LibGfx: Remove bit casting in OpenType OS2 table after construction

Userland/Libraries/LibGfx/Font/OpenType/Font.cpp

@@ -213,7 +213,7 @@ ErrorOr<NonnullRefPtr<Font>> Font::try_load_from_offset(ReadonlyBytes buffer, u3
 
  Optional<OS2> os2;
  if (opt_os2_slice.has_value())
- os2 = OS2(opt_os2_slice.value());
+ os2 = TRY(OS2::from_slice(opt_os2_slice.value()));
 
  Optional<Kern> kern {};
  if (opt_kern_slice.has_value())

Userland/Libraries/LibGfx/Font/OpenType/Tables.cpp

@@ -345,46 +345,76 @@ Optional<i16> Kern::read_glyph_kerning_format0(ReadonlyBytes slice, u16 left_gly
  return pair->value;
 }
 
+ErrorOr<OS2> OS2::from_slice(ReadonlyBytes slice)
+{
+ // All OS2 tables begin with a version.
+ if (slice.size() < sizeof(BigEndian<u16>))
+ return Error::from_string_literal("Could not load OS2: Not enough data");
+ u16 version = *bit_cast<BigEndian<u16> const*>(slice.data());
+
+ // NOTE: We assume that this table only ever has new fields added to the end in future versions.
+ switch (version) {
+ case 0: {
+ if (slice.size() < sizeof(Version0))
+ return Error::from_string_literal("Could not load OS2 v0: Not enough data");
+ return OS2(bit_cast<Version0 const*>(slice.data()));
+ }
+ case 1: {
+ if (slice.size() < sizeof(Version1))
+ return Error::from_string_literal("Could not load OS2 v1: Not enough data");
+ return OS2(bit_cast<Version1 const*>(slice.data()));
+ }
+ case 2:
+ default: {
+ if (slice.size() < sizeof(Version2))
+ return Error::from_string_literal("Could not load OS2 v2: Not enough data");
+ return OS2(bit_cast<Version2 const*>(slice.data()));
+ }
+ }
+}
+
 u16 OS2::weight_class() const
 {
- return header().us_weight_class;
+ return m_data.visit([](auto* any) { return any->us_weight_class; });
 }
 
 u16 OS2::width_class() const
 {
- return header().us_width_class;
+ return m_data.visit([](auto* any) { return any->us_width_class; });
 }
 
 u16 OS2::selection() const
 {
- return header().fs_selection;
+ return m_data.visit([](auto* any) { return any->fs_selection; });
 }
 
 i16 OS2::typographic_ascender() const
 {
- return header().s_typo_ascender;
+ return m_data.visit([](auto* any) { return any->s_typo_ascender; });
 }
 
 i16 OS2::typographic_descender() const
 {
- return header().s_typo_descender;
+ return m_data.visit([](auto* any) { return any->s_typo_descender; });
 }
 
 i16 OS2::typographic_line_gap() const
 {
- return header().s_typo_line_gap;
+ return m_data.visit([](auto* any) { return any->s_typo_line_gap; });
 }
 
 bool OS2::use_typographic_metrics() const
 {
- return header().fs_selection & 0x80;
+ return m_data.visit([](auto* any) { return any->fs_selection & 0x80; });
 }
 
 Optional<i16> OS2::x_height() const
 {
- if (header().version < 2)
- return {};
- return header_v2().sx_height;
+ return m_data.visit(
+ []<typename T> requires(requires { T::sx_height; })(T * data)->Optional<i16> {
+ return data->sx_height;
+ },
+ [](auto*) { return Optional<i16>(); });
 }
 
 ErrorOr<CBLC> CBLC::from_slice(ReadonlyBytes slice)

Userland/Libraries/LibGfx/Font/OpenType/Tables.h

@@ -272,6 +272,8 @@ class Hmtx {
 // OS/2: OS/2 and Windows Metrics Table
 class OS2 {
 public:
+ static ErrorOr<OS2> from_slice(ReadonlyBytes);
+
  u16 weight_class() const;
  u16 width_class() const;
  u16 selection() const;
@@ -283,11 +285,6 @@ class OS2 {
 
  [[nodiscard]] Optional<i16> x_height() const;
 
- explicit OS2(ReadonlyBytes slice)
- : m_slice(slice)
- {
- }
-
 private:
  struct [[gnu::packed]] Version0 {
  BigEndian<u16> version;
@@ -338,14 +335,12 @@ class OS2 {
  };
  static_assert(AssertSize<Version2, 96>());
 
- Version0 const& header() const { return *bit_cast<Version0 const*>(m_slice.data()); }
- Version2 const& header_v2() const
+ explicit OS2(Variant<Version0 const*, Version1 const*, Version2 const*> data)
+  : m_data(move(data))
  {
- VERIFY(header().version >= 2);
- return *bit_cast<Version2 const*>(m_slice.data());
  }
 
- ReadonlyBytes m_slice;
+ Variant<Version0 const*, Version1 const*, Version2 const*> m_data;
 };
 
 // https://learn.microsoft.com/en-us/typography/opentype/spec/name