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Update simple-graph implementation to track discovery and finish time 

+ Allows result of topological sort to match examples shown in MIT Algorithms
+ Correct order of initialization for all graphs and adjacent nodes in graph.cpp
+ Provide overloaded DFS for beginning at a specific node within the graph
This commit is contained in:
Shaun Reed 2021-07-10 13:13:50 -04:00
parent 3d0dfa63d1
commit 166d998508
3 changed files with 155 additions and 62 deletions
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112
cpp/algorithms/graphs/simple/graph.cpp
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@ -1,10 +1,10 @@
/*#############################################################################
## Author: Shaun Reed ##
## Legal: All Content (c) 2021 Shaun Reed, all rights reserved ##
## About: Driver program to test a simple graph implementation ##
## ##
## Contact: shaunrd0@gmail.com | URL: www.shaunreed.com | GitHub: shaunrd0 ##
###############################################################################
/*##############################################################################
## Author: Shaun Reed ##
## Legal: All Content (c) 2021 Shaun Reed, all rights reserved ##
## About: Driver program to test a simple graph implementation ##
## ##
## Contact: shaunrd0@gmail.com | URL: www.shaunreed.com | GitHub: shaunrd0 ##
################################################################################
*/
#include "lib-graph.hpp"
@ -13,17 +13,17 @@
int main (const int argc, const char * argv[])
{
// We could initialize the graph with some localNodes...
std::map<int, std::set<int>> localNodes{
{1, {2, 5}}, // Node 1
{2, {1, 6}}, // Node 2
{3, {4, 6, 7}},
{4, {3, 7, 8}},
std::unordered_map<int, std::unordered_set<int>> localNodes{
{8, {6, 4}},
{7, {8, 6, 4, 3}},
{6, {7, 3, 2}},
{5, {1}},
{6, {2, 3, 7}},
{7, {3, 4, 6, 8}},
{8, {4, 6}},
{4, {8, 7, 3}},
{3, {7, 6, 4}},
{2, {6, 1}}, // Node 2...
{1, {5, 2}}, // Node 1
};
// Graph bfsGraph(localNodes);
Graph exampleGraph(localNodes);
std::cout << "\n\n##### Breadth First Search #####\n";
@ -31,14 +31,14 @@ int main (const int argc, const char * argv[])
// Initialize a example graph for Breadth First Search
Graph bfsGraph (
{
{1, {2, 5}}, // Node 1
{2, {1, 6}}, // Node 2...
{3, {4, 6, 7}},
{4, {3, 7, 8}},
{8, {6, 4}},
{7, {8, 6, 4, 3}},
{6, {7, 3, 2}},
{5, {1}},
{6, {2, 3, 7}},
{7, {3, 4, 6, 8}},
{8, {4, 6}},
{4, {8, 7, 3}},
{3, {7, 6, 4}},
{2, {6, 1}}, // Node 2...
