sx-git Phase 3: commit DAG — log/ancestry/merge-base SX-side + Datalog bridge (TDD)

Topo log = reverse DFS postorder over parent edges from commit objects;
reflexive is-ancestor?, all-LCA merge-bases (criss-cross verified). Datalog
bridge exports (git-parent child parent) facts under a minimal 2-rule
ancestor closure, cross-checked against the SX walk. 30/30, total 106/106.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/git/tests/dag.sx

@@ -0,0 +1,172 @@
+; Phase 3 — dag: log (topo), ancestry, is-ancestor?, reachability, merge-base,
+; and the minimal Datalog ancestor-closure bridge.
+; Fixtures: a linear chain, a diamond merge, a criss-cross (two LCAs),
+; and two unrelated roots.
+
+(define gdt-db (persist/mem-backend))
+(define gdt (git/repo gdt-db))
+
+(define
+  gdt-commit!
+  (fn
+    (msg parents)
+    (git/write gdt (git/commit (git/write-blob gdt msg) parents {:message msg}))))
+
+; linear chain c1 <- c2 <- c3
+(define gdt-c1 (gdt-commit! "c1" (list)))
+(define gdt-c2 (gdt-commit! "c2" (list gdt-c1)))
+(define gdt-c3 (gdt-commit! "c3" (list gdt-c2)))
+
+; diamond: b1 <- p, b1 <- q, m = merge(p q)
+(define gdt-b1 (gdt-commit! "b1" (list)))
+(define gdt-p (gdt-commit! "p" (list gdt-b1)))
+(define gdt-q (gdt-commit! "q" (list gdt-b1)))
+(define gdt-m (gdt-commit! "m" (list gdt-p gdt-q)))
+
+; criss-cross: base <- a, base <- b; x = merge(a b), y = merge(a b) via
+; distinct messages so x != y
+(define gdt-base (gdt-commit! "base" (list)))
+(define gdt-a (gdt-commit! "a" (list gdt-base)))
+(define gdt-b (gdt-commit! "b" (list gdt-base)))
+(define gdt-x (gdt-commit! "x" (list gdt-a gdt-b)))
+(define gdt-y (gdt-commit! "y" (list gdt-a gdt-b)))
+
+; unrelated root
+(define gdt-lone (gdt-commit! "lone" (list)))
+
+; ---- parents ----
+(git-test
+  "parents of a root commit"
+  (= (git/parents gdt gdt-c1) (list))
+  true)
+(git-test
+  "parents of a merge commit"
+  (= (git/parents gdt gdt-m) (list gdt-p gdt-q))
+  true)
+(git-test
+  "parents of a non-commit cid"
+  (= (git/parents gdt "sx1:junk") (list))
+  true)
+
+; ---- reachability ----
+(git-test
+  "reachable includes self"
+  (git/dag-member? gdt-c3 (git/reachable gdt gdt-c3))
+  true)
+(git-test
+  "reachable spans the chain"
+  (= (git/reachable gdt gdt-c3) (list gdt-c1 gdt-c2 gdt-c3))
+  true)
+(git-test
+  "reachable covers both diamond legs"
+  (len (git/reachable gdt gdt-m))
+  4)
+(git-test
+  "reachable-all merges from several heads without duplicates"
+  (len (git/reachable-all gdt (list gdt-x gdt-y)))
+  5)
+
+; ---- log: children before parents ----
+(git-test
+  "log on a chain is newest-first"
+  (= (git/log gdt gdt-c3) (list gdt-c3 gdt-c2 gdt-c1))
+  true)
+(git-test
+  "log messages read newest-first"
+  (= (git/log-messages gdt gdt-c3) (list "c3" "c2" "c1"))
+  true)
+(git-test
+  "log of a diamond is a valid topo order"
+  (= (git/log gdt gdt-m) (list gdt-m gdt-q gdt-p gdt-b1))
+  true)
+(git-test
+  "log of a root is just the root"
+  (= (git/log gdt gdt-c1) (list gdt-c1))
+  true)
+
+; ---- ancestors / is-ancestor? ----
+(git-test
+  "ancestors excludes self"
+  (git/dag-member? gdt-c3 (git/ancestors gdt gdt-c3))
+  false)
+(git-test
+  "ancestors of the chain tip"
+  (= (git/ancestors gdt gdt-c3) (list gdt-c1 gdt-c2))
+  true)
+(git-test
+  "ancestors of a merge include both legs and the base"
+  (len (git/ancestors gdt gdt-m))
+  3)
+(git-test
+  "is-ancestor? along the chain"
+  (git/is-ancestor? gdt gdt-c1 gdt-c3)
+  true)
+(git-test
+  "is-ancestor? is directed"
+  (git/is-ancestor? gdt gdt-c3 gdt-c1)
+  false)
+(git-test
+  "is-ancestor? is reflexive"
+  (git/is-ancestor? gdt gdt-c2 gdt-c2)
+  true)
+(git-test
+  "is-ancestor? false across unrelated history"
+  (git/is-ancestor? gdt gdt-c1 gdt-lone)
+  false)
+
+; ---- merge-base ----
+(git-test
+  "merge-base of the diamond legs is the base"
+  (git/merge-base gdt gdt-p gdt-q)
+  gdt-b1)
+(git-test
+  "merge-base when one side is an ancestor is that side"
+  (git/merge-base gdt gdt-c1 gdt-c3)
+  gdt-c1)
+(git-test "merge-base with self" (git/merge-base gdt gdt-c3 gdt-c3) gdt-c3)
+(git-test
+  "merge-base of unrelated commits is nil"
+  (git/merge-base gdt gdt-c3 gdt-lone)
+  nil)
+(git-test
+  "merge-base is symmetric"
+  (equal? (git/merge-base gdt gdt-p gdt-q) (git/merge-base gdt gdt-q gdt-p))
+  true)
+(git-test
+  "criss-cross yields BOTH best common ancestors"
+  (=
+    (git/merge-bases gdt gdt-x gdt-y)
+    (artdag/sort-strings (list gdt-a gdt-b)))
+  true)
+(git-test
+  "dominated common ancestor is not a merge-base"
+  (git/dag-member? gdt-base (git/merge-bases gdt gdt-x gdt-y))
+  false)
+
+; ---- Datalog bridge ----
+(git-test
+  "dag-facts exports one fact per parent edge"
+  (len (git/dag-facts gdt (list gdt-m)))
+  4)
+(git-test
+  "datalog ancestors match the SX-side walk"
+  (let
+    ((db (git/dag-db gdt (list gdt-m))))
+    (=
+      (git/ancestors-dl db gdt-m)
+      (artdag/sort-strings (git/ancestors gdt gdt-m))))
+  true)
+(git-test
+  "datalog is-ancestor? positive"
+  (git/is-ancestor-dl? (git/dag-db gdt (list gdt-m)) gdt-b1 gdt-m)
+  true)
+(git-test
+  "datalog is-ancestor? negative"
+  (git/is-ancestor-dl? (git/dag-db gdt (list gdt-m)) gdt-m gdt-b1)
+  false)
+(git-test
+  "datalog closure spans the chain"
+  (=
+    (git/ancestors-dl (git/dag-db gdt (list gdt-c3)) gdt-c3)
+    (artdag/sort-strings (list gdt-c1 gdt-c2)))
+  true)