adjacency_list.h

#pragma once
 
#include <numeric>
#include <queue>
#include <utility>
#include <vector>
 
#include <assert.h>
 
namespace cgv{
namespace math{
 
struct edge
{
    size_t start = 0;
    size_t end = 0;
};
 
template<typename T>
struct weighted_edge : public edge
{
    using weight_type = T;
 
    T weight = {};
};
 
template<typename EdgeT>
struct vertex
{
    using edge_type = EdgeT;
 
    std::vector<edge_type> edges;
};
 
enum class EdgeOrientation {
    Undirected, // the graph contains only undirected edges
    Directed,   // the graph contains only directed edges
};
 
template<typename VertexT>
class adjacency_list {
public:
    using vertex_type = VertexT;
    using edge_type = typename VertexT::edge_type;
 
    adjacency_list(EdgeOrientation edge_orientation = EdgeOrientation::Undirected) : _edge_orientation(edge_orientation) {}
 
    adjacency_list(size_t vertex_count, EdgeOrientation edge_orientation = EdgeOrientation::Undirected) : _edge_orientation(edge_orientation) {
        _vertices.resize(vertex_count);
    }
 
    void resize(size_t vertex_count) {
        remove_all_edges();
        _vertices.resize(vertex_count);
    }
 
    bool is_directed() const {
        return _edge_orientation == EdgeOrientation::Directed;
    }
 
    void clear() {
        _vertices.clear();
    }
 
    bool empty() const {
        return _vertices.empty();
    }
 
    size_t vertex_count() const {
        return _vertices.size();
    }
 
    size_t edge_count() const {
        size_t count = std::accumulate(_vertices.begin(), _vertices.end(), 0, [](const vertex_type& vertex, size_t count) { return count + vertex.edges.size(); });
        // Undirected edges are stored twice so we need to divide the count by 2
        if(!is_directed())
            return count / 2;
        return count;
    }
 
    void remove_all_edges() {
        std::for_each(_vertices.begin(), _vertices.end(), [](vertex_type& vertex) { vertex.edges.clear(); });
    }
 
    vertex_type& vertex(size_t i) {
        return _vertices[i];
    }
 
    const vertex_type& vertex(size_t i) const {
        return _vertices[i];
    }
 
    const std::vector<edge_type> to_edge_list() const {
        std::vector<edge_type> edges;
        for(const auto& vertex : _vertices)
            std::copy(vertex.edges.begin(), vertex.edges.end(), std::inserter(edges, edges.end()));
        return edges;
    }
 
    size_t add_vertex(const vertex_type& vertex) {
        _vertices.push_back(vertex);
        return _vertices.size() - 1;
    }
 
    bool add_edge(const edge_type& edge) {
        if(edge_exists(edge.start, edge.end))
            return false;
 
        if(edge.start < vertex_count() && edge.end < vertex_count()) {
            vertex(edge.start).edges.push_back(edge);
            if(!is_directed()) {
                // build a reverse edg by copying the new edge and its properties and swapping its start and end indices
                edge_type reverse_edge = edge;
                std::swap(reverse_edge.start, reverse_edge.end);
                vertex(edge.end).edges.push_back(reverse_edge);
            }
            return true;
        }
        return false;
    }
 
    bool add_edge(size_t start, size_t end) {
        return add_edge({ start, end });
    }
 
    bool edge_exists(size_t start, size_t end) const {
        for(const edge_type& edge : vertex(start).edges) {
            if(edge.end == end)
                return true;
        }
        return false;
    }
 
    bool is_cyclic() const {
        if(is_directed()) {
            // If a directed graph cannot be sorted topologically it contains a cycle
            return !topological_sort_impl();
        } else {
            // Keep track of visited vertices
            std::vector<bool> visited(vertex_count(), false);
 
