From 2e72de2281034b69a7133533e12713f4243cd020 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stefan=20M=C3=BCller?= <stefan.mueller@aksdb.de>
Date: Sat, 4 Oct 2025 22:23:08 +0200
Subject: [PATCH] Add experimental Graph implementation

---
 src/main.rs | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)

diff --git a/src/main.rs b/src/main.rs
index e7a11a9..bff4b7a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,76 @@
+#[derive(Copy, Clone, PartialEq, Eq)]
+struct Vertex(usize);
+
+#[derive(Copy, Clone, PartialEq, Eq)]
+struct Incidence(usize);
+
+struct VertexIncidenceHeader {
+    incidence_count: usize,
+    first_incidence: Option<Incidence>,
+}
+
+// TODO: Visibility of Graph members.
+struct Graph {
+    // TODO: Index 'vertex_incidence_headers' by a 'Vertex' instead of 'Vertex.0'?
+    vertex_incidence_headers: Vec<VertexIncidenceHeader>,
+    next_incidences: Vec<Option<Incidence>>,
+    incidence_vertices: Vec<Vertex>,
+}
+
+// TODO: Add iterator of vertex neighbors, see 'fn add_adjacent()'.
+impl Graph {
+    fn vertex_count(&self) -> usize {
+        self.vertex_incidence_headers.len()
+    }
+
+    fn edge_count(&self) -> usize {
+        self.next_incidences.len() / 2
+    }
+
+    fn degree(&self, v: Vertex) -> usize {
+        self.vertex_incidence_headers[v.0].incidence_count
+    }
+
+    // TODO: 'fn are_adjacent()' could probably be done with a neighbor vertex iterator.
+    fn are_adjacent(&self, v1: Vertex, v2: Vertex) -> bool {
+        let Some(next) = self.vertex_incidence_headers[v1.0].first_incidence else {
+            return false;
+        };
+        loop {
+            if self.incidence_vertices[next.0] == v2 {
+                return true;
+            }
+            let Some(next) = self.next_incidences[next.0] else {
+                return false;
+            };
+        }
+    }
+
+    // TODO: 'fn add_vertex()' should not be publicly accessible for all graph implementations.
+    // TODO: 'fn add_vertex()' needs variants for custom vertex data.
+    fn add_vertex(&mut self) -> Vertex {
+        self.vertex_incidence_headers.push(VertexIncidenceHeader {
+            incidence_count: 0,
+            first_incidence: None,
+        });
+        Vertex(self.vertex_incidence_headers.len() - 1)
+    }
+
+    // TODO: 'fn add_edge()' should not be publicly accessible for all graph implementations.
+    // TODO: 'fn add_edge()' needs variants for custom edge data.
+    fn add_edge(&mut self, v1: Vertex, v2: Vertex) {
+        self.add_incidence(v1, v2);
+        self.add_incidence(v2, v1);
+    }
+
+    fn add_incidence(&mut self, v1: Vertex, v2: Vertex) {
+        self.incidence_vertices.push(v2);
+        self.next_incidences.push(self.vertex_incidence_headers[v1.0].first_incidence.take());
+        self.vertex_incidence_headers[v1.0].incidence_count += 1;
+        self.vertex_incidence_headers[v1.0].first_incidence = Some(Incidence(self.incidence_vertices.len() - 1));
+    }
+}
+
 fn main() {
     println!("Hello, world!");
 }