word ladder 8 char max

typedef int word_id;
typedef unordered_multimap<uint64_t, word_id> graph_t;

void build_graph(graph_t & G, vector<string> words)
{
    uint64_t basket = 0;
    for(int w_id = 0; w_id < words.size(); ++w_id)
    {
        const string & S = words[i];
        const size_t L = S.length();

        assert(L <= 8);

        uint64_t word = *((uint64_t*)S.c_str());
        for(int ch = 0; ch < L; ++ch)
        {
            uint64_t mask = &(0xFF << (ch * 8));
            basket = mask & word;

            G.insert(basket, w_id);
        }
    }
}