add hamilton path/cycle validation

src/graph.rs

@@ -116,6 +116,36 @@ where
         None
     }
 
+    /// check if a provided path is a hamilton path within the graph,
+    /// meaning it contains all of the graphs vertices exactly once.
+    ///
+    /// an empty path is considered hamiltonian for empty graphs.
+    pub fn is_hamilton_path(&self, path: &[V]) -> bool {
+        match self.vertex_count() {
+            0 => path.is_empty(),
+            n => {
+                let edges_exist = path.windows(2).all(|w| self.has_edge((&w[0], &w[1])));
+                let has_all_verts_once =
+                    path.len() == n && self.vertices == path.iter().cloned().collect();
+
+                edges_exist && has_all_verts_once
+            }
+        }
+    }
+
+    /// check if a provided path is a hamilton cycle within the graph,
+    /// meaning it is a cycle and contains all of the graphs vertices exactly once.
+    ///
+    /// this assumes a path only containing the start/end vertex once,
+    /// so a valid 2-vertex-loop would be represented as `[v, w]` and not `[v, w, v]`.
+    ///
+    /// empty paths are never considered a cycle.
+    pub fn is_hamilton_cycle(&self, path: &[V]) -> bool {
+        !path.is_empty()
+            && self.is_hamilton_path(path)
+            && self.has_edge((path.last().unwrap(), path.first().unwrap()))
+    }
+
     pub fn iter(&self) -> impl Iterator<Item = &V> {
         self.vertices.iter()
     }
@@ -344,6 +374,36 @@ where
         None
     }
 
+    /// check if a provided path is a hamilton path within the graph,
+    /// meaning it contains all of the graphs vertices exactly once.
+    ///
+    /// an empty path is considered hamiltonian for empty graphs.
+    pub fn is_hamilton_path(&self, path: &[V]) -> bool {
+        match self.vertex_count() {
+            0 => path.is_empty(),
+            n => {
+                let edges_exist = path.windows(2).all(|w| self.has_edge((&w[0], &w[1])));
+                let has_all_verts_once =
+                    path.len() == n && self.vertices == path.iter().cloned().collect();
+
+                edges_exist && has_all_verts_once
+            }
+        }
+    }
+
+    /// check if a provided path is a hamilton cycle within the graph,
+    /// meaning it is a cycle and contains all of the graphs vertices exactly once.
+    ///
+    /// this assumes a path only containing the start/end vertex once,
+    /// so a valid 2-vertex-loop would be represented as `[v, w]` and not `[v, w, v]`.
+    ///
+    /// empty paths are never considered a cycle.
+    pub fn is_hamilton_cycle(&self, path: &[V]) -> bool {
+        !path.is_empty()
+            && self.is_hamilton_path(path)
+            && self.has_edge((path.last().unwrap(), path.first().unwrap()))
+    }
+
     pub fn iter(&self) -> impl Iterator<Item = &V> {
         self.vertices.iter()
     }

src/main.rs

@@ -104,6 +104,66 @@ fn main() {
         "Dijkstra should find cheapest way in weighted example graph"
     );
 
+    let path = ['a', 'b', 'c', 'd', 'e', 'f'];
+    assert!(
+        g.is_hamilton_path(&path),
+        "Valid hamilton path in graph should be recognized as such"
+    );
+
+    let path = ['a', 'd', 'c', 'b', 'e', 'f'];
+    assert!(
+        !g.is_hamilton_path(&path),
+        "A path with a non-existent edge is not hamiltonian"
+    );
+
+    let path = ['a', 'b', 'c', 'd', 'e'];
+    assert!(
+        !g.is_hamilton_path(&path),
+        "A path with missing vertices is not hamiltonian"
+    );
+
+    let path = ['f', 'e', 'd', 'c', 'b', 'a', 'b'];
+    assert!(
+        !g.is_hamilton_path(&path),
+        "A path with duplicate vertices is not hamiltonian"
+    );
+
+    // test graph:
+    //      ┏━━━━━1━━━━━┓
+    //   -> 0     ┃     3
+    //      ┗━━━━━2━━━━━┛
+
+    let g = graph! {
+        0: 1, 2;
+        1: 0, 2, 3;
+        2: 0, 1, 3;
+        3: 1, 2;
+    };
+
+    let path = [0, 1, 3, 2];
+    assert!(
+        g.is_hamilton_cycle(&path),
+        "Valid hamilton cycle in graph should be recognized as such"
+    );
+
+    let path = [0, 3, 1, 2];
+    assert!(
+        !g.is_hamilton_cycle(&path),
+        "A path with a non-existent edge is not a hamilton cycle"
+    );
+
+    let path = [0, 1, 2, 3];
+    assert!(
+        !g.is_hamilton_cycle(&path),
+        "Hamilton path that does not loop is not a hamilton cycle"
+    );
+
+    let path = [0, 1, 2];
+    assert!(
+        !g.is_hamilton_cycle(&path),
+        "Cycle that does not contain all vertices is not a hamilton cycle"
+    );
+
     // yay
     println!("All tests passed.");
 }