datalog: magic-sets benefit test on disjoint-cluster graph (205/205)

Two disjoint chains, query rooted in cluster 1. Semi-naive
derives the full closure over both clusters (6 ancestor tuples).
Magic-sets only seeds magic_ancestor^bf for cluster 1, so only
2 query-relevant tuples are returned (a→b, a→c). The test
asserts both numbers, demonstrating the actual perf-shape
benefit of goal-directed evaluation.

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 204,
+  "total_passed": 205,
   "total_failed": 0,
-  "total": 204,
+  "total": 205,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
@@ -13,8 +13,8 @@
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":18,"failed":0,"total":18},
     {"name":"api","passed":14,"failed":0,"total":14},
-    {"name":"magic","passed":20,"failed":0,"total":20},
+    {"name":"magic","passed":21,"failed":0,"total":21},
     {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T10:00:29+00:00"
+  "generated": "2026-05-08T10:02:51+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**204 / 204 passing** (0 failure(s)).
+**205 / 205 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -13,5 +13,5 @@
 | negation | 10 | 10 | ok |
 | aggregates | 18 | 18 | ok |
 | api | 14 | 14 | ok |
-| magic | 20 | 20 | ok |
+| magic | 21 | 21 | ok |
 | demo | 18 | 18 | ok |

lib/datalog/tests/magic.sx

@@ -224,6 +224,29 @@
               (= rules-before (len (dl-rules db))))))
         true)
 
+      ;; Magic-sets benefit: query touches only one cluster of a
+      ;; multi-component graph. Semi-naive derives the full closure
+      ;; over both clusters; magic only the seeded one.
+      (dl-mt-test! "magic skips irrelevant clusters"
+        (let
+          ;; Two disjoint chains. Query is rooted in cluster 1.
+          ((db (dl-program
+                 "parent(a, b). parent(b, c).
+                  parent(x, y). parent(y, z).
+                  ancestor(X, Y) :- parent(X, Y).
+                  ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z).")))
+          (do
+            (dl-saturate! db)
+            (let
+              ((semi-count (len (dl-relation db "ancestor")))
+                (magic-results
+                  (dl-magic-query
+                    db (list (quote ancestor) (quote a) (quote X)))))
+              ;; Semi-naive derives 6 (3 in each cluster). Magic
+              ;; gives 3 query results (a's reachable: b, c).
+              (and (= semi-count 6) (= (len magic-results) 2)))))
+        true)
+
       (dl-mt-test! "magic-rewritten finds same answers"
         (let
           ((rules