algorithm-engineering/src/sorter.cpp

#include "sorter.hpp"

#include <algorithm>
#include <iterator>
#include <cassert>
#include <iostream>
#include <bitset>
#include <climits>

#define DEBUG false

namespace ae {

void sorter::sort(container& data) {
  for (auto i = 1uz; i < data.placeholder_.size(); ++i) {
    std::ranges::copy(data.placeholder_[i], std::back_inserter(data.placeholder_[0]));
    data.placeholder_[i].clear();
  }
  #if DEBUG
    std::vector<container::element_type> copy;
    std::ranges::copy(data.placeholder_[0], std::back_inserter(copy));
    std::sort(copy.begin(), copy.end());
  #endif
  sorter::msd_inplace_radix_sort(data.placeholder_[0], 0, [&](auto span) {sorter::robin_hood_sort(span);});

  #if DEBUG
    for (int i = 0; i < copy.size(); i++) {
      if (copy[i] != data.placeholder_[0][i])
      std::cerr << i << " " << "sorted: " << copy[i] << " actual:" << data.placeholder_[0][i] << std::endl;
    }
  #endif

}

void sorter::msd_inplace_radix_sort(
  std::span<container::element_type> range, 
  size_t passes, 
  const std::function<void(std::span<container::element_type> bucket)>& bucket_sort
) {
  if (std::begin(range) >= std::end(range)) {
    return;
  }
  if (sorter::RADIX_ITERATIONS == passes) {
    switch (range.size()) {
      case 1: return;
      case 2: 
        if (range[0] >= range[1]) {
          std::swap(range[0], range[1]);
        }
        return;
      default:
        bucket_sort(range);
        return;
    }
    if (range.size() > 1) {
      bucket_sort(range);
    }
    return;
  }

  auto lower = std::begin(range);
  auto upper = std::end(range);
  while (lower < upper) {
    
    if (*lower & (1L << (sizeof(container::element_type) * CHAR_BIT - passes - 1))) {
      // The <passes>-left bit is set, so move to the beginning of the end section and decrement the upper iterator
      --upper;
      std::swap(*upper, *lower);
    } else {
      ++lower;
    }
  }
  #if DEBUG
    std::cerr << "pass: " << passes << " begin: " << &*std::begin(range) << " end: " << &*std::end(range) << " lower: " << &*lower << std::endl;
  #endif
  sorter::msd_inplace_radix_sort(std::span<container::element_type> (std::begin(range), lower), passes + 1, bucket_sort);
  sorter::msd_inplace_radix_sort(std::span<container::element_type> (lower, std::end(range)), passes + 1, bucket_sort);
}
void sorter::robin_hood_sort(std::span<container::element_type> bucket) {
  const auto size = bucket.size() + sorter::OVERHEAD_SIZE;
  const auto mask = ((1L) << (sizeof(container::element_type) * CHAR_BIT - sorter::RADIX_ITERATIONS)) - 1;
  std::vector<container::element_type> space(size, -1L);
  for (auto element : bucket) {
    auto masked_element = (element & mask);
    auto index = ((masked_element) * bucket.size()) / mask;
    if (space[index] == -1) {
      space[index] = element;
    } else {
      #if DEBUG
        std::cerr << "Linear probing of " << element << " at index " << index << ". Current element " << space[index] << std::endl;
      #endif
      auto i = index;
      // linear probing
      while (i < size - 1 && space[i] != -1) {++i;};
      #if DEBUG
        std::cerr << "Inserting " << element << " at index " << i << " instead of " << index << std::endl;
      #endif
      space[i] = element;
    }
  }

  #if DEBUG
    std::cerr << "Unsorted\n";
    for (auto element : space) {
      std::cerr << element << " ";
    }
    std::cerr << std::endl;
  #endif

  // One final pass to correct linear probing errors
  for (auto i = 1; i < size; ++i) {
    auto j = i;
    while ((uint64_t) space[j-1] > space[j] && j > 0) {
      std::swap((space[j]),space[j-1]);
      j--;
    }
  }

  #if DEBUG
    std::cerr << "Original\n";
    for (auto element : bucket) {
      std::cerr << element << " ";
    }
    std::cerr << std::endl;

    std::cerr << "Checking if sorted\n";
    for (auto element : space) {
      std::cerr << element << " ";
    }
    std::cerr << std::endl;
  #endif

  // copy data back into original range
  auto i = 0;
  for (auto element = std::begin(bucket); element < std::end(bucket); ++element) {
    *element = space[i];
    ++i;
  }
}

}  // namespace ae