Freedom xdomain strategy (#2719)

* 统一 `domainStrategy` 行为.

* aliases NG.

* 化简.

* 调整.

* Let it crash.

* Update proto

---------

Co-authored-by: rui0572 <125641819+rui0572@users.noreply.github.com>

proxy/freedom/config.go

@@ -1,5 +1,44 @@
 package freedom
 
-func (c *Config) useIP() bool {
-	return c.DomainStrategy == Config_USE_IP || c.DomainStrategy == Config_USE_IP4 || c.DomainStrategy == Config_USE_IP6
+var strategy = [][]byte{
+	//              name        strategy,   prefer, fallback
+	{0, 0, 0}, //   AsIs        none,       /,      /
+	{1, 0, 0}, //   UseIP       use,        both,   none
+	{1, 4, 0}, //   UseIPv4     use,        4,      none
+	{1, 6, 0}, //   UseIPv6     use,        6,      none
+	{1, 4, 6}, //   UseIPv4v6   use,        4,      6
+	{1, 6, 4}, //   UseIPv6v4   use,        6,      4
+	{2, 0, 0}, //   ForceIP     force,      both,   none
+	{2, 4, 0}, //   ForceIPv4   force,      4,      none
+	{2, 6, 0}, //   ForceIPv6   force,      6,      none
+	{2, 4, 6}, //   ForceIPv4v6 force,      4,      6
+	{2, 6, 4}, //   ForceIPv6v4 force,      6,      4
+}
+
+func (c *Config) hasStrategy() bool {
+	return strategy[c.DomainStrategy][0] != 0
+}
+
+func (c *Config) forceIP() bool {
+	return strategy[c.DomainStrategy][0] == 2
+}
+
+func (c *Config) preferIP4() bool {
+	return strategy[c.DomainStrategy][1] == 4 || strategy[c.DomainStrategy][1] == 0
+}
+
+func (c *Config) preferIP6() bool {
+	return strategy[c.DomainStrategy][1] == 6 || strategy[c.DomainStrategy][1] == 0
+}
+
+func (c *Config) hasFallback() bool {
+	return strategy[c.DomainStrategy][2] != 0
+}
+
+func (c *Config) fallbackIP4() bool {
+	return strategy[c.DomainStrategy][2] == 4
+}
+
+func (c *Config) fallbackIP6() bool {
+	return strategy[c.DomainStrategy][2] == 6
 }