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 // Copyright 2024 The Abseil Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     https://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
 #ifndef ABSL_DEBUGGING_INTERNAL_BOUNDED_UTF8_LENGTH_SEQUENCE_H_
 #define ABSL_DEBUGGING_INTERNAL_BOUNDED_UTF8_LENGTH_SEQUENCE_H_
 
 #include <cstdint>
 
 #include "absl/base/config.h"
 #include "absl/numeric/bits.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // A sequence of up to max_elements integers between 1 and 4 inclusive, whose
 // insertion operation computes the sum of all the elements before the insertion
 // point.  This is useful in decoding Punycode, where one needs to know where in
 // a UTF-8 byte stream the n-th code point begins.
 //
 // BoundedUtf8LengthSequence is async-signal-safe and suitable for use in
 // symbolizing stack traces in a signal handler, provided max_elements is not
 // improvidently large.  For inputs of lengths accepted by the Rust demangler,
 // up to a couple hundred code points, InsertAndReturnSumOfPredecessors should
 // run in a few dozen clock cycles, on par with the other arithmetic required
 // for Punycode decoding.
 template <uint32_t max_elements>
 class BoundedUtf8LengthSequence {
  public:
   // Constructs an empty sequence.
   BoundedUtf8LengthSequence() = default;
 
   // Inserts `utf_length` at position `index`, shifting any existing elements at
   // or beyond `index` one position to the right.  If the sequence is already
   // full, the rightmost element is discarded.
   //
   // Returns the sum of the elements at positions 0 to `index - 1` inclusive.
   // If `index` is greater than the number of elements already inserted, the
   // excess positions in the range count 1 apiece.
   //
   // REQUIRES: index < max_elements and 1 <= utf8_length <= 4.
   uint32_t InsertAndReturnSumOfPredecessors(
       uint32_t index, uint32_t utf8_length) {
     // The caller shouldn't pass out-of-bounds inputs, but if it does happen,
     // clamp the values and try to continue.  If we're being called from a
     // signal handler, the last thing we want to do is crash.  Emitting
     // malformed UTF-8 is a lesser evil.
     if (index >= max_elements) index = max_elements - 1;
     if (utf8_length == 0 || utf8_length > 4) utf8_length = 1;
 
     const uint32_t word_index = index/32;
     const uint32_t bit_index = 2 * (index % 32);
     const uint64_t ones_bit = uint64_t{1} << bit_index;
 
     // Compute the sum of predecessors.
     //   - Each value from 1 to 4 is represented by a bit field with value from
     //     0 to 3, so the desired sum is index plus the sum of the
     //     representations actually stored.
     //   - For each bit field, a set low bit should contribute 1 to the sum, and
     //     a set high bit should contribute 2.
     //   - Another way to say the same thing is that each set bit contributes 1,
     //     and each set high bit contributes an additional 1.
     //   - So the sum we want is index + popcount(everything) + popcount(bits in
     //     odd positions).
     const uint64_t odd_bits_mask = 0xaaaaaaaaaaaaaaaa;
     const uint64_t lower_seminibbles_mask = ones_bit - 1;
     const uint64_t higher_seminibbles_mask = ~lower_seminibbles_mask;
     const uint64_t same_word_bits_below_insertion =
         rep_[word_index] & lower_seminibbles_mask;
     int full_popcount = absl::popcount(same_word_bits_below_insertion);
     int odd_popcount =
         absl::popcount(same_word_bits_below_insertion & odd_bits_mask);
     for (uint32_t j = word_index; j > 0; --j) {
       const uint64_t word_below_insertion = rep_[j - 1];
       full_popcount += absl::popcount(word_below_insertion);
       odd_popcount += absl::popcount(word_below_insertion & odd_bits_mask);
     }
     const uint32_t sum_of_predecessors =
         index + static_cast<uint32_t>(full_popcount + odd_popcount);
 
     // Now insert utf8_length's representation, shifting successors up one
     // place.
     for (uint32_t j = max_elements/32 - 1; j > word_index; --j) {
       rep_[j] = (rep_[j] << 2) | (rep_[j - 1] >> 62);
     }
     rep_[word_index] =
         (rep_[word_index] & lower_seminibbles_mask) |
         (uint64_t{utf8_length - 1} << bit_index) |
         ((rep_[word_index] & higher_seminibbles_mask) << 2);
 
     return sum_of_predecessors;
   }
 
  private:
   // If the (32 * i + j)-th element of the represented sequence has the value k
   // (0 <= j < 32, 1 <= k <= 4), then bits 2 * j and 2 * j + 1 of rep_[i]
   // contain the seminibble (k - 1).
   //
   // In particular, the zero-initialization of rep_ makes positions not holding
   // any inserted element count as 1 in InsertAndReturnSumOfPredecessors.
   //
   // Example: rep_ = {0xb1, ... the rest zeroes ...} represents the sequence
   // (2, 1, 4, 3, ... the rest 1's ...).  Constructing the sequence of Unicode
   // code points "Àa🂻中" = {U+00C0, U+0061, U+1F0BB, U+4E2D} (among many
   // other examples) would yield this value of rep_.
   static_assert(max_elements > 0 && max_elements % 32 == 0,
                 "max_elements must be a positive multiple of 32");
   uint64_t rep_[max_elements/32] = {};
 };
 
 }  // namespace debugging_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_BOUNDED_UTF8_LENGTH_SEQUENCE_H_

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