/* MPI MST reference implementation. Prim's algorithm */
extern "C" void* MST(graph_t *G) {
trees.clear();
int rank = G->rank, size = G->nproc;
vertex_id_t TotVertices = G->n;
// marked edges are those that lead to vertices already in the tree
vector<uint8_t> marked_edges(G->local_m, 0);
// marked vertices are local edges already in the tree
vector<uint8_t> marked_vertices(G->local_n, 0);
// start with first vertex on first node
vertex_id_t root = 0;
do {
// start a new tree
trees.push_back(vector<edge_id_t>());
// local queue of vertices
vector<vertex_id_t> queue;
// keep track of last added edge to mark edges
vertex_id_t last_vertex = root;
while (true) {
if (VERTEX_OWNER(last_vertex, TotVertices, size) == G->rank) {
// last vertex is ours - put it in the queue and mark
vertex_id_t last_local_vertex = VERTEX_LOCAL(last_vertex, TotVertices, size, rank);
marked_vertices[last_local_vertex] = 1;
queue.push_back(last_local_vertex);
}
// mark edges that lead to the last added vertex
for (edge_id_t j = 0; j < G->local_m; j++) {
if (G->endV[j] == last_vertex) {
marked_edges[j] = 1;
}
}
// determine our best candidate edge
struct {
double weight;
int rank;
edge_id_t edge;
} best;
best.weight = DBL_MAX;
for (vertex_id_t i = 0; i < queue.size(); i++) {
for (edge_id_t j = G->rowsIndices[queue[i]]; j < G->rowsIndices[queue[i] + 1]; j++) {
// skip marked edges
if (!marked_edges[j]) {
// check if this edge is better than what we have up to now
if (best.weight == DBL_MAX || G->weights[j] < best.weight) {
best.weight = G->weights[j];
best.edge = j;
}
}
}
}
// reduce and determine global best edge
best.rank = G->rank;
MPI_Allreduce(MPI_IN_PLACE, &best, 1, MPI_DOUBLE_INT, MPI_MINLOC, MPI_COMM_WORLD);
if (best.weight == DBL_MAX) {
// no suitable edge found, finish this tree
break;
} else {
if (best.rank == G->rank) {
// we have the best edge
trees.back().push_back(edge_to_global(best.edge,G));
last_vertex = G->endV[best.edge];
}
MPI_Bcast(&last_vertex, 1, MPI_UINT32_T, best.rank, MPI_COMM_WORLD);
}
}
// find root of a new tree
root = UINT32_MAX;
for (vertex_id_t i = 0; i < G->local_n; i++) {
if (!marked_vertices[i]) {
root = VERTEX_TO_GLOBAL(i, TotVertices, size, rank);
break;
}
}
MPI_Allreduce(MPI_IN_PLACE, &root, 1, MPI_UINT32_T, MPI_MIN, MPI_COMM_WORLD);
} while (root!=UINT32_MAX);
return &trees;
}
