[libc] utf8 to 32 CharacterConverter

libc/src/__support/wchar/character_converter.cpp

@@ -28,39 +28,30 @@ int CharacterConverter::push(char8_t utf8_byte) {
   // Checking the first byte if first push
   if (state->bytes_processed == 0 && state->total_bytes == 0) {
     state->partial = static_cast<char32_t>(0);
-    int numOnes = cpp::countl_one(utf8_byte);
-    switch (numOnes) {
+    uint8_t numOnes = static_cast<uint8_t>(cpp::countl_one(utf8_byte));
     // 1 byte total
-    case 0:
+    if (numOnes == 0) {
       state->total_bytes = 1;
-      break;
-    // 2 bytes total
-    case 2:
-      state->total_bytes = 2;
-      utf8_byte &= 0x1F;
-      break;
-    // 3 bytes total
-    case 3:
-      state->total_bytes = 3;
-      utf8_byte &= 0x0F;
-      break;
-    // 4 bytes total
-    case 4:
-      state->total_bytes = 4;
-      utf8_byte &= 0x07;
-      break;
+    }
+    // 2 through 4 bytes total
+    else if (numOnes >= 2 && numOnes <= 4) {
+      state->total_bytes = numOnes;
+      utf8_byte &= (0x7F >> numOnes);
+    }
     // Invalid first byte
-    default:
+    else {
       return -1;
     }
     state->partial = static_cast<char32_t>(utf8_byte);
     state->bytes_processed++;
     return 0;
   }
   // Any subsequent push
+  // Adding 6 more bits so need to left shift
+  const int shift_amount = 6;
   if (cpp::countl_one(utf8_byte) == 1 && !isComplete()) {
     char32_t byte = utf8_byte & 0x3F;
-    state->partial = state->partial << 6;
+    state->partial = state->partial << shift_amount;
     state->partial |= byte;
     state->bytes_processed++;
     return 0;

libc/test/src/__support/wchar/utf8_to_32_test.cpp

@@ -31,7 +31,8 @@ TEST(LlvmLibcCharacterConverterUTF8To32Test, TwoBytes) {
   LIBC_NAMESPACE::internal::mbstate state;
   state.bytes_processed = 0;
   state.total_bytes = 0;
-  const char ch[2] = {static_cast<char>(0xC2), static_cast<char>(0x8E)}; // Ž
+  const char ch[2] = {static_cast<char>(0xC2),
+                      static_cast<char>(0x8E)}; // Ž car symbol
 
   LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
   char_conv.push(static_cast<char8_t>(ch[0]));
@@ -47,7 +48,7 @@ TEST(LlvmLibcCharacterConverterUTF8To32Test, ThreeBytes) {
   state.bytes_processed = 0;
   state.total_bytes = 0;
   const char ch[3] = {static_cast<char>(0xE2), static_cast<char>(0x88),
-                      static_cast<char>(0x91)}; // ∑
+                      static_cast<char>(0x91)}; // ∑ sigma symbol
 
   LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
   char_conv.push(static_cast<char8_t>(ch[0]));
@@ -64,7 +65,8 @@ TEST(LlvmLibcCharacterConverterUTF8To32Test, FourBytes) {
   state.bytes_processed = 0;
   state.total_bytes = 0;
   const char ch[4] = {static_cast<char>(0xF0), static_cast<char>(0x9F),
-                      static_cast<char>(0xA4), static_cast<char>(0xA1)}; // 🤡
+                      static_cast<char>(0xA4),
+                      static_cast<char>(0xA1)}; // 🤡 clown emoji
 
   LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
   char_conv.push(static_cast<char8_t>(ch[0]));