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//! Gem-free graph predicate (ALGO-PR-098).
//!
//! A graph is gem-free if it contains no induced subgraph isomorphic
//! to the gem graph (also called the fan `F_{1,3}`): a `P_4` plus a
//! universal vertex adjacent to all four path vertices. The gem has
//! 5 vertices and 7 edges.
//!
//! For directed graphs, the function returns `false`.
use crate::core::{Graph, IgraphResult};
/// Check whether a graph is gem-free.
///
/// The gem (fan `F_{1,3}`) is `P_4` plus a vertex adjacent to all
/// four path vertices. A gem-free graph has no induced gem.
///
/// Returns `false` for directed graphs.
///
/// # Examples
///
/// ```
/// use rust_igraph::{Graph, is_gem_free};
///
/// // Triangle is gem-free (too few vertices for a gem)
/// let mut g = Graph::with_vertices(3);
/// g.add_edge(0, 1).unwrap();
/// g.add_edge(1, 2).unwrap();
/// g.add_edge(2, 0).unwrap();
/// assert!(is_gem_free(&g).unwrap());
/// ```
pub fn is_gem_free(graph: &Graph) -> IgraphResult<bool> {
if graph.is_directed() {
return Ok(false);
}
let n = graph.vcount();
if n < 5 {
return Ok(true);
}
let n_usize = n as usize;
let mut adj = vec![vec![false; n_usize]; n_usize];
let mut nbrs_list: Vec<Vec<u32>> = Vec::with_capacity(n_usize);
for v in 0..n {
let nbrs = graph.neighbors(v)?;
for &w in &nbrs {
adj[v as usize][w as usize] = true;
}
nbrs_list.push(nbrs);
}
// A gem is: vertices {u, a, b, c, d} where a-b-c-d is an induced P_4
// and u is adjacent to all of {a, b, c, d}.
// Strategy: for each vertex u with degree ≥ 4, check if N(u)
// contains an induced P_4.
for u in 0..n {
let u_idx = u as usize;
if nbrs_list[u_idx].len() < 4 {
continue;
}
let nbrs_u = &nbrs_list[u_idx];
// Check all ordered 4-tuples (a, b, c, d) from N(u) for induced P_4
// a-b adjacent, b-c adjacent, c-d adjacent,
// a-c NOT adjacent, b-d NOT adjacent, a-d NOT adjacent
for (i, &a) in nbrs_u.iter().enumerate() {
let ai = a as usize;
for &b in &nbrs_u[i + 1..] {
let bi = b as usize;
if !adj[ai][bi] {
continue;
}
// a-b adjacent; look for c adjacent to b but not a
for &c in nbrs_u {
if c == a || c == b {
continue;
}
let ci = c as usize;
if !adj[bi][ci] || adj[ai][ci] {
continue;
}
// a-b-c is an induced P_3; look for d adjacent to c
// but not a and not b
for &d in nbrs_u {
if d == a || d == b || d == c {
continue;
}
let di = d as usize;
if adj[ci][di] && !adj[bi][di] && !adj[ai][di] {
return Ok(false);
}
}
}
}
}
}
Ok(true)
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn empty_graph() {
let g = Graph::with_vertices(0);
assert!(is_gem_free(&g).unwrap());
}
#[test]
fn small_graphs() {
let g = Graph::with_vertices(4);
assert!(is_gem_free(&g).unwrap());
}
#[test]
fn triangle() {
let mut g = Graph::with_vertices(3);
g.add_edge(0, 1).unwrap();
g.add_edge(1, 2).unwrap();
g.add_edge(2, 0).unwrap();
assert!(is_gem_free(&g).unwrap());
}
#[test]
fn gem_graph() {
// Gem: P_4 = 1-2-3-4, universal vertex 0
let mut g = Graph::with_vertices(5);
g.add_edge(0, 1).unwrap();
g.add_edge(0, 2).unwrap();
g.add_edge(0, 3).unwrap();
g.add_edge(0, 4).unwrap();
g.add_edge(1, 2).unwrap();
g.add_edge(2, 3).unwrap();
g.add_edge(3, 4).unwrap();
assert!(!is_gem_free(&g).unwrap());
}
#[test]
fn k5() {
// K_5 is gem-free: every 5-vertex induced subgraph is K_5,
// which has too many edges for a gem
let mut g = Graph::with_vertices(5);
for i in 0..5u32 {
for j in (i + 1)..5 {
g.add_edge(i, j).unwrap();
}
}
assert!(is_gem_free(&g).unwrap());
}
#[test]
fn star_k15() {
// Star K_{1,4}: vertex 0 connected to 1,2,3,4
// N(0) = {1,2,3,4}, all mutually non-adjacent → no P_4 in N(0)
let mut g = Graph::with_vertices(5);
g.add_edge(0, 1).unwrap();
g.add_edge(0, 2).unwrap();
g.add_edge(0, 3).unwrap();
g.add_edge(0, 4).unwrap();
assert!(is_gem_free(&g).unwrap());
}
#[test]
fn c5_gem_free() {
let mut g = Graph::with_vertices(5);
g.add_edge(0, 1).unwrap();
g.add_edge(1, 2).unwrap();
g.add_edge(2, 3).unwrap();
g.add_edge(3, 4).unwrap();
g.add_edge(4, 0).unwrap();
assert!(is_gem_free(&g).unwrap());
}
#[test]
fn path_p5_gem_free() {
let mut g = Graph::with_vertices(5);
g.add_edge(0, 1).unwrap();
g.add_edge(1, 2).unwrap();
g.add_edge(2, 3).unwrap();
g.add_edge(3, 4).unwrap();
assert!(is_gem_free(&g).unwrap());
}
#[test]
fn wheel_w5_not_gem_free() {
// W_5: hub 0, rim 1-2-3-4-5-1
let mut g = Graph::with_vertices(6);
g.add_edge(0, 1).unwrap();
g.add_edge(0, 2).unwrap();
g.add_edge(0, 3).unwrap();
g.add_edge(0, 4).unwrap();
g.add_edge(0, 5).unwrap();
g.add_edge(1, 2).unwrap();
g.add_edge(2, 3).unwrap();
g.add_edge(3, 4).unwrap();
g.add_edge(4, 5).unwrap();
g.add_edge(5, 1).unwrap();
// Hub 0 has N(0) = {1,2,3,4,5}
// In N(0): 1-2-3-4 is P_4 (1-2 adj, 2-3 adj, 3-4 adj,
// 1-3 not adj, 2-4 not adj, 1-4 not adj via rim) → gem
assert!(!is_gem_free(&g).unwrap());
}
#[test]
fn directed_returns_false() {
let mut g = Graph::new(3, true).unwrap();
g.add_edge(0, 1).unwrap();
g.add_edge(1, 2).unwrap();
g.add_edge(2, 0).unwrap();
assert!(!is_gem_free(&g).unwrap());
}
#[test]
fn fan_3_plus_universal() {
// Universal vertex 0, path 1-2-3, plus extra vertex 4 connected to 0 only
// N(0) = {1,2,3,4}. In N(0): 1-2-3 but 4 isolated from 1,2,3
// No P_4 in N(0) since 4 is isolated
let mut g = Graph::with_vertices(5);
g.add_edge(0, 1).unwrap();
g.add_edge(0, 2).unwrap();
g.add_edge(0, 3).unwrap();
g.add_edge(0, 4).unwrap();
g.add_edge(1, 2).unwrap();
g.add_edge(2, 3).unwrap();
assert!(is_gem_free(&g).unwrap());
}
}