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