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use crate::{
HyperedgeIndex,
HyperedgeKey,
HyperedgeTrait,
Hypergraph,
VertexTrait,
errors::HypergraphError,
};
impl<V, HE> Hypergraph<V, HE>
where
V: VertexTrait,
HE: HyperedgeTrait,
{
/// Removes a hyperedge by index.
pub fn remove_hyperedge(
&mut self,
hyperedge_index: HyperedgeIndex,
) -> Result<(), HypergraphError<V, HE>> {
let internal_index = self.get_internal_hyperedge(hyperedge_index)?;
let HyperedgeKey { vertices, .. } =
self.hyperedges.get_index(internal_index).cloned().ok_or(
HypergraphError::InternalHyperedgeIndexNotFound(internal_index),
)?;
// Find the last index.
let last_index = self.hyperedges.len() - 1;
// Swap and remove by index.
self.hyperedges.swap_remove_index(internal_index);
// Update the mapping for the removed hyperedge.
self.hyperedges_mapping.left.remove(&internal_index);
self.hyperedges_mapping.right.remove(&hyperedge_index);
// Remove the hyperedge from the vertices.
for vertex in vertices {
match self.vertices.get_index_mut(vertex) {
Some((_, index_set)) => {
index_set.swap_remove(&internal_index);
}
None => return Err(HypergraphError::InternalVertexIndexNotFound(vertex)),
}
}
// Given the following bi-mapping with three hyperedges, i.e. an
// initial set of hyperedges [0, 1, 2]. Let's assume in this example
// that the first hyperedge will be removed:
//
// left | Right
// ---------------------------------------------------------
// 0usize -> HyperedgeIndex(0) | HyperedgeIndex(0) -> 0usize
// 1usize -> HyperedgeIndex(1) | HyperedgeIndex(1) -> 1usize
// 2usize -> HyperedgeIndex(2) | HyperedgeIndex(2) -> 2usize
//
// In the previous steps, the current hyperedge has been already nuked.
// So we now have:
//
// left | Right
// ---------------------------------------------------------
// xxxxxxxxxxxxxxxxxxxxxxxxxxx | xxxxxxxxxxxxxxxxxxxxxxxxxxx
// 1usize -> HyperedgeIndex(1) | HyperedgeIndex(1) -> 1usize
// 2usize -> HyperedgeIndex(2) | HyperedgeIndex(2) -> 2usize
//
// The next step will be to insert the swapped index on the right:
//
// left | Right
// ---------------------------------------------------------
// xxxxxxxxxxxxxxxxxxxxxxxxxxx | xxxxxxxxxxxxxxxxxxxxxxxxxxx
// 1usize -> HyperedgeIndex(1) | HyperedgeIndex(1) -> 1usize
// 2usize -> HyperedgeIndex(2) | HyperedgeIndex(2) -> 0usize
//
// Now remove the index which no longer exists on the left:
//
// left | Right
// ---------------------------------------------------------
// xxxxxxxxxxxxxxxxxxxxxxxxxxx | xxxxxxxxxxxxxxxxxxxxxxxxxxx
// 1usize -> HyperedgeIndex(1) | HyperedgeIndex(1) -> 1usize
// xxxxxxxxxxxxxxxxxxxxxxxxxxx | HyperedgeIndex(2) -> 0usize
//
// Insert the swapped index on the left:
//
// left | Right
// ---------------------------------------------------------
// 0usize -> HyperedgeIndex(2) | xxxxxxxxxxxxxxxxxxxxxxxxxxx
// 1usize -> HyperedgeIndex(1) | HyperedgeIndex(1) -> 1usize
// xxxxxxxxxxxxxxxxxxxxxxxxxxx | HyperedgeIndex(2) -> 0usize
//
// If the index to remove wasn't the last one, the last hyperedge has
// been swapped in place of the removed one. Thus we need to update
// the mapping accordingly.
if internal_index != last_index {
// Get the index of the swapped hyperedge.
let swapped_hyperedge_index = self.get_hyperedge(last_index)?;
// Proceed with the aforementioned operations.
self.hyperedges_mapping
.right
.insert(swapped_hyperedge_index, internal_index);
self.hyperedges_mapping.left.remove(&last_index);
self.hyperedges_mapping
.left
.insert(internal_index, swapped_hyperedge_index);
// Get the vertices of the swapped hyperedge.
let HyperedgeKey {
vertices: swapped_vertices,
..
} = self.hyperedges.get_index(internal_index).cloned().ok_or(
HypergraphError::InternalHyperedgeIndexNotFound(internal_index),
)?;
// Update the impacted vertices accordingly.
for vertex in swapped_vertices {
match self.vertices.get_index_mut(vertex) {
Some((_, index_set)) => {
// Perform an insertion of the current hyperedge and a
// removal of the swapped one.
index_set.insert(internal_index);
index_set.swap_remove(&last_index);
}
None => return Err(HypergraphError::InternalVertexIndexNotFound(vertex)),
}
}
}
// Return a unit.
Ok(())
}
}