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/* Copyright (c) 2012, 2025, Oracle and/or its affiliates.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License, version 2.0,
as published by the Free Software Foundation.
This program is designed to work with certain software (including
but not limited to OpenSSL) that is licensed under separate terms,
as designated in a particular file or component or in included license
documentation. The authors of MySQL hereby grant you an additional
permission to link the program and your derivative works with the
separately licensed software that they have either included with
the program or referenced in the documentation.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License, version 2.0, for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*/
#ifndef MERGE_SORT_INCLUDED
#define MERGE_SORT_INCLUDED
/**
@file
@brief
Merge sort and insert sort implementations. These sorting functions
are primarily intended for sorting of JOIN_TABs before the greedy
search algorithm is applied. Since the JOIN_TAB comparison functions
(Join_tab_compare*) are not transitive, the resulting order depends
on the sorting implementation to a certain degree.
Since the std::stable_sort and std::sort implementations differ
between platforms, the result of sorting JOIN_TABs may also differ.
In turn, the query execution plan would differ between platforms and
that is a problem with mtr tests (EXPLAIN output would vary).
If you intend to sort something transitive (which means almost
everything except JOIN_TABs) you should most likely use one of the
std sorting functions instead of this.
*/
#include <assert.h>
#include <queue>
/**
Sorts the elements in the range [first,last) into ascending order
using insertion sort.
@param first First element in an array of pointers to be sorted
@param last Element after the last element in an array of pointers
to be sorted
@param comp Comparison function object that, taking two pointers
of the same type as those contained in the range,
returns true if the first argument goes before the
second argument in the specific strict weak ordering
it defines, and false otherwise.
In our case comp should be a function object with an operator:
bool operator()(Element_type*, Element_type*)
*/
template <typename Element_type, typename Comp_func>
void insert_sort(Element_type **first, Element_type **last, Comp_func comp) {
for (Element_type **high_water_mark = first + 1; high_water_mark < last;
high_water_mark++) {
for (Element_type **cur = high_water_mark; cur > first; cur--) {
if (comp(*(cur - 1), *cur)) break;
Element_type *tmp = *(cur - 1);
*(cur - 1) = *cur;
*cur = tmp;
}
}
}
/**
Sorts the elements in the range [first,last) into ascending order
using merge sort.
@param first First element in an array of pointers to be sorted
@param last Element after the last element in an array of pointers
to be sorted
@param comp Comparison function object that, taking two pointers
of the same type as those contained in the range,
returns true if the first argument goes before the
second argument in the specific strict weak ordering
it defines, and false otherwise.
In our case comp should be a function object with an operator:
bool operator()(Element_type*, Element_type*)
*/
template <typename Element_type, typename Comp_func>
void merge_sort(Element_type **first, Element_type **last, Comp_func comp) {
const uint elements = static_cast<uint>(last - first);
/*
Tests showed that the value 5 was a good number for JOIN_TAB
ordering, which is the primary use case for this function
*/
if (elements < 5) {
insert_sort(first, last, comp);
return;
}
Element_type **middle = first + (elements) / 2;
merge_sort(first, middle, comp);
merge_sort(middle, last, comp);
std::queue<Element_type *> merged;
Element_type **cur1 = first;
Element_type **cur2 = middle;
for (uint i = 0; i < elements; i++) {
assert(cur1 < middle || cur2 < last);
if (cur1 == middle)
merged.push(*cur2++);
else if (cur2 == last)
merged.push(*cur1++);
else if (comp(*cur1, *cur2))
merged.push(*cur1++);
else
merged.push(*cur2++);
}
Element_type **result = first;
while (!merged.empty()) {
*result++ = merged.front();
merged.pop();
}
}
#endif /* MERGE_SORT_INCLUDED */
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