IPv4Address: constexpr support

Problem:
- IPv4Address class cannot be used in a compile-time context.
- A union is used by initializing one of the members and reading the
  non-active member. This is undefined behavior and not permitted in a
  `constexpr` context.

Solution:
- Eliminate undefined behavior by changing to a simple `u32` for
  storage instead of the union and performing mask/shift calculations
  for obtaining the individual octets.
- Decorate functions with `constexpr` where possible. Currently string
  formatting and optionals are not `constexpr`-capable so functions
  using those are left out.
- Modify tests to validate functionality in a `constexpr` context in
  addition to the run-time tests already being run. This ensures that
  functionality is the same in both contexts.

AK/IPv4Address.h

@@ -33,69 +33,72 @@
 #include <AK/StringView.h>
 #include <AK/Vector.h>
 
-typedef u32 in_addr_t;
-
 namespace AK {
 
 class [[gnu::packed]] IPv4Address
 {
+ enum class SubnetClass : int {
+ A = 0,
+ B,
+ C,
+ D
+ };
+
 public:
- IPv4Address() { }
- IPv4Address(const u8 data[4])
+ using in_addr_t = u32;
+
+ constexpr IPv4Address() = default;
+
+ constexpr IPv4Address(u32 a, u32 b, u32 c, u32 d)
  {
- m_data[0] = data[0];
- m_data[1] = data[1];
- m_data[2] = data[2];
- m_data[3] = data[3];
+ m_data = (d << 24) | (c << 16) | (b << 8) | a;
  }
- IPv4Address(u8 a, u8 b, u8 c, u8 d)
+
+ constexpr IPv4Address(const u8 data[4])
  {
- m_data[0] = a;
- m_data[1] = b;
- m_data[2] = c;
- m_data[3] = d;
+ m_data = (u32(data[3]) << 24) | (u32(data[2]) << 16) | (u32(data[1]) << 8) | u32(data[0]);
  }
- IPv4Address(NetworkOrdered<u32> address)
- : m_data_as_u32(address)
+
+ constexpr IPv4Address(NetworkOrdered<u32> address)
+ : m_data(address)
  {
  }
 
- u8 operator[](int i) const
+ constexpr u8 operator[](int i) const
  {
  ASSERT(i >= 0 && i < 4);
- return m_data[i];
+ return octet(SubnetClass(i));
  }
 
  String to_string() const
  {
- return String::formatted("{}.{}.{}.{}", m_data[0], m_data[1], m_data[2], m_data[3]);
+ return String::formatted("{}.{}.{}.{}",
+ octet(SubnetClass::A),
+ octet(SubnetClass::B),
+ octet(SubnetClass::C),
+ octet(SubnetClass::D));
  }
 
  static Optional<IPv4Address> from_string(const StringView& string)
  {
  if (string.is_null())
  return {};
- auto parts = string.split_view('.');
 
- u32 a;
- u32 b;
- u32 c;
- u32 d;
+ const auto parts = string.split_view('.');
+
+ u32 a {};
+ u32 b {};
+ u32 c {};
+ u32 d {};
 
  if (parts.size() == 1) {
- a = 0;
- b = 0;
- c = 0;
  d = parts[0].to_uint().value_or(256);
  } else if (parts.size() == 2) {
  a = parts[0].to_uint().value_or(256);
- b = 0;
- c = 0;
  d = parts[1].to_uint().value_or(256);
  } else if (parts.size() == 3) {
  a = parts[0].to_uint().value_or(256);
  b = parts[1].to_uint().value_or(256);
- c = 0;
  d = parts[2].to_uint().value_or(256);
  } else if (parts.size() == 4) {
  a = parts[0].to_uint().value_or(256);
@@ -108,32 +111,37 @@ class [[gnu::packed]] IPv4Address
 
  if (a > 255 || b > 255 || c > 255 || d > 255)
  return {};
- return IPv4Address((u8)a, (u8)b, (u8)c, (u8)d);
+ return IPv4Address(a, b, c, d);
  }
 
- in_addr_t to_in_addr_t() const { return m_data_as_u32; }
- u32 to_u32() const { return m_data_as_u32; }
+ constexpr in_addr_t to_in_addr_t() const { return m_data; }
+ constexpr u32 to_u32() const { return m_data; }
 
- bool operator==(const IPv4Address& other) const { return m_data_as_u32 == other.m_data_as_u32; }
- bool operator!=(const IPv4Address& other) const { return m_data_as_u32 != other.m_data_as_u32; }
+ constexpr bool operator==(const IPv4Address& other) const = default;
+ constexpr bool operator!=(const IPv4Address& other) const = default;
 