{1, {5, 2}}, // Node 1
}
);
// The graph traversed in this example is seen in MIT Intro to Algorithms
@ -50,12 +50,12 @@ int main (const int argc, const char * argv[])
// Initialize an example graph for Depth First Search
Graph dfsGraph (
{
{1, {2, 4}},
{2, {5}},
{3, {5, 6}},
{4, {2}},
{5, {4}},
{6, {6}},
{5, {4}},
{4, {2}},
{3, {6, 5}},
{2, {5}},
{1, {4, 2}},
}
);
// The graph traversed in this example is seen in MIT Intro to Algorithms
@ -67,32 +67,62 @@ int main (const int argc, const char * argv[])
// Initialize an example graph for Topological Sort
Graph topologicalGraph (
{
{1, {4, 5}},
{2, {5}},
{3, {}},
{4, {5, 7}},
{5, {}},
{6, {7, 8}},
{7, {9}},
{8, {9}},
{9, {}},
{8, {9}},
{7, {9}},
{6, {7, 8}},
{5, {}},
{4, {7, 5}},
{3, {}},
{2, {5}},
{1, {5, 4}},
}
);
auto order = topologicalGraph.TopologicalSort(topologicalGraph.GetNode(6));
std::cout << "\nTopological order: ";
while (!order.empty()) {
std::cout << order.back() << " ";
order.pop_back();
}
std::cout << std::endl << std::endl;
// If we want the topological order to match what is seen in the book
// + We have to initialize the graph carefully to get this result -
// Because this is an unordered_(map/set) initialization is reversed
// + So the order of nodes on the container below is 6,7,8,9,3,1,4,5,2
// + The same concept applies to their adjacent nodes (7,8 initializes to 8,7)
// + In object-graph implementation, I use vectors this does not apply there
Graph topologicalGraph2 (
{
{2, {5}}, // socks
{5, {}}, // shoes
{4, {7, 5}}, // pants
{1, {5, 4}}, // undershorts
{3, {}}, // watch
{9, {}}, // jacket
{7, {9}}, // belt
{8, {9}}, // tie
{6, {7, 8}}, // shirt
}
);
// The graph traversed in this example is seen in MIT Intro to Algorithms
// + Chapter 22, Figure 22.7 on Topological Sort
// + Each node was replaced with a value from left-to-right, top-to-bottom
// + Undershorts = 1, Socks = 2, Watch = 3, Pants = 4, etc...
std::vector<int> order = topologicalGraph.TopologicalSort();
std::vector<int> order2 =
topologicalGraph2.TopologicalSort(topologicalGraph2.NodeBegin());
// Because this is a simple graph with no objects to store finishing time
// + The result is only one example of valid topological order
// + There are other valid orders; Final result differs from one in the book
std::cout << "\n\nTopological order: ";
while (!order.empty()) {
std::cout << order.back() << " ";
order.pop_back();
std::cout << "\nTopological order: ";
while (!order2.empty()) {
std::cout << order2.back() << " ";
order2.pop_back();
}
std::cout << std::endl;
std::cout << std::endl;
return 0;
}

71
cpp/algorithms/graphs/simple/lib-graph.cpp
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@ -8,6 +8,7 @@
################################################################################
*/
#include <algorithm>
#include "lib-graph.hpp"
@ -55,6 +56,7 @@ void Graph::DFS()
{
// Track the nodes we have discovered
std::vector<bool> discovered(nodes_.size(), false);
int time = 0;
// Visit each node in the graph
for (const auto &node : nodes_) {
@ -65,14 +67,55 @@ void Graph::DFS()
// Mark the node as visited so we don't visit it twice
discovered[node.first - 1] = true;
// Visiting the undiscovered node will check it's adjacent nodes
DFSVisit(node.first, discovered);
DFSVisit(time, node.first, discovered);
}
}
}
void Graph::DFSVisit(int startNode, std::vector<bool> &discovered)
void Graph::DFS(Node::iterator startIter)
{
// Track the nodes we have discovered
std::vector<bool> discovered(nodes_.size(), false);
int time = 0;
auto startNode = GetNode(startIter->first);
// beginning at startNode, visit each node in the graph until we reach the end
while (startIter != nodes_.end()) {
std::cout << "Visiting node " << startIter->first << std::endl;
// If the startIter is undiscovered, visit it
if (!discovered[startIter->first - 1]) {
std::cout << "Found undiscovered node: " << startIter->first << std::endl;
// Visiting the undiscovered node will check it's adjacent nodes
discovered[startIter->first - 1] = true;
DFSVisit(time, startIter->first, discovered);
}
startIter++;
