CGI/exercise2/include/sample_set.h

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2018-09-06 12:35:43 +00:00
//
// This source code is property of the Computer Graphics and Visualization
// chair of the TU Dresden. Do not distribute!
// Copyright (C) 2015-2017 CGV TU Dresden - All Rights Reserved
//
#pragma once
#include <vector>
#include <unordered_map>
#include <random>
/*
Usage:
//C++0x Random Engine Mersenne Twister
std::mt19937 eng;
//a set data structure with amortize constant time insertion, removal, and uniform random sampling from its elements
sample_set<int> my_set;
my_set.reserve(3);
my_set.insert(1);
my_set.insert(2);
my_set.insert(6);
int rand_elem1 = my_set.sample(eng);//1,2 or 6
my_set.remove(2);
int rand_elem2 = my_set.sample(eng); //1 or 6
*/
template <typename T>
struct sample_set
{
typedef int element_index;
std::vector<T> elements;
std::unordered_map<T,element_index> index_lut;
//create empty set
sample_set(){}
//reserve memory for n elements
void reserve(size_t n)
{
elements.reserve(n);
index_lut.rehash(n);
}
//insert element elem into set
void insert(const T& elem)
{
//guard against duplicates
if (index_lut.find(elem) == index_lut.end())
{
element_index idx = (element_index)elements.size();
elements.push_back(elem);
index_lut[elem]=idx;
}
}
//remove element elem from set
bool remove(const T& elem)
{
auto it = index_lut.find(elem);
if(it == index_lut.end())
{
return false;
}
int i = it->second;
std::swap(elements[i],elements.back());
elements.pop_back();
index_lut.erase(it);
if(unsigned(i) < elements.size())
{
index_lut[elements[i]]=i;
}
return true;
}
//draw a sample from set
template <typename Engine>
const T& sample(Engine& eng)
{
int b = (element_index)(elements.size()-1);
std::uniform_int_distribution<int> uniform_dist(0,b);
element_index idx = uniform_dist(eng);
return elements[idx];
}
//returns number of elements in set
size_t size() const
{
return elements.size();
}
//returns true if set is empty
bool empty() const
{
return elements.empty();
}
};