test(mnemosyne): graph cluster analysis tests — 22 tests

- graph_clusters: empty, orphans, linked pairs, separate clusters, topics, density
- hub_entries: ordering, limit, inbound/outbound counting
- bridge_entries: triangle (none), chain (B is bridge), small cluster filtered
- rebuild_links: creates links, threshold override, persistence

nexus/mnemosyne/tests/test_graph_clusters.py

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+"""Tests for Mnemosyne graph cluster analysis features.
+
+Tests: graph_clusters, hub_entries, bridge_entries, rebuild_links.
+"""
+
+import pytest
+from pathlib import Path
+import tempfile
+
+from nexus.mnemosyne.archive import MnemosyneArchive
+from nexus.mnemosyne.entry import ArchiveEntry
+
+
+@pytest.fixture
+def archive():
+    """Create a fresh archive in a temp directory."""
+    with tempfile.TemporaryDirectory() as tmp:
+        path = Path(tmp) / "test_archive.json"
+        a = MnemosyneArchive(archive_path=path)
+        yield a
+
+
+def _make_entry(title="Test", content="test content", topics=None):
+    return ArchiveEntry(title=title, content=content, topics=topics or [])
+
+
+class TestGraphClusters:
+    """Test graph_clusters() connected component discovery."""
+
+    def test_empty_archive(self, archive):
+        clusters = archive.graph_clusters()
+        assert clusters == []
+
+    def test_single_orphan(self, archive):
+        archive.add(_make_entry("Lone entry"), auto_link=False)
+        # min_size=1 includes orphans
+        clusters = archive.graph_clusters(min_size=1)
+        assert len(clusters) == 1
+        assert clusters[0]["size"] == 1
+        assert clusters[0]["density"] == 0.0
+
+    def test_single_orphan_filtered(self, archive):
+        archive.add(_make_entry("Lone entry"), auto_link=False)
+        clusters = archive.graph_clusters(min_size=2)
+        assert clusters == []
+
+    def test_two_linked_entries(self, archive):
+        """Two manually linked entries form a cluster."""
+        e1 = archive.add(_make_entry("Alpha dogs", "canine training"), auto_link=False)
+        e2 = archive.add(_make_entry("Beta cats", "feline behavior"), auto_link=False)
+        # Manual link
+        e1.links.append(e2.id)
+        e2.links.append(e1.id)
+        archive._save()
+
+        clusters = archive.graph_clusters(min_size=2)
+        assert len(clusters) == 1
+        assert clusters[0]["size"] == 2
+        assert clusters[0]["internal_edges"] == 1
+        assert clusters[0]["density"] == 1.0  # 1 edge out of 1 possible
+
+    def test_two_separate_clusters(self, archive):
+        """Two disconnected groups form separate clusters."""
+        a1 = archive.add(_make_entry("AI models", "neural networks"), auto_link=False)
+        a2 = archive.add(_make_entry("AI training", "gradient descent"), auto_link=False)
+        b1 = archive.add(_make_entry("Cooking pasta", "italian recipes"), auto_link=False)
+        b2 = archive.add(_make_entry("Cooking sauces", "tomato basil"), auto_link=False)
+
+        # Link cluster A
+        a1.links.append(a2.id)
+        a2.links.append(a1.id)
+        # Link cluster B
+        b1.links.append(b2.id)
+        b2.links.append(b1.id)
+        archive._save()
+
+        clusters = archive.graph_clusters(min_size=2)
+        assert len(clusters) == 2
+        sizes = sorted(c["size"] for c in clusters)
+        assert sizes == [2, 2]
+
+    def test_cluster_topics(self, archive):
+        """Cluster includes aggregated topics."""
