hypergraph 1.3.9

Hypergraph is data structure library to create a directed hypergraph in which an hyperedge can join any number of vertices.
Documentation 
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use crate::{HyperedgeTrait, Hypergraph, VertexIndex, VertexTrait};

impl<V, HE> Hypergraph<V, HE>
where
    V: VertexTrait,
    HE: HyperedgeTrait,
{
    // This private method is infallible since adding the same vertex
    // will return the existing index.
    pub(crate) fn add_vertex_index(&mut self, internal_index: usize) -> VertexIndex {
        match self.vertices_mapping.left.get(&internal_index) {
            Some(vertex_index) => *vertex_index,
            None => {
                let vertex_index = VertexIndex(self.vertices_count);

                if self
                    .vertices_mapping
                    .left
                    .insert(internal_index, vertex_index)
                    .is_none()
                {
                    // Update the counter only for the first insertion.
                    self.vertices_count += 1;
                }

                self.vertices_mapping
                    .right
                    .insert(vertex_index, internal_index);

                vertex_index
            }
        }
    }
}