gistfile1.txt

template <typename Map>
concept is_associative = requires(Map& map, Map::key_type& k) {
    typename Map::key_type;
    typename Map::mapped_type;
    typename Map::value_type;
    map[k];
};


template <typename Graph>
struct NodeIndexFactory {
    constexpr auto operator()() {
        if constexpr (is_associative<Graph>) {
            using adj_nodes = Graph::mapped_type;
            return rng::range_value_t<adj_nodes>();
        } else {
            using adj_nodes = rng::range_value_t<Graph>;
            return rng::range_value_t<adj_nodes>();
        }
    }
};

template <class Graph>
using node_index_t = decltype(NodeIndexFactory<Graph>()());

template <typename G, typename NodeId = node_index_t<G>>
concept Graph = requires(G& g, NodeId& n) {
    // Note: A Graph is a structure to model pairwise relations between objects (nodes) -- wiki
    // This Graph concept has the bare minimum functionality for an adjacency graph
    // Specifically, graphs only store the pairwise relations for each node and don't require
    // maintenance functions (e.g. constructing or maintaining edges)
    // A graph is random access range (supports []) with value_type of a range with a value_type that can be
    requires rng::range<G>;
    requires rng::range<decltype(g[n])>;          // g[i] returns a range
    { *g[n].begin() } -> convertible_to<NodeId>;  // node_index type can be used as an index into the graph
};