+        e1 = archive.add(_make_entry("Alpha", "content", topics=["ai", "models"]), auto_link=False)
+        e2 = archive.add(_make_entry("Beta", "content", topics=["ai", "training"]), auto_link=False)
+        e1.links.append(e2.id)
+        e2.links.append(e1.id)
+        archive._save()
+
+        clusters = archive.graph_clusters(min_size=2)
+        assert "ai" in clusters[0]["top_topics"]
+
+    def test_density_calculation(self, archive):
+        """Triangle (3 nodes, 3 edges) has density 1.0."""
+        e1 = archive.add(_make_entry("A", "aaa"), auto_link=False)
+        e2 = archive.add(_make_entry("B", "bbb"), auto_link=False)
+        e3 = archive.add(_make_entry("C", "ccc"), auto_link=False)
+        # Fully connected triangle
+        for e, others in [(e1, [e2, e3]), (e2, [e1, e3]), (e3, [e1, e2])]:
+            for o in others:
+                e.links.append(o.id)
+        archive._save()
+
+        clusters = archive.graph_clusters(min_size=2)
+        assert len(clusters) == 1
+        assert clusters[0]["internal_edges"] == 3
+        assert clusters[0]["density"] == 1.0  # 3 edges / 3 possible
+
+    def test_chain_density(self, archive):
+        """A-B-C chain has density 2/3 (2 edges out of 3 possible)."""
+        e1 = archive.add(_make_entry("A", "aaa"), auto_link=False)
+        e2 = archive.add(_make_entry("B", "bbb"), auto_link=False)
+        e3 = archive.add(_make_entry("C", "ccc"), auto_link=False)
+        # Chain: A-B-C
+        e1.links.append(e2.id)
+        e2.links.extend([e1.id, e3.id])
+        e3.links.append(e2.id)
+        archive._save()
+
+        clusters = archive.graph_clusters(min_size=2)
+        assert abs(clusters[0]["density"] - 2/3) < 0.01
+
+
+class TestHubEntries:
+    """Test hub_entries() degree centrality ranking."""
+
+    def test_empty(self, archive):
+        assert archive.hub_entries() == []
+
+    def test_no_links(self, archive):
+        archive.add(_make_entry("Lone"), auto_link=False)
+        assert archive.hub_entries() == []
+
+    def test_hub_ordering(self, archive):
+        """Entry with most links is ranked first."""
+        e1 = archive.add(_make_entry("Hub", "central node"), auto_link=False)
+        e2 = archive.add(_make_entry("Spoke 1", "content"), auto_link=False)
+        e3 = archive.add(_make_entry("Spoke 2", "content"), auto_link=False)
+        e4 = archive.add(_make_entry("Spoke 3", "content"), auto_link=False)
+
+        # e1 connects to all spokes
+        e1.links.extend([e2.id, e3.id, e4.id])
+        e2.links.append(e1.id)
+        e3.links.append(e1.id)
+        e4.links.append(e1.id)
+        archive._save()
+
+        hubs = archive.hub_entries()
+        assert len(hubs) == 4
+        assert hubs[0]["entry"].id == e1.id
+        assert hubs[0]["degree"] == 3
+
+    def test_limit(self, archive):
+        e1 = archive.add(_make_entry("A", ""), auto_link=False)
+        e2 = archive.add(_make_entry("B", ""), auto_link=False)
+        e1.links.append(e2.id)
+        e2.links.append(e1.id)
+        archive._save()
+
+        assert len(archive.hub_entries(limit=1)) == 1
+
+    def test_inbound_outbound(self, archive):
+        """Inbound counts links TO an entry, outbound counts links FROM it."""
+        e1 = archive.add(_make_entry("Source", ""), auto_link=False)
+        e2 = archive.add(_make_entry("Target", ""), auto_link=False)
+        # Only e1 links to e2
+        e1.links.append(e2.id)
+        archive._save()
+
+        hubs = archive.hub_entries()
+        h1 = next(h for h in hubs if h["entry"].id == e1.id)
+        h2 = next(h for h in hubs if h["entry"].id == e2.id)
+        assert h1["inbound"] == 0
+        assert h1["outbound"] == 1
+        assert h2["inbound"] == 1
+        assert h2["outbound"] == 0
+
+
+class TestBridgeEntries:
+    """Test bridge_entries() articulation point detection."""
