mirror of https://github.com/bsnes-emu/bsnes.git
83 lines
2.4 KiB
C++
Executable File
83 lines
2.4 KiB
C++
Executable File
#pragma once
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#include <algorithm>
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#include <nall/utility.hpp>
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//class: merge sort
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//average: O(n log n)
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//worst: O(n log n)
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//memory: O(n)
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//stack: O(log n)
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//stable?: yes
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//note: merge sort was chosen over quick sort, because:
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//* it is a stable sort
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//* it lacks O(n^2) worst-case overhead
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//* it usually runs faster than quick sort anyway
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//note: insertion sort is generally more performant than selection sort
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#define NALL_MERGE_SORT_INSERTION
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//#define NALL_MERGE_SORT_SELECTION
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namespace nall {
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template<typename T, typename Comparator> auto sort(T list[], uint size, const Comparator& lessthan) -> void {
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if(size <= 1) return; //nothing to sort
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//sort smaller blocks using an O(n^2) algorithm (which for small sizes, increases performance)
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if(size < 64) {
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//insertion sort requires a copy (via move construction)
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#if defined(NALL_MERGE_SORT_INSERTION)
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for(int i = 1, j; i < size; i++) {
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T copy(move(list[i]));
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for(j = i - 1; j >= 0; j--) {
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if(!lessthan(copy, list[j])) break;
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list[j + 1] = move(list[j]);
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}
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list[j + 1] = move(copy);
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}
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//selection sort requires a swap
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#elif defined(NALL_MERGE_SORT_SELECTION)
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for(uint i = 0; i < size; i++) {
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uint min = i;
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for(uint j = i + 1; j < size; j++) {
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if(lessthan(list[j], list[min])) min = j;
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}
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if(min != i) swap(list[i], list[min]);
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}
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#endif
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return;
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}
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//split list in half and recursively sort both
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uint middle = size / 2;
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sort(list, middle, lessthan);
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sort(list + middle, size - middle, lessthan);
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//left and right are sorted here; perform merge sort
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//use placement new to avoid needing T to be default-constructable
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auto buffer = memory::allocate<T>(size);
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uint offset = 0, left = 0, right = middle;
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while(left < middle && right < size) {
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if(!lessthan(list[right], list[left])) {
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new(buffer + offset++) T(move(list[left++]));
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} else {
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new(buffer + offset++) T(move(list[right++]));
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}
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}
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while(left < middle) new(buffer + offset++) T(move(list[left++]));
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while(right < size ) new(buffer + offset++) T(move(list[right++]));
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for(uint i = 0; i < size; i++) {
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list[i] = move(buffer[i]);
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buffer[i].~T();
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}
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memory::free(buffer);
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}
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template<typename T> auto sort(T list[], uint size) -> void {
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return sort(list, size, [](const T& l, const T& r) { return l < r; });
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}
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}
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