algorithm-engineering/src/sorter.cpp

#include "sorter.hpp"

#include "single_task_handler.hpp"
#include "thread_pool.hpp"

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

#define DEBUG false

namespace ae {

sorter::sorter(uint32_t num = 1) {
  sorter::num_threads = num;
  if (num > 1) {
    sorter::handler = (TaskHandler*) new ThreadPool(num);
  } else {
    sorter::handler = (TaskHandler*) new SingleTaskHandler();
  }
}

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

    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);});
  while (sorter::handler->size() > 0 || sorter::handler->isWorking()) {};
  #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_binary( 
  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_binary(std::span<container::element_type> (std::begin(range), lower), passes + 1, bucket_sort);
  sorter::msd_inplace_radix_sort_binary(std::span<container::element_type> (lower, std::end(range)), passes + 1, bucket_sort);
}

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 (range.size() <= sorter::SMALL_SORT_THRESHHOLD) {
    bucket_sort(range);
    return;
  }

  // We first determine the number of elements per bucket
  // This is one pass additional pass over the elements and needs O(buckets) additional space, so in one configuration constant overhead
  uint32_t bucket_sizes[sorter::RADIX_BUCKETS] = { 0 };
  auto upper_bucket_mask = ((1L << sorter::RADIX_SIZE) - 1) << (sizeof(container::element_type) * CHAR_BIT - sorter::RADIX_SIZE * (1 + passes));

  auto mask_bucket = [&](container::element_type* element){ return (*element & upper_bucket_mask) >> (sizeof(container::element_type) * CHAR_BIT - sorter::RADIX_SIZE) * (1 + passes); };

  for (auto element : range) {
    auto bucket = mask_bucket(&element);
    bucket_sizes[bucket]++;
  }
  #if DEBUG
    std::cerr << "Bucket sizes: ";
    for (auto bucket : bucket_sizes) {
      std::cerr << bucket << " ";
    }
    std::cerr << std::endl;
  #endif

  // We now point each bucket to its start location in the range
  container::element_type* buckets_end[sorter::RADIX_BUCKETS];
  container::element_type* buckets_start[sorter::RADIX_BUCKETS];
  #if DEBUG
    std::cerr << "Starting bucket" << std::endl;
  #endif
  auto count = 0;
  for (int i = 0; i < sorter::RADIX_BUCKETS; ++i) {
    buckets_end[i] = &range[count];
    buckets_start[i] = &range[count];
    #if DEBUG
      std::cerr << "bucket " << i << " at " << count << std::endl;
    #endif
    count += bucket_sizes[i];
  }
  #if DEBUG
    std::cerr << "finish" << std::endl;
  #endif
  // Loop over the elements and swap them into the correct buckets.
  // This will look at each element exactly once.
  auto element = &range[0];
  while (element < &*std::end(range)) {
    uint32_t bucket = mask_bucket(element);

    // Check if we are currently in the bounds of the corresponding bucket
    if (&*element >= buckets_start[bucket] && &*element < buckets_end[bucket]) {
      // The element is in the correct bucket, we skip to the end of the bucket
      element = buckets_end[bucket];
    } else {
      // The element is not in the correct bucket; swap
      std::swap(*element, *buckets_end[bucket]);
      buckets_end[bucket]++;
    }
  }
  #if DEBUG
    for (int i = 0; i < range.size(); i++) {
      std::cerr << i << " reordered:" << range[i] << std::endl;
    }
    std::cerr << "Finish reordering elements" << std::endl;
    std::cerr << "Bucket elements at begin of bucket" << std::endl;
    for (auto bucket : buckets_start) {
      std::cerr << *bucket << " bucket " << mask_bucket(bucket) << std::endl;
    }
    std::cerr << std::endl;
  #endif

  for (auto i = 0; i < sorter::RADIX_BUCKETS - 1; ++i) {
    assert(buckets_end[i] == buckets_start[i + 1]);
  }
  assert(buckets_end[sorter::RADIX_BUCKETS - 1] == &*std::end(range));
  #if DEBUG
    std::cerr << "Ranges of buckets are correct" << std::endl;
  #endif

  // sort each bucket recursively
  for (auto i = 0; i < sorter::RADIX_BUCKETS; i++) {
    #if DEBUG
      std::cerr << "Putting in task with depth " << passes << " of bucket " << i << std::endl;
    #endif
    auto start = buckets_start[i];
    auto end = buckets_end[i];
    sorter::handler->add([start, end, &bucket_sort, passes, this, i](){
      #if DEBUG
        std::cerr << "Starting task with depth " << passes << " of bucket " << i << std::endl;
      #endif
      sorter::msd_inplace_radix_sort(std::span<container::element_type> (start, end), passes + 1, bucket_sort);
      #if DEBUG
        std::cerr << "Finishing task with depth " << passes << " of bucket " << i << std::endl;
      #endif
    });
  }
}

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