243 lines
8.0 KiB
Rust
243 lines
8.0 KiB
Rust
//! The topology graph.
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use crate::error::{TopologyError, TopologyErrorKind};
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/// One link in a topology.
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#[derive(Debug, Clone, Copy, PartialEq)]
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pub struct Edge {
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/// The index of the first endpoint.
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pub from: usize,
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/// The index of the second endpoint.
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pub to: usize,
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/// The cost of traversing the link.
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///
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/// The meaning is the caller's: a delay in seconds, a hop count, or any
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/// other measure a routing algorithm minimises. It must be finite and
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/// positive, so that adding edges never shortens a path.
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pub cost: f64,
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}
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impl Edge {
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/// Creates an edge.
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pub const fn new(from: usize, to: usize, cost: f64) -> Self {
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Self { from, to, cost }
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}
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/// Returns the same edge with its endpoints exchanged.
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///
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/// A link between two satellites carries traffic both ways, so most
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/// topologies add both directions.
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pub const fn reversed(self) -> Self {
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Self {
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from: self.to,
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to: self.from,
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cost: self.cost,
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}
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}
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}
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/// A graph of nodes and links at one instant.
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///
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/// The node count is fixed at construction and nodes are identified by their
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/// index. Edges are directed: adding an edge from one node to another does
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/// not add the reverse, so a caller that wants a bidirectional link adds
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/// both directions. This is deliberate, because an inter-satellite link may
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/// have different capacity in each direction and a ground link is often
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/// asymmetric.
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#[derive(Debug, Clone, PartialEq, Default)]
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pub struct Topology {
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node_count: usize,
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edges: Vec<Edge>,
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}
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impl Topology {
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/// Creates an empty topology with the given number of nodes.
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pub const fn new(node_count: usize) -> Self {
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Self {
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node_count,
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edges: Vec::new(),
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}
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}
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/// Returns the number of nodes.
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pub const fn node_count(&self) -> usize {
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self.node_count
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}
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/// Returns the number of edges.
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pub fn edge_count(&self) -> usize {
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self.edges.len()
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}
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/// Returns `true` when there are no edges.
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pub fn is_empty(&self) -> bool {
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self.edges.is_empty()
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}
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/// Returns the edges.
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pub fn edges(&self) -> &[Edge] {
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&self.edges
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}
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/// Adds a directed edge.
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///
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/// Returns an error when either node index is out of range, when the
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/// cost is not finite and positive, or when the edge already exists.
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/// Rejecting a duplicate rather than replacing it keeps a topology free
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/// of silent contradictions about the cost of a link.
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pub fn add_edge(&mut self, from: usize, to: usize, cost: f64) -> Result<(), TopologyError> {
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if from >= self.node_count || to >= self.node_count {
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return Err(TopologyError::new(TopologyErrorKind::NoSuchNode)
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.with_detail(format!("nodes {from} and {to} of {}", self.node_count)));
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}
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if !cost.is_finite() || cost <= 0.0 {
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return Err(TopologyError::new(TopologyErrorKind::InvalidCost)
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.with_detail(format!("cost {cost}")));
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}
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if self.has_edge(from, to) {
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return Err(TopologyError::new(TopologyErrorKind::DuplicateEdge)
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.with_detail(format!("edge {from} to {to}")));
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}
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self.edges.push(Edge::new(from, to, cost));
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Ok(())
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}
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/// Adds an edge in both directions with the same cost.
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///
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/// If the second direction fails after the first succeeds, the first is
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/// rolled back, so the topology is left unchanged when the call reports
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/// an error.
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pub fn add_bidirectional_edge(
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&mut self,
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first: usize,
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second: usize,
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cost: f64,
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) -> Result<(), TopologyError> {
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self.add_edge(first, second, cost)?;
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if let Err(error) = self.add_edge(second, first, cost) {
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// Remove the first direction so the topology is unchanged.
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self.edges
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.retain(|edge| !(edge.from == first && edge.to == second));
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return Err(error);
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}
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Ok(())
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}
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/// Returns `true` when an edge exists from one node to another.
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pub fn has_edge(&self, from: usize, to: usize) -> bool {
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self.edges
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.iter()
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.any(|edge| edge.from == from && edge.to == to)
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}
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/// Returns the cost of the edge between two nodes.
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///
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/// Returns `None` when there is no such edge.
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pub fn cost(&self, from: usize, to: usize) -> Option<f64> {
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self.edges
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.iter()
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.find(|edge| edge.from == from && edge.to == to)
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.map(|edge| edge.cost)
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}
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/// Removes the edge between two nodes.
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///
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/// Returns an error when there is no such edge.
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pub fn remove_edge(&mut self, from: usize, to: usize) -> Result<(), TopologyError> {
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let before = self.edges.len();
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self.edges
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.retain(|edge| !(edge.from == from && edge.to == to));
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if self.edges.len() == before {
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return Err(TopologyError::new(TopologyErrorKind::NoSuchEdge)
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.with_detail(format!("edge {from} to {to}")));
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}
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Ok(())
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}
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/// Returns the indices of the nodes reachable from a node in one hop.
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///
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/// The result is in the order the edges were added, so it is
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/// deterministic for a given construction sequence.
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pub fn neighbours(&self, node: usize) -> Vec<usize> {
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self.edges
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.iter()
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.filter(|edge| edge.from == node)
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.map(|edge| edge.to)
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.collect()
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}
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/// Returns the number of edges leaving a node.
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pub fn out_degree(&self, node: usize) -> usize {
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self.edges.iter().filter(|edge| edge.from == node).count()
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}
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/// Returns the number of edges arriving at a node.
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pub fn in_degree(&self, node: usize) -> usize {
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self.edges.iter().filter(|edge| edge.to == node).count()
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}
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/// Returns the adjacency of every node.
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///
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/// The result has one entry per node, in node order, each listing the
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/// neighbours of that node.
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pub fn adjacency(&self) -> Vec<Vec<usize>> {
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(0..self.node_count)
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.map(|node| self.neighbours(node))
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.collect()
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}
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/// Returns the number of connected components, treating the edges as
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/// undirected.
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///
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/// This answers how many separate networks the topology forms, which is
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/// the first thing to check about a constellation snapshot: a
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/// constellation that has split into pieces can still carry traffic
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/// within each piece but not between them.
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pub fn connected_components(&self) -> usize {
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if self.node_count == 0 {
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return 0;
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}
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let mut visited = vec![false; self.node_count];
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let mut components = 0;
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for start in 0..self.node_count {
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if visited[start] {
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continue;
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}
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components += 1;
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// A breadth first walk over the edges in both directions.
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let mut stack = vec![start];
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visited[start] = true;
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while let Some(node) = stack.pop() {
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for edge in &self.edges {
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let neighbour = if edge.from == node {
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edge.to
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} else if edge.to == node {
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edge.from
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} else {
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continue;
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};
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if !visited[neighbour] {
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visited[neighbour] = true;
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stack.push(neighbour);
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}
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}
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}
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}
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components
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}
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/// Returns `true` when every node can reach every other node, treating
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/// the edges as undirected.
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pub fn is_connected(&self) -> bool {
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self.node_count <= 1 || self.connected_components() == 1
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}
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/// Returns the total cost of every edge.
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///
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/// This is a crude measure of the network's extent, useful for comparing
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/// snapshots rather than for routing.
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pub fn total_cost(&self) -> f64 {
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self.edges.iter().map(|edge| edge.cost).sum()
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}
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}
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