// Chooses the correct edge to follow given possible options in the cycle joining phase
int Graph::selectEdge(std::vector<int> & connections)
{
std::vector<int> firstPri;
std::vector<int> secondPri;
std::vector<int> thirdPri;
for(auto&& edgeIndex : connections)
{
auto edge = edges[edgeIndex];
if (!inStack(edge.cycleID))
{
firstPri.push_back(edgeIndex); // First priority: Return unused edge from a new cycle
}
if (edge.cycleID == cycleStack.end()[-1])
{
secondPri.push_back(edgeIndex); // Second priority: Return unused edge from current cycle
}
if (edge.cycleID == cycleStack.end()[-2])
{
thirdPri.push_back(edgeIndex); // Third priority: Return unused edge from previous cycle
}
}
if (!firstPri.empty()) return firstPri[0];
else if (!secondPri.empty()) return secondPri[0];
else if (!thirdPri.empty()) return thirdPri[0];
output << "CRITICAL FAILURE :(" << std::endl;
return -1;
}
// Manages the cycle stack
void Graph::updateCycle(int edgeIndex, int toNode)
{
auto edge = edges[edgeIndex];
int fromNode = edge.getOtherEndpoint(toNode);
if (!inStack(edge.cycleID)) // if current cycle is not on the stack
{
cycleStack.push_back(edge.cycleID);
cycleStartNodes[edge.cycleID] = fromNode;
}
if (toNode == cycleStartNodes[edge.cycleID]) // If the cycle is finished
{
cycleStack.pop_back();
}
}
bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
if (prerequisites.empty() || numCourses == 0) return true;
// build adjacency list
vector<unordered_set<int>> adj(numCourses);
for (int i = 0; i < prerequisites.size(); i++) {
// e.second -> e.first, i.e. e.second is a prerequisite for e.first
pair<int, int> e = prerequisites[i];
adj[e.second].insert(e.first);
}
// detect directed cycle in adj
vector<bool> visited(numCourses, false);
vector<bool> inStack(numCourses, false);
for (int i = 0; i < numCourses; i++) {
if (!visited[i]) {
if (dfs(adj, i, visited, inStack)) {
// a cycle exists: cannot finish
return false;
}
}
}
// no cycle detected: can finish
return true;
}