BLI_bit_span_to_index_ranges.hh

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/* SPDX-FileCopyrightText: 2024 Blender Authors
 *
 * SPDX-License-Identifier: GPL-2.0-or-later */
 
#pragma once
 
#include <limits>
 
#include "BLI_bit_span.hh"
#include "BLI_index_ranges_builder.hh"
#include "BLI_math_bits.h"
#include "BLI_simd.hh"
 
namespace blender::bits {
 
template<typename IntT>
inline void bits_to_index_ranges(const BitSpan bits, IndexRangesBuilder<IntT> &builder)
{
  if (bits.is_empty()) {
    return;
  }
 
  /* -1 because we also need to store the end of the last range. */
  constexpr int64_t max_index = std::numeric_limits<IntT>::max() - 1;
  UNUSED_VARS_NDEBUG(max_index);
 
  auto append_range = [&](const IndexRange range) {
    BLI_assert(range.last() <= max_index);
    builder.add_range(IntT(range.start()), IntT(range.one_after_last()));
  };
 
  auto process_bit_int = [&](const BitInt value,
                             const int64_t start_bit,
                             const int64_t bits_num,
                             const int64_t start) {
    /* The bits in the mask are the ones we should look at. */
    const BitInt mask = mask_range_bits(start_bit, bits_num);
    const BitInt masked_value = mask & value;
    if (masked_value == 0) {
      /* Do nothing. */
      return;
    }
    if (masked_value == mask) {
      /* All bits are set. */
      append_range(IndexRange::from_begin_size(start, bits_num));
      return;
    }
    const int64_t bit_i_to_output_offset = start - start_bit;
 
    /* Iterate over ranges of 1s. For example, if the bits are 0b000111110001111000, the loop
     * below requires two iterations. The worst case for this is when the there are very many small
     * ranges of 1s (e.g. 0b10101010101). So far it seems like the overhead of detecting such
     * cases is higher than the potential benefit of using another algorithm. */
    BitInt current_value = masked_value;
    while (current_value != 0) {
      /* Find start of next range of 1s. */
      const int64_t first_set_bit_i = int64_t(bitscan_forward_uint64(current_value));
      /* This mask is used to find the end of the 1s range. */
      const BitInt find_unset_value = ~(current_value | mask_first_n_bits(first_set_bit_i) |
                                        ~mask);
      if (find_unset_value == 0) {
        /* In this case, the range one 1s extends to the end of the current integer. */
        const IndexRange range = IndexRange::from_begin_end(first_set_bit_i, start_bit + bits_num);
        append_range(range.shift(bit_i_to_output_offset));
        break;
      }
      /* Find the index of the first 0 after the range of 1s. */
      const int64_t next_unset_bit_i = int64_t(bitscan_forward_uint64(find_unset_value));
      /* Store the range of 1s. */
      const IndexRange range = IndexRange::from_begin_end(first_set_bit_i, next_unset_bit_i);
      append_range(range.shift(bit_i_to_output_offset));
      /* Remove the processed range of 1s so that it is ignored in the next iteration. */
      current_value &= ~mask_first_n_bits(next_unset_bit_i);
    }
    return;
  };
 
  const BitInt *data = bits.data();
  const IndexRange bit_range = bits.bit_range();
 
  /* As much as possible we want to process full 64-bit integers at once. However, the bit-span may
   * not be aligned, so it's first split up into aligned and unaligned sections. */
  const AlignedIndexRanges ranges = split_index_range_by_alignment(bit_range, bits::BitsPerInt);
 
  /* Process the first (partial) integer in the bit-span. */
  if (!ranges.prefix.is_empty()) {
    const BitInt first_int = *int_containing_bit(data, bit_range.start());
    process_bit_int(
        first_int, BitInt(ranges.prefix.start()) & BitIndexMask, ranges.prefix.size(), 0);
  }
 
  /* Process all the full integers in the bit-span. */
  if (!ranges.aligned.is_empty()) {
    const BitInt *start = int_containing_bit(data, ranges.aligned.start());
    const int64_t ints_to_check = ranges.aligned.size() / BitsPerInt;
    int64_t int_i = 0;
 
/* Checking for chunks of 0 bits can be speedup using intrinsics quite significantly. */
#if BLI_HAVE_SSE2
    for (; int_i + 1 < ints_to_check; int_i += 2) {
      /* Loads the next 128 bit. */
      const __m128i group = _mm_loadu_si128(reinterpret_cast<const __m128i *>(start + int_i));
      /* Checks if all the 128 bits are zero. */
      const bool group_is_zero = _mm_testz_si128(group, group);
      if (group_is_zero) {
        continue;
      }
      /* If at least one of them is not zero, process the two integers separately. */
      for (int j = 0; j < 2; j++) {
        process_bit_int(
            start[int_i + j], 0, BitsPerInt, ranges.prefix.size() + (int_i + j) * BitsPerInt);
      }
    }
#endif
 
    /* Process the remaining integers. */
    for (; int_i < ints_to_check; int_i++) {
      process_bit_int(start[int_i], 0, BitsPerInt, ranges.prefix.size() + int_i * BitsPerInt);
    }
  }
 
  /* Process the final few bits that don't fill up a full integer. */
  if (!ranges.suffix.is_empty()) {
    const BitInt last_int = *int_containing_bit(data, bit_range.last());
    process_bit_int(
        last_int, 0, ranges.suffix.size(), ranges.prefix.size() + ranges.aligned.size());
  }
}
 
}  // namespace blender::bits