content: snapshot cache over op-log replay (snapshot.sx) + 20 tests (338/338)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/content/snapshot.sx

@@ -0,0 +1,90 @@
+;; content-on-sx — snapshot cache over the op-log replay.
+;;
+;; Snapshots are a CACHE, never primary state: the op log stays the source of
+;; truth. A snapshot stores a materialised document at a sequence in the persist
+;; KV; cached reads start from it and replay only the tail of ops, so they return
+;; a document IDENTICAL to a full replay — just faster. Drop the snapshot and
+;; nothing is lost.
+;;
+;; Requires (loaded by harness): store.sx (+ doc.sx, persist event/log/kv/api).
+
+(define content/-snap-key (fn (doc-id) (str "content-snap:" doc-id)))
+
+;; take a snapshot of the current head at the current version. Returns the seq.
+(define
+  content/snapshot!
+  (fn
+    (b doc-id)
+    (let
+      ((seq (content/version-count b doc-id)))
+      (begin (persist/kv-put b (content/-snap-key doc-id) {:doc (content/head b doc-id) :seq seq}) seq))))
+
+(define
+  content/-snapshot
+  (fn
+    (b doc-id)
+    (if
+      (persist/kv-has? b (content/-snap-key doc-id))
+      (persist/kv-get b (content/-snap-key doc-id))
+      nil)))
+
+(define
+  content/snapshot-seq
+  (fn
+    (b doc-id)
+    (let
+      ((s (content/-snapshot b doc-id)))
+      (if (= s nil) 0 (get s :seq)))))
+
+(define
+  content/has-snapshot?
+  (fn (b doc-id) (persist/kv-has? b (content/-snap-key doc-id))))
+
+(define
+  content/drop-snapshot!
+  (fn (b doc-id) (persist/kv-delete b (content/-snap-key doc-id))))
+
+;; ── cached reads (transparent: identical result to store.sx replay) ──
+(define
+  content/-tail-ops
+  (fn
+    (b doc-id from to)
+    (map
+      (fn (ev) (persist/event-data ev))
+      (filter
+        (fn
+          (ev)
+          (and
+            (> (persist/event-seq ev) from)
+            (<= (persist/event-seq ev) to)))
+        (content/log b doc-id)))))
+
+(define
+  content/head-cached
+  (fn
+    (b doc-id)
+    (let
+      ((snap (content/-snapshot b doc-id)))
+      (if
+        (= snap nil)
+        (content/head b doc-id)
+        (doc-apply-all
+          (get snap :doc)
+          (content/-tail-ops
+            b
+            doc-id
+            (get snap :seq)
+            (content/version-count b doc-id)))))))
+
+(define
+  content/at-cached
+  (fn
+    (b doc-id seq)
+    (let
+      ((snap (content/-snapshot b doc-id)))
+      (if
+        (or (= snap nil) (< seq (get snap :seq)))
+        (content/at b doc-id seq)
+        (doc-apply-all
+          (get snap :doc)
+          (content/-tail-ops b doc-id (get snap :seq) seq))))))