IPAddressV6::kToFullyQualifiedSize.

Summary:
[Folly] IPAddressV6::kToFullyQualifiedSize.

The scenario: One has a string known to be a fully-qualified IPv4 or fully-qualified IPv6 address, but one does not know which of the two it is.

The dirty but fast solution: just check its size.

Test Plan: Build. Unit tests.

Reviewed By: mshneer@fb.com

Subscribers: folly-diffs@, dougw, yfeldblum, chalfant, brettp

FB internal diff: D2016634

Signature: t1:2016634:1429831808:08ecb28eab3060988f9613748ed8dabf3724bf13

folly/IPAddressV6.h

@@ -82,6 +82,10 @@ class IPAddressV6 : boost::totally_ordered<IPAddressV6> {
   // Binary prefix for 6to4 networks
   static const uint32_t PREFIX_6TO4;
 
+  // Size of std::string returned by toFullyQualified.
+  static constexpr size_t kToFullyQualifiedSize =
+    8 /*words*/ * 4 /*hex chars per word*/ + 7 /*separators*/;
+
   /**
    * Create a new IPAddress instance from the provided binary data.
    * @throws IPAddressFormatException if the input length is not 16 bytes.

folly/test/IPAddressTest.cpp

@@ -409,6 +409,11 @@ TEST(IPAddress, ToFullyQualifiedLocal) {
   EXPECT_EQ("0000:0000:0000:0000:0000:0000:0000:0001", ip.toFullyQualified())
       << ip;
 }
+TEST(IPAddress, ToFullyQualifiedSize) {
+  auto actual = IPAddressV6::kToFullyQualifiedSize;
+  auto expected = IPAddress("::").toFullyQualified().size();
+  EXPECT_EQ(expected, actual);
+}
 
 // test v4-v6 mapped addresses
 TEST_P(IPAddressMappedTest, MappedEqual) {