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//! Everything related to graphs: edges, nodes and, of course, graphs. //! //! The most important part of this module is [`Graph`](Graph). //! However, if you want to manually build graphs, //! you will need to use [`Edge`](Edge). use crate::Sampler; use indexmap::IndexSet; use serde::{Deserialize, Serialize}; /// A (variable, constraint) pair. /// /// Since variables and constraints are different sets of nodes, /// it is possible to have an edge with the same value for variable and constraint. #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, Serialize, Deserialize)] pub struct Edge { pub variable: usize, pub constraint: usize, } impl Edge { /// Creates a new edge for the given variable and constraint. pub fn new(variable: usize, constraint: usize) -> Self { Self { variable, constraint, } } } /// A bipartite regular graph. /// /// A graph is a set of variables and constraints together with /// a set of edges. /// An edge is a (variable, constraint) pair. /// /// A graph can be build manually or from a [`Sampler`](Sampler). /// /// # Example /// /// ``` /// use bigs::graph::{Edge, Graph}; /// /// let mut graph = Graph::new(); /// /// graph.insert_edge(Edge::new(0, 0)); // Edge between variable 0 and constraint 0. /// graph.insert_edge(Edge::new(0, 1)); // Edge between variable 0 and constraint 1. /// graph.insert_edge(Edge::new(1, 2)); // Edge between variable 1 and constraint 2. /// graph.insert_edge(Edge::new(1, 3)); // Edge between variable 1 and constraint 3. /// /// assert_eq!(graph.number_of_variables(), 2); /// assert_eq!(graph.number_of_constraints(), 4); /// assert_eq!(graph.number_of_edges(), 4); /// /// for variable in graph.variables() { /// assert_eq!(variable.degree(), 2); /// } /// /// for constraint in graph.constraints() { /// assert_eq!(constraint.degree(), 1); /// } /// ``` /// /// # Performance tips /// /// Don't use unnecessary large labels for variable and constraint. /// The construction assume that you will use labels 0 to n - 1 if you /// want n variables and the same for constraints. /// /// If for whatever reason you want to assign a node with a large label, /// notes that this will allocate the needed memory for all nodes up to that label. /// /// ``` /// use bigs::graph::{Edge, Graph}; /// /// let mut graph = Graph::new(); /// graph.insert_edge(Edge::new(0, 42)); /// /// assert_eq!(graph.number_of_variables(), 1); /// assert_eq!(graph.number_of_constraints(), 43); /// ``` #[derive(Debug, PartialEq, Eq, Clone, Serialize, Deserialize)] pub struct Graph { variable_neighbors: Vec<IndexSet<usize>>, constraint_neighbors: Vec<IndexSet<usize>>, edges: IndexSet<Edge>, } impl Graph { /// Creates a new empty graph. pub fn new() -> Self { Self { variable_neighbors: Vec::new(), constraint_neighbors: Vec::new(), edges: IndexSet::new(), } } /// Checks if the given edge is in the graph. pub fn contains_edge(&self, edge: Edge) -> bool { self.edges.contains(&edge) } /// Inserts the given edge in the graph and returns true if the /// edge was not already in the graph. /// /// If the edge variable is greater or equal to the number of variables in the graph, /// the number of variables will be incremented by the difference. /// Same holds for constraints. /// /// # Example /// /// ``` /// use bigs::graph::{Edge, Graph}; /// /// let mut graph = Graph::new(); /// assert_eq!(graph.number_of_variables(), 0); /// assert_eq!(graph.number_of_constraints(), 0); /// assert_eq!(graph.number_of_edges(), 0); /// /// graph.insert_edge(Edge::new(0, 0)); /// assert_eq!(graph.number_of_variables(), 1); /// assert_eq!(graph.number_of_constraints(), 1); /// assert_eq!(graph.number_of_edges(), 1); /// /// graph.insert_edge(Edge::new(5, 6)); /// assert_eq!(graph.number_of_variables(), 6); /// assert_eq!(graph.number_of_constraints(), 7); /// assert_eq!(graph.number_of_edges(), 2); /// /// assert_eq!(graph.insert_edge(Edge::new(0, 0)), false); /// ``` pub fn insert_edge(&mut self, edge: Edge) -> bool { if self.edges.insert(edge) { self.insert_variable(edge); self.insert_constraint(edge); true } else { false } } fn insert_variable(&mut self, edge: Edge) { if edge.variable >= self.number_of_variables() { self.variable_neighbors.extend(vec![ IndexSet::new(); edge.variable - self.number_of_variables() + 1 ]); } self.variable_neighbors[edge.variable].insert(edge.constraint); } fn insert_constraint(&mut self, edge: Edge) { if edge.constraint >= self.number_of_constraints() { self.constraint_neighbors.extend(vec![ IndexSet::new(); edge.constraint - self.number_of_constraints() + 1 ]); } self.constraint_neighbors[edge.constraint].insert(edge.variable); } /// Removes the given edge from the graph if it exists and returns true. /// Else, returns false. /// /// However, this do not update the number of variables and constraints in the graph. /// /// # Example /// /// ``` /// use bigs::graph::{Edge, Graph}; /// /// let mut graph = Graph::new(); /// assert_eq!(graph.number_of_variables(), 0); /// assert_eq!(graph.number_of_constraints(), 0); /// assert_eq!(graph.number_of_edges(), 0); /// /// graph.insert_edge(Edge::new(0, 0)); /// assert_eq!(graph.number_of_variables(), 1); /// assert_eq!(graph.number_of_constraints(), 1); /// assert_eq!(graph.number_of_edges(), 1); /// /// graph.remove_edge(Edge::new(0, 0)); /// assert_eq!