使用 Boost::graph 随机访问顶点

Question

我正在尝试使用 OpenMP 并行迭代提升图的顶点。这似乎需要一个支持随机访问元素的迭代器（例如，itr[i] 获取ith 元素）。但是，vertices(g) 返回的迭代器（vertex_iterator）似乎不支持这一点。有没有一种高效、干净的方法来实现这一点？理想情况下，我只想要一个标准的循环，例如：

for (int i = 0; i < num_vertices; i++) {
  vertex v = itr[i];
  // Compute on vertex
}

它将与 OpenMP 合作。谢谢！

Answer 1

使用adjacency_list<..., vecS, ...> 或adjacency_matrix 将通过具有整数类型的顶点描述符来实现这一点。

稍微开箱即用，看看Parallel Boost Graph Library（Parallel BGL）。它很可能会做你想要的（以及更多）但更好？

小演示

Live On Coliru

示例输出（在我的系统上）：

Generated 50000000 vertices in 1879ms
Using 8 threads.
Sum of volumes for 50000000 vertices in 94ms: 2.5603e+10

完整列表：

#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/random.hpp>
#include <chrono>
#include <iostream>
#include <omp.h>
#include <random>

static std::mt19937 prng { std::random_device{}() };

struct MyVertex {
    uintmax_t volume = [] { static std::uniform_int_distribution<int> pick(0, 1024); return pick(prng); }();
};

using namespace boost;
using G = adjacency_list<vecS, vecS, directedS, MyVertex>;

G generate() {
    using namespace std::chrono;
    auto start = high_resolution_clock::now();

    G g;
    generate_random_graph(g, 50000000, 0, prng);

    auto end = high_resolution_clock::now();
    std::cerr << "Generated " << num_vertices(g) << " vertices " << "in " << duration_cast<milliseconds>(end-start).count() << "ms\n";

    return g;
}

int main() {

    auto const g = generate();

    using namespace std::chrono;
    auto start = high_resolution_clock::now();
    double sum = 0;
#pragma omp parallel
    {
#pragma omp single
        std::cerr << "Using " << omp_get_num_threads() << " threads.\n";

#pragma omp for reduction(+:sum)
        for (G::vertex_descriptor u = 0; u < num_vertices(g); ++u) {
            sum += g[vertex(u, g)].volume;
        }
    }

    auto end = high_resolution_clock::now();
    std::cerr << "Sum of volumes for " << num_vertices(g)                                << " vertices "
              << "in "                 << duration_cast<milliseconds>(end-start).count() << "ms: " << sum << "\n";
}