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//! Sort vertex IDs by degree (ALGO-PR-054).
//!
//! Counterpart of `igraph_sort_vertex_ids_by_degree()` from
//! `references/igraph/src/properties/degrees.c:712`.
//!
//! Returns vertex IDs sorted by their degree in ascending or
//! descending order.
use crate::algorithms::properties::degree::DegreeMode;
use crate::core::{Graph, IgraphError, IgraphResult, VertexId};
/// Sort order for vertex degree sorting.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SortOrder {
/// Smallest degree first.
Ascending,
/// Largest degree first.
Descending,
}
/// Return vertex IDs sorted by their degree.
///
/// - `mode`: which degree to use (`Out`, `In`, or `All`). For
/// undirected graphs all modes are equivalent.
/// - `order`: ascending (smallest degree first) or descending.
///
/// Returns a `Vec<VertexId>` containing all vertex IDs in the
/// requested order. Ties are broken by vertex ID (stable sort).
///
/// # Examples
///
/// ```
/// use rust_igraph::{Graph, sort_vertices_by_degree, DegreeMode, SortOrder};
///
/// // Star: center vertex has degree 3, leaves have degree 1.
/// let mut g = Graph::with_vertices(4);
/// g.add_edge(0, 1).unwrap();
/// g.add_edge(0, 2).unwrap();
/// g.add_edge(0, 3).unwrap();
/// let sorted = sort_vertices_by_degree(&g, DegreeMode::All, SortOrder::Descending).unwrap();
/// assert_eq!(sorted[0], 0); // highest degree vertex first
/// ```
pub fn sort_vertices_by_degree(
graph: &Graph,
mode: DegreeMode,
order: SortOrder,
) -> IgraphResult<Vec<VertexId>> {
let n = graph.vcount();
if n == 0 {
return Ok(Vec::new());
}
let n_usize = n as usize;
let ecount = graph.ecount();
let directed = graph.is_directed();
let mut deg = vec![0u32; n_usize];
if ecount > 0 {
let m_u32 =
u32::try_from(ecount).map_err(|_| IgraphError::Internal("ecount exceeds u32::MAX"))?;
for eid in 0..m_u32 {
let (from, to) = graph.edge(eid)?;
let f = from as usize;
let t = to as usize;
if directed {
match mode {
DegreeMode::Out => {
deg[f] += 1;
}
DegreeMode::In => {
deg[t] += 1;
}
DegreeMode::All => {
deg[f] += 1;
deg[t] += 1;
}
}
} else if from == to {
deg[f] += 2;
} else {
deg[f] += 1;
deg[t] += 1;
}
}
}
let mut ids: Vec<VertexId> = (0..n).collect();
match order {
SortOrder::Ascending => {
ids.sort_by(|&a, &b| deg[a as usize].cmp(°[b as usize]));
}
SortOrder::Descending => {
ids.sort_by(|&a, &b| deg[b as usize].cmp(°[a as usize]));
}
}
Ok(ids)
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn empty_graph() {
let g = Graph::with_vertices(0);
let s = sort_vertices_by_degree(&g, DegreeMode::All, SortOrder::Ascending).unwrap();
assert!(s.is_empty());
}
#[test]
fn no_edges_ascending() {
let g = Graph::with_vertices(5);
let s = sort_vertices_by_degree(&g, DegreeMode::All, SortOrder::Ascending).unwrap();
// All degree 0, stable sort → original order.
assert_eq!(s, vec![0, 1, 2, 3, 4]);
}
#[test]
fn star_descending() {
let mut g = Graph::with_vertices(4);
g.add_edge(0, 1).unwrap();
g.add_edge(0, 2).unwrap();
g.add_edge(0, 3).unwrap();
let s = sort_vertices_by_degree(&g, DegreeMode::All, SortOrder::Descending).unwrap();
// deg = [3, 1, 1, 1]. Descending: 0 first, then 1,2,3.
assert_eq!(s[0], 0);
assert_eq!(s.len(), 4);
}
#[test]
fn star_ascending() {
let mut g = Graph::with_vertices(4);
g.add_edge(0, 1).unwrap();
g.add_edge(0, 2).unwrap();
g.add_edge(0, 3).unwrap();
let s = sort_vertices_by_degree(&g, DegreeMode::All, SortOrder::Ascending).unwrap();
// deg = [3, 1, 1, 1]. Ascending: 1,2,3 first (degree 1), then 0 (degree 3).
assert_eq!(s[3], 0);
}
#[test]
fn path_5_ascending() {
let mut g = Graph::with_vertices(5);
for i in 0..4u32 {
g.add_edge(i, i + 1).unwrap();
}
let s = sort_vertices_by_degree(&g, DegreeMode::All, SortOrder::Ascending).unwrap();
// deg = [1, 2, 2, 2, 1]. Sorted ascending: 0,4 (deg 1), then 1,2,3 (deg 2).
assert!(s[0] == 0 || s[0] == 4);
assert!(s[1] == 0 || s[1] == 4);
assert!(s[0] != s[1]);
}
#[test]
fn directed_out_degree() {
// 0→1, 0→2, 1→2
let mut g = Graph::new(3, true).unwrap();
g.add_edge(0, 1).unwrap();
g.add_edge(0, 2).unwrap();
g.add_edge(1, 2).unwrap();
let s = sort_vertices_by_degree(&g, DegreeMode::Out, SortOrder::Descending).unwrap();
// out-deg = [2, 1, 0]. Desc: 0, 1, 2.
assert_eq!(s, vec![0, 1, 2]);
}
#[test]
fn directed_in_degree() {
// 0→1, 0→2, 1→2
let mut g = Graph::new(3, true).unwrap();
g.add_edge(0, 1).unwrap();
g.add_edge(0, 2).unwrap();
g.add_edge(1, 2).unwrap();
let s = sort_vertices_by_degree(&g, DegreeMode::In, SortOrder::Descending).unwrap();
// in-deg = [0, 1, 2]. Desc: 2, 1, 0.
assert_eq!(s, vec![2, 1, 0]);
}
#[test]
fn k4_stable_order() {
let mut g = Graph::with_vertices(4);
for u in 0..4u32 {
for v in (u + 1)..4 {
g.add_edge(u, v).unwrap();
}
}
let s = sort_vertices_by_degree(&g, DegreeMode::All, SortOrder::Ascending).unwrap();
// All same degree → stable sort preserves original order.
assert_eq!(s, vec![0, 1, 2, 3]);
}
#[test]
fn self_loop_counted() {
let mut g = Graph::with_vertices(3);
g.add_edge(0, 0).unwrap(); // self-loop: deg(0) += 2
g.add_edge(0, 1).unwrap(); // deg(0) += 1, deg(1) += 1
g.add_edge(1, 2).unwrap(); // deg(1) += 1, deg(2) += 1
let s = sort_vertices_by_degree(&g, DegreeMode::All, SortOrder::Descending).unwrap();
// deg = [3, 2, 1]. Desc: 0, 1, 2.
assert_eq!(s, vec![0, 1, 2]);
}
}