+
+    def test_empty(self, archive):
+        assert archive.bridge_entries() == []
+
+    def test_no_bridges_in_triangle(self, archive):
+        """Fully connected triangle has no articulation points."""
+        e1 = archive.add(_make_entry("A", ""), auto_link=False)
+        e2 = archive.add(_make_entry("B", ""), auto_link=False)
+        e3 = archive.add(_make_entry("C", ""), auto_link=False)
+        for e, others in [(e1, [e2, e3]), (e2, [e1, e3]), (e3, [e1, e2])]:
+            for o in others:
+                e.links.append(o.id)
+        archive._save()
+
+        assert archive.bridge_entries() == []
+
+    def test_bridge_in_chain(self, archive):
+        """A-B-C chain: B is the articulation point."""
+        e1 = archive.add(_make_entry("A", ""), auto_link=False)
+        e2 = archive.add(_make_entry("B", ""), auto_link=False)
+        e3 = archive.add(_make_entry("C", ""), auto_link=False)
+        e1.links.append(e2.id)
+        e2.links.extend([e1.id, e3.id])
+        e3.links.append(e2.id)
+        archive._save()
+
+        bridges = archive.bridge_entries()
+        assert len(bridges) == 1
+        assert bridges[0]["entry"].id == e2.id
+        assert bridges[0]["components_after_removal"] == 2
+
+    def test_no_bridges_in_small_cluster(self, archive):
+        """Two-node clusters are too small for bridge detection."""
+        e1 = archive.add(_make_entry("A", ""), auto_link=False)
+        e2 = archive.add(_make_entry("B", ""), auto_link=False)
+        e1.links.append(e2.id)
+        e2.links.append(e1.id)
+        archive._save()
+
+        assert archive.bridge_entries() == []
+
+
+class TestRebuildLinks:
+    """Test rebuild_links() full recomputation."""
+
+    def test_empty_archive(self, archive):
+        assert archive.rebuild_links() == 0
+
+    def test_creates_links(self, archive):
+        """Rebuild creates links between similar entries."""
+        archive.add(_make_entry("Alpha dogs canine training", "obedience training"), auto_link=False)
+        archive.add(_make_entry("Beta dogs canine behavior", "behavior training"), auto_link=False)
+        archive.add(_make_entry("Cat food feline nutrition", "fish meals"), auto_link=False)
+
+        total = archive.rebuild_links()
+        assert total > 0
+
+        # Check that dog entries are linked to each other
+        entries = list(archive._entries.values())
+        dog_entries = [e for e in entries if "dog" in e.title.lower()]
+        assert any(len(e.links) > 0 for e in dog_entries)
+
+    def test_override_threshold(self, archive):
+        """Lower threshold creates more links."""
+        archive.add(_make_entry("Alpha dogs", "training"), auto_link=False)
+        archive.add(_make_entry("Beta cats", "training"), auto_link=False)
+        archive.add(_make_entry("Gamma birds", "training"), auto_link=False)
+
+        # Very low threshold = more links
+        low_links = archive.rebuild_links(threshold=0.01)
+
+        # Reset
+        for e in archive._entries.values():
+            e.links = []
+
+        # Higher threshold = fewer links
+        high_links = archive.rebuild_links(threshold=0.9)
+
+        assert low_links >= high_links
+
+    def test_rebuild_persists(self, archive):
+        """Rebuild saves to disk."""
+        archive.add(_make_entry("Alpha dogs", "training"), auto_link=False)
+        archive.add(_make_entry("Beta dogs", "training"), auto_link=False)
+        archive.rebuild_links()
+
+        # Reload and verify links survived
+        archive2 = MnemosyneArchive(archive_path=archive.path)
+        entries = list(archive2._entries.values())
+        total_links = sum(len(e.links) for e in entries)
+        assert total_links > 0