(graph.number_of_variables(), 1); /// assert_eq!(graph.number_of_constraints(), 1); /// assert_eq!(graph.number_of_edges(), 0); /// ``` pub fn remove_edge(&mut self, edge: Edge) -> bool { if self.edges.remove(&edge) { self.variable_neighbors[edge.variable].remove(&edge.constraint); self.constraint_neighbors[edge.constraint].remove(&edge.variable); true } else { false } } /// Returns an iterator over all edges in the graph in some possibly random order. pub fn edges(&self) -> impl Iterator<Item = Edge> + '_ { self.edges.iter().cloned() } /// Returns the number of variables in the graph. /// /// That is, the one more than the highest variable label inserted in the graph. pub fn number_of_variables(&self) -> usize { self.variable_neighbors.len() } /// Returns the number of constraints in the graph. /// /// That is, the one more than the highest constraint label inserted in the graph. pub fn number_of_constraints(&self) -> usize { self.constraint_neighbors.len() } /// Returns the number of edges in the graph. pub fn number_of_edges(&self) -> usize { self.edges.len() } /// Returns an iterator over all variables in the graph in increasing label order. /// /// # Example /// /// ``` /// use bigs::graph::{Edge, Graph}; /// use indexmap::indexset; /// /// let mut graph = Graph::new(); /// /// graph.insert_edge(Edge::new(0, 0)); // Edge between variable 0 and constraint 0. /// graph.insert_edge(Edge::new(0, 1)); // Edge between variable 0 and constraint 1. /// graph.insert_edge(Edge::new(1, 2)); // Edge between variable 1 and constraint 2. /// graph.insert_edge(Edge::new(1, 3)); // Edge between variable 1 and constraint 3. /// /// let mut iter = graph.variables(); /// /// let first_variable = iter.next().unwrap(); /// assert_eq!(first_variable.label(), 0); /// assert_eq!(first_variable.degree(), 2); /// assert_eq!(first_variable.neighbors(), &indexset! { 0, 1 }); /// /// let second_variable = iter.next().unwrap(); /// assert_eq!(second_variable.label(), 1); /// assert_eq!(second_variable.degree(), 2); /// assert_eq!(second_variable.neighbors(), &indexset! { 2, 3 }); /// /// assert!(iter.next().is_none()); /// ``` pub fn variables(&self) -> Nodes { Nodes { iter: self.variable_neighbors.iter().enumerate(), kind: NodeKind::Variable, } } /// Returns an iterator over all constraints in the graph in increasing label order. /// /// # Example /// /// ``` /// use bigs::graph::{Edge, Graph}; /// use indexmap::indexset; /// /// let mut graph = Graph::new(); /// /// graph.insert_edge(Edge::new(0, 0)); // Edge between variable 0 and constraint 0. /// graph.insert_edge(Edge::new(0, 1)); // Edge between variable 0 and constraint 1. /// graph.insert_edge(Edge::new(1, 2)); // Edge between variable 1 and constraint 2. /// graph.insert_edge(Edge::new(1, 3)); // Edge between variable 1 and constraint 3. /// /// let mut iter = graph.constraints(); /// /// let first_constraint = iter.next().unwrap(); /// assert_eq!(first_constraint.label(), 0); /// assert_eq!(first_constraint.degree(), 1); /// assert_eq!(first_constraint.neighbors(), &indexset! { 0 }); /// /// let second_constraint = iter.next().unwrap(); /// assert_eq!(second_constraint.label(), 1); /// assert_eq!(second_constraint.degree(), 1); /// assert_eq!(second_constraint.neighbors(), &indexset! { 0 }); /// /// assert!(iter.next().is_some()); /// assert!(iter.next().is_some()); /// assert!(iter.next().is_none()); /// ``` pub fn constraints(&self) -> Nodes { Nodes { iter: self.constraint_neighbors.iter().enumerate(), kind: NodeKind::Constraint, } } pub(crate) fn from_sampler(sampler: &Sampler) -> Self { Self { variable_neighbors: vec![ IndexSet::with_capacity(sampler.variable_degree()); sampler.number_of_variables() ], constraint_neighbors: vec![ IndexSet::with_capacity(sampler.constraint_degree()); sampler.number_of_constraints() ], edges: IndexSet::with_capacity(sampler.number_of_edges()), } } } /// An iterator for a set of nodes in a graph. /// /// This is created via the [`Graph::variables`](Graph::variables) /// or the [`Graph::constraints`](Graph::constraints) methods. pub struct Nodes<'g> { iter: std::iter::Enumerate<std::slice::Iter<'g, IndexSet<usize>>>, kind: NodeKind, } impl<'g> Iterator for Nodes<'g> { type Item = Node<'g>; fn next(&mut self) -> Option<Self::Item> { self.iter.next().map(|(label, neighbors)| Node { neighbors, label, kind: self.kind, }) } } /// A node in the graph. /// /// This is used to iterates throught the nodes of a graph. pub struct Node<'g> { neighbors: &'g IndexSet<usize>, label: usize, kind: NodeKind, } impl<'g> Node<'g> { /// Returns the set of labels of the neighbors of the node. pub fn neighbors(&self) -> &IndexSet<usize> { self.neighbors } /// Returns the label of the node. pub fn label(&self) -> usize { self.label } /// Returns the degree of the node. /// That is, the number of neighbors. pub fn degree(&self) -> usize { self.neighbors.len() } /// Checks if a node a neighbor with the given label pub fn has_neighbor(&self, label: usize) -> bool { self.neighbors.contains(&label) } /// Checks if a node is a variable. pub fn is_variable(&self) -> bool { self.kind == NodeKind::Variable } /// Checks if a node is a constraint. pub fn is_constraint(&self) -> bool { self.kind == NodeKind::Constraint } } #[derive(Clone, Copy, PartialEq, Eq)] enum NodeKind { Variable, Constraint, }