}
// Once we reach the last node, check the beginning for unchecked nodes
startIter = nodes_.begin();
// Once we reach the initial startNode, we have checked all nodes
while (startIter->first != startNode->first) {
std::cout << "Visiting node " << startIter->first << std::endl;
// If the startIter is undiscovered, visit it
if (!discovered[startIter->first - 1]) {
std::cout << "Found undiscovered node: " << startIter->first << std::endl;
// Visiting the undiscovered node will check it's adjacent nodes
discovered[startIter->first - 1] = true;
DFSVisit(time, startIter->first, discovered);
}
startIter++;
}
}
void Graph::DFSVisit(int &time, int startNode, std::vector<bool> &discovered)
{
time++;
discoveryTime[startNode - 1] = std::make_pair(startNode, time);
// Check the adjacent nodes of the startNode
// + Do not offset startNode by 1, since we use it as a key to a map
for (auto &adjacent : nodes_[startNode]) {
@ -83,31 +126,25 @@ void Graph::DFSVisit(int startNode, std::vector<bool> &discovered)
discovered[adjacent - 1] = true;
// Visiting the undiscovered node will check it's adjacent nodes
DFSVisit(adjacent, discovered);
DFSVisit(time, adjacent, discovered);
}
}
time++;
finishTime[startNode - 1] = std::make_pair(startNode, time);
}
std::vector<int> Graph::TopologicalSort()
std::vector<int> Graph::TopologicalSort(Node::iterator startNode)
{
DFS(startNode);
std::vector<int> topologicalOrder;
// Track the nodes we have discovered
std::vector<bool> discovered(nodes_.size(), false);
std::vector<std::pair<int, int>> finishOrder(finishTime);
// Visit each node in the graph
for (const auto &node : nodes_) {
std::cout << "Visiting node " << node.first << std::endl;
// If the node is undiscovered, visit it
// + Offset by 1 to account for 0 index of discovered vector
if (!discovered[node.first - 1]) {
std::cout << "Found undiscovered node: " << node.first << std::endl;
std::sort(finishOrder.begin(), finishOrder.end(), Graph::FinishedSort);
// Visiting the undiscovered node will check it's adjacent nodes
TopologicalVisit(node.first, discovered, topologicalOrder);
}
}
for (const auto &node : finishOrder) topologicalOrder.push_back(node.first);
// The topologicalOrder is read right-to-left in the final result
// + Output is handled in main as FILO, similar to a stack

34
cpp/algorithms/graphs/simple/lib-graph.hpp
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@ -15,22 +15,48 @@
#include <queue>
#include <set>
#include <vector>
#include <unordered_map>
#include <unordered_set>
class Graph {
public:
explicit Graph(std::map<int, std::set<int>> nodes) : nodes_(std::move(nodes)) {}
std::map<int, std::set<int>> nodes_;
using Node = std::unordered_map<int, std::unordered_set<int>>;
explicit Graph(Node nodes) : nodes_(std::move(nodes))
{
discoveryTime.resize(nodes_.size());
finishTime.resize(nodes_.size(), std::make_pair(0,0));
}
void BFS(int startNode);
void DFS();
void DFSVisit(int startNode, std::vector<bool> &discovered);
void DFS(Node::iterator startNode);
void DFSVisit(int &time, int startNode, std::vector<bool> &discovered);
std::vector<int> TopologicalSort();
std::vector<int> TopologicalSort(Node::iterator startNode);
void TopologicalVisit(
int startNode, std::vector<bool> &discovered, std::vector<int> &order
);
// Define a comparator for std::sort
// + This will help to sort nodes by finished time after traversal
static bool FinishedSort(std::pair<int, int> &node1, decltype(node1) &node2)
{ return node1.second < node2.second;}
inline Node::iterator NodeBegin() { return nodes_.begin();}
// A non-const accessor for direct access to a node with the number value i
inline Node::iterator GetNode(int i) { return nodes_.find(i);}
private:
// Unordered to avoid container reorganizing elements
// + Since this would alter the order nodes are traversed in
Node nodes_;
// Where the first element in the following two pairs is the node number
// And the second element is the discovery / finish time
std::vector<std::pair<int, int>> discoveryTime;
std::vector<std::pair<int, int>> finishTime;
};
#endif // LIB_GRAPH_HPP