bsnes/nall/sort.hpp

#pragma once

#include <algorithm>
#include <nall/utility.hpp>

//class:   merge sort
//average: O(n log n)
//worst:   O(n log n)
//memory:  O(n)
//stack:   O(log n)
//stable?: yes

//note: merge sort was chosen over quick sort, because:
//* it is a stable sort
//* it lacks O(n^2) worst-case overhead

#define NALL_SORT_INSERTION
//#define NALL_SORT_SELECTION

namespace nall {

template<typename T, typename Comparator> auto sort(T list[], unsigned size, const Comparator& lessthan) -> void {
  if(size <= 1) return;  //nothing to sort

  //use insertion sort to quickly sort smaller blocks
  if(size < 64) {
    #if defined(NALL_SORT_INSERTION)
    for(signed i = 1, j; i < size; i++) {
      T copy = std::move(list[i]);
      for(j = i - 1; j >= 0; j--) {
        if(!lessthan(copy, list[j])) break;
        list[j + 1] = std::move(list[j]);
      }
      list[j + 1] = std::move(copy);
    }
    #elif defined(NALL_SORT_SELECTION)
    for(unsigned i = 0; i < size; i++) {
      unsigned min = i;
      for(unsigned j = i + 1; j < size; j++) {
        if(lessthan(list[j], list[min])) min = j;
      }
      if(min != i) std::swap(list[i], list[min]);
    }
    #endif
    return;
  }

  //split list in half and recursively sort both
  unsigned middle = size / 2;
  sort(list, middle, lessthan);
  sort(list + middle, size - middle, lessthan);

  //left and right are sorted here; perform merge sort
  T* buffer = new T[size];
  unsigned offset = 0, left = 0, right = middle;
  while(left < middle && right < size) {
    if(!lessthan(list[right], list[left])) {
      buffer[offset++] = std::move(list[left++]);
    } else {
      buffer[offset++] = std::move(list[right++]);
    }
  }
  while(left < middle) buffer[offset++] = std::move(list[left++]);
  while(right < size)  buffer[offset++] = std::move(list[right++]);

  for(unsigned i = 0; i < size; i++) list[i] = std::move(buffer[i]);
  delete[] buffer;
}

template<typename T> auto sort(T list[], unsigned size) -> void {
  return sort(list, size, [](const T& l, const T& r) { return l < r; });
}

}