            // Perform BFS from every unvisited node
            for(size_t i = 0; i < vertex_count(); ++i) {
                if(!visited[i]) {
                    // If BFS finds a cycle
                    if(has_cycle_undirected_breadth_first(i, visited))
                        return true;
                }
            }
 
            // If no cycle is found in any component
            return false;
        }
    }
 
    // return the graph vertex indices in topological order; only works for directed graphs; if the graph is undirected or contains a cycle an empty list is returned
    std::vector<size_t> topological_sort() const {
        std::vector<size_t> vertices;
        if(!topological_sort_impl(&vertices))
            return {};
        return vertices;
    }
 
private:
    // search for a cycle from a given start vertex in an undirected graph and return true if the graph contains a cycle
    bool has_cycle_undirected_breadth_first(size_t start, std::vector<bool>& visited) const {
        // Queue stores { current vertex, parent vertex }
        std::queue<std::pair<size_t, size_t>> edge_queue;
 
        // maker for invalid indices akin to std::string::npos
        constexpr size_t nindex = static_cast<size_t>(-1);
 
        // Start node has no parent
        edge_queue.push({ start, nindex });
        visited[start] = true;
 
        while(!edge_queue.empty()) {
            size_t node = edge_queue.front().first;
            size_t parent = edge_queue.front().second;
            edge_queue.pop();
            
            // Traverse all neighbors of current node
            for(const edge_type& edge : vertex(node).edges) {
 
                // If neighbor is not visited, mark it visited and push to queue
                if(!visited[edge.end]) {
                    visited[edge.end] = true;
                    edge_queue.push({ edge.end, node });
                } else if(edge.end != parent) {
                    // If neighbor is visited and not parent, a cycle is detected
                    return true;
                }
            }
        }
 
        // No cycle found starting from this node
        return false;
    }
 
    // return true if the graph vertices can be sorted in topological order; only works for directed graphs; if the graph is undirected or contains a cycle false is returned;
    // if ordered_vertices is supplied it will be filled with the vertex indices in topological order if the function returned true
    bool topological_sort_impl(std::vector<size_t>* ordered_vertices = nullptr) const {
        if(!is_directed())
            return false;
 
        // Array to store in-degree of each vertex
        std::vector<size_t> in_degree(vertex_count(), 0);
 
        // Compute in-degrees of all vertices
        for(const vertex_type& vertex : _vertices) {
            for(const edge_type& edge : vertex.edges)
                in_degree[edge.end]++;
        }
 
        std::queue<size_t> vertex_queue;
 
        // Add all vertices with in-degree 0 to the queue
        for(size_t i = 0; i < in_degree.size(); ++i) {
            if(in_degree[i] == 0)
                vertex_queue.push(i);
        }
 
        if(ordered_vertices)
            ordered_vertices->reserve(vertex_count());
 
        // Count of visited (processed) nodes
        size_t visited_count = 0;
 
        // Perform breadth first serach (Topological Sort)
        while(!vertex_queue.empty()) {
            size_t i = vertex_queue.front();
            vertex_queue.pop();
            visited_count++;
            if(visited_count > vertex_count())
                return false;
 
            // Add the vertex to the output list
            if(ordered_vertices)
                ordered_vertices->push_back(i);
 
            // Reduce in-degree of neighbors
            for(const auto& edge : vertex(i).edges) {
                in_degree[edge.end]--;
                if(in_degree[edge.end] == 0) {
                    // Add to queue when in-degree becomes 0
                    vertex_queue.push(edge.end);
                }
            }
        }
 
        // If visited nodes != total nodes, a cycle exists
        return visited_count == vertex_count();
    }
 
    std::vector<vertex_type> _vertices;
    EdgeOrientation _edge_orientation = EdgeOrientation::Undirected;
};
 
using graph = adjacency_list<vertex<edge>>;
template<typename T>
using weighted_graph = adjacency_list<vertex<weighted_edge<T>>>;
 
} // namespace math
} // namespace cgv