[libc-commits] [libc] [libc] utf8 to 32 CharacterConverter (PR #143973)

[Brooks Moses via libc-commits]

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@@ -22,13 +25,65 @@ bool CharacterConverter::isComplete() {
   return state->bytes_processed == state->total_bytes;
 }
 
-int CharacterConverter::push(char8_t utf8_byte) {}
+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);
+    uint8_t numOnes = static_cast<uint8_t>(cpp::countl_one(utf8_byte));
+    // 1 byte total
+    if (numOnes == 0) {
+      state->total_bytes = 1;
+    }
+    // 2 through 4 bytes total
+    else if (numOnes >= 2 && numOnes <= 4) {
+      /* Since the format is 110xxxxx, 1110xxxx, and 11110xxx for 2, 3, and 4,
+      we will make the base mask with 7 ones and right shift it as necessary. */
+      constexpr size_t significant_bits = 7;
+      state->total_bytes = numOnes;
+      utf8_byte &=
+          (mask_trailing_ones<uint32_t, significant_bits>() >> numOnes);
+    }
+    // Invalid first byte
+    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
+  constexpr size_t ENCODED_BITS_PER_UTF8 = 6;
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brooksmoses wrote:

Here your constant is all-caps, on line 41 it's lower-case.  Be consistent?

https://github.com/llvm/llvm-project/pull/143973