONFIG_MQTT_LOG_LEVEL);
  */
 static int unpack_uint8(struct buf_ctx *buf, uint8_t *val)
 {
-	NET_DBG(">> cur:%p, end:%p", (void *)buf->cur, (void *)buf->end);
+	uint8_t *cur = buf->cur;
+	uint8_t *end = buf->end;
+
+	NET_DBG(">> cur:%p, end:%p", (void *)cur, (void *)end);
 
-	if ((buf->end - buf->cur) < sizeof(uint8_t)) {
+	if ((end - cur) < sizeof(uint8_t)) {
 		return -EINVAL;
 	}
 
-	*val = *(buf->cur++);
+	*val = cur[0];
+	buf->cur = (cur + sizeof(uint8_t));
 
 	NET_DBG("<< val:%02x", *val);
 
@@ -55,14 +59,17 @@ static int unpack_uint8(struct buf_ctx *buf, uint8_t *val)
  */
 static int unpack_uint16(struct buf_ctx *buf, uint16_t *val)
 {
-	NET_DBG(">> cur:%p, end:%p", (void *)buf->cur, (void *)buf->end);
+	uint8_t *cur = buf->cur;
+	uint8_t *end = buf->end;
+
+	NET_DBG(">> cur:%p, end:%p", (void *)cur, (void *)end);
 
-	if ((buf->end - buf->cur) < sizeof(uint16_t)) {
+	if ((end - cur) < sizeof(uint16_t)) {
 		return -EINVAL;
 	}
 
-	*val = *(buf->cur++) << 8; /* MSB */
-	*val |= *(buf->cur++); /* LSB */
+	*val = sys_get_be16(cur);
+	buf->cur = (cur + sizeof(uint16_t));
 
 	NET_DBG("<< val:%04x", *val);