bsnes/nall/merge-sort.hpp

83 lines
2.4 KiB
C++
Executable File

#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
//* it usually runs faster than quick sort anyway
//note: insertion sort is generally more performant than selection sort
#define NALL_MERGE_SORT_INSERTION
//#define NALL_MERGE_SORT_SELECTION
namespace nall {
template<typename T, typename Comparator> auto sort(T list[], uint size, const Comparator& lessthan) -> void {
if(size <= 1) return; //nothing to sort
//sort smaller blocks using an O(n^2) algorithm (which for small sizes, increases performance)
if(size < 64) {
//insertion sort requires a copy (via move construction)
#if defined(NALL_MERGE_SORT_INSERTION)
for(int i = 1, j; i < size; i++) {
T copy(move(list[i]));
for(j = i - 1; j >= 0; j--) {
if(!lessthan(copy, list[j])) break;
list[j + 1] = move(list[j]);
}
list[j + 1] = move(copy);
}
//selection sort requires a swap
#elif defined(NALL_MERGE_SORT_SELECTION)
for(uint i = 0; i < size; i++) {
uint min = i;
for(uint j = i + 1; j < size; j++) {
if(lessthan(list[j], list[min])) min = j;
}
if(min != i) swap(list[i], list[min]);
}
#endif
return;
}
//split list in half and recursively sort both
uint middle = size / 2;
sort(list, middle, lessthan);
sort(list + middle, size - middle, lessthan);
//left and right are sorted here; perform merge sort
//use placement new to avoid needing T to be default-constructable
auto buffer = memory::allocate<T>(size);
uint offset = 0, left = 0, right = middle;
while(left < middle && right < size) {
if(!lessthan(list[right], list[left])) {
new(buffer + offset++) T(move(list[left++]));
} else {
new(buffer + offset++) T(move(list[right++]));
}
}
while(left < middle) new(buffer + offset++) T(move(list[left++]));
while(right < size ) new(buffer + offset++) T(move(list[right++]));
for(uint i = 0; i < size; i++) {
list[i] = move(buffer[i]);
buffer[i].~T();
}
memory::free(buffer);
}
template<typename T> auto sort(T list[], uint size) -> void {
return sort(list, size, [](const T& l, const T& r) { return l < r; });
}
}