diff options
Diffstat (limited to 'absl/debugging/internal/bounded_utf8_length_sequence.h')
| -rw-r--r-- | absl/debugging/internal/bounded_utf8_length_sequence.h | 126 |
1 files changed, 126 insertions, 0 deletions
diff --git a/absl/debugging/internal/bounded_utf8_length_sequence.h b/absl/debugging/internal/bounded_utf8_length_sequence.h new file mode 100644 index 00000000..188e06c4 --- /dev/null +++ b/absl/debugging/internal/bounded_utf8_length_sequence.h @@ -0,0 +1,126 @@ +// 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_ |