- bool is_zero() const
+ constexpr bool is_zero() const
  {
- return m_data_as_u32 == 0;
+ return m_data == 0u;
  }
 
 private:
- union {
- u8 m_data[4];
- u32 m_data_as_u32 { 0 };
- };
+ constexpr u32 octet(const SubnetClass subnet) const
+ {
+ NetworkOrdered<u32> address(m_data);
+ constexpr auto bits_per_byte = 8;
+ const auto bits_to_shift = bits_per_byte * int(subnet);
+ return (m_data >> bits_to_shift) & 0x0000'00FF;
+ }
+
+ u32 m_data {};
 };
 
 static_assert(sizeof(IPv4Address) == 4);
 
 template<>
 struct Traits<IPv4Address> : public GenericTraits<IPv4Address> {
- static unsigned hash(const IPv4Address& address) { return string_hash((const char*)&address, sizeof(address)); }
+ static constexpr unsigned hash(const IPv4Address& address) { return int_hash(address.to_u32()); }
 };
 
 inline const LogStream& operator<<(const LogStream& stream, const IPv4Address& value)

AK/Tests/TestIPv4Address.cpp

@@ -31,31 +31,45 @@
 
 TEST_CASE(should_default_contructor_with_0s)
 {
- const IPv4Address addr {};
+ constexpr IPv4Address addr {};
+
+ static_assert(addr.is_zero());
 
  EXPECT(addr.is_zero());
 }
 
 TEST_CASE(should_construct_from_c_array)
 {
- const u8 a[4] = { 1, 2, 3, 4 };
- const IPv4Address addr(a);
+ constexpr auto addr = [] {
+ const u8 a[4] = { 1, 2, 3, 4 };
+ return IPv4Address(a);
+ }();
+
+ static_assert(!addr.is_zero());
 
  EXPECT(!addr.is_zero());
 }
 
 TEST_CASE(should_construct_from_u32)
 {
- const NetworkOrdered<u32> a = 0x11'22'33'44;
- const IPv4Address addr(a);
+ constexpr auto addr = [] {
+ const NetworkOrdered<u32> a = 0x11'22'33'44;
+ return IPv4Address(a);
+ }();
+
+ static_assert(!addr.is_zero());
 
  EXPECT(!addr.is_zero());
 }
 
 TEST_CASE(should_get_octets_by_byte_offset)
 {
- const u8 a[4] = { 1, 25, 39, 42 };
- const IPv4Address addr(a);
+ constexpr IPv4Address addr(1, 25, 39, 42);
+
+ static_assert(1 == addr[0]);
+ static_assert(25 == addr[1]);
+ static_assert(39 == addr[2]);
+ static_assert(42 == addr[3]);
 
  EXPECT_EQ(1, addr[0]);
  EXPECT_EQ(25, addr[1]);
@@ -65,8 +79,7 @@ TEST_CASE(should_get_octets_by_byte_offset)
 
 TEST_CASE(should_convert_to_string)
 {
- const u8 a[4] = { 1, 25, 39, 42 };
- const IPv4Address addr(a);
+ constexpr IPv4Address addr(1, 25, 39, 42);
 
  EXPECT_EQ("1.25.39.42", addr.to_string());
 }
@@ -131,26 +144,29 @@ TEST_CASE(should_fill_a_b_d_octets_from_3_parts)
 
 TEST_CASE(should_convert_to_in_addr_t)
 {
- const u8 a[4] = { 1, 2, 3, 4 };
- const IPv4Address addr(a);
+ constexpr IPv4Address addr(1, 2, 3, 4);
+
+ static_assert(0x04'03'02'01u == addr.to_in_addr_t());
 
  EXPECT_EQ(0x04'03'02'01u, addr.to_in_addr_t());
 }
 
 TEST_CASE(should_convert_to_u32)
 {
- const u8 a[4] = { 1, 2, 3, 4 };
- const IPv4Address addr(a);
+ constexpr IPv4Address addr(1, 2, 3, 4);
+
+ static_assert(0x04'03'02'01u == addr.to_in_addr_t());
 
  EXPECT_EQ(0x04'03'02'01u, addr.to_u32());
 }
 
 TEST_CASE(should_compare)
 {
- const u8 a[4] = { 1, 2, 3, 4 };
- const u8 b[4] = { 1, 2, 3, 5 };
- const IPv4Address addr_a(a);
- const IPv4Address addr_b(b);
+ constexpr IPv4Address addr_a(1, 2, 3, 4);
+ constexpr IPv4Address addr_b(1, 2, 3, 5);
+
+ static_assert(addr_a != addr_b);
+ static_assert(addr_a == addr_a);
 
  EXPECT(addr_a != addr_b);
  EXPECT(addr_a == addr_a);