Has anyone seen a 2-Sat implementation
I have been looking for a while, but I just can't seem to find any implementation of the 2-Sat algorithm.
I am working in c++ with the boost library (which h开发者_如何学编程as a strongly connected component module) and need some guidance to either create an efficient 2-Sat program or find an existing library for me to utilise through c++.
I suppose you know how to model a 2-Sat problem to solve it with SCC. The way I handle vars and its negation isn't very elegant, but allows a short implementation: Given n variables numbered from 0 to n-1, in the clauses -i means the negation of variable i, and in the graph i+n means the same (am I clear ?)
#include <boost/config.hpp>
#include <iostream>
#include <vector>
#include <boost/graph/strong_components.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/foreach.hpp>
typedef std::pair<int, int> clause;
//Properties of our graph. By default oriented graph
typedef boost::adjacency_list<> Graph;
const int nb_vars = 5;
int var_to_node(int var)
{
if(var < 0)
return (-var + nb_vars);
else
return var;
}
int main(int argc, char ** argv)
{
std::vector<clause> clauses;
clauses.push_back(clause(1,2));
clauses.push_back(clause(2,-4));
clauses.push_back(clause(1,4));
clauses.push_back(clause(1,3));
clauses.push_back(clause(-2,4));
//Creates a graph with twice as many nodes as variables
Graph g(nb_vars * 2);
//Let's add all the edges
BOOST_FOREACH(clause c, clauses)
{
int v1 = c.first;
int v2 = c.second;
boost::add_edge(
var_to_node(-v1),
var_to_node(v2),
g);
boost::add_edge(
var_to_node(-v2),
var_to_node(v1),
g);
}
// Every node will belong to a strongly connected component
std::vector<int> component(num_vertices(g));
std::cout << strong_components(g, &component[0]) << std::endl;
// Let's check if there is variable having it's negation
// in the same SCC
bool satisfied = true;
for(int i=0; i<nb_vars; i++)
{
if(component[i] == component[i+nb_vars])
satisfied = false;
}
if(satisfied)
std::cout << "Satisfied!" << std::endl;
else
std::cout << "Not satisfied!" << std::endl;
}
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