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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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