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+;;; cj-cache-lib.el --- Generic TTL cache with build-guard -*- lexical-binding: t; -*-
+
+;; Author: Craig Jennings <c@cjennings.net>
+
+;;; Commentary:
+
+;; Generic "rebuild a long-running computation behind a TTL" cache,
+;; with a building-flag guard that prevents duplicate concurrent
+;; rebuilds in async-build scenarios.
+;;
+;; Used by org-agenda-config and org-refile-config which previously
+;; carried parallel hand-rolled implementations of this exact shape.
+;; See docs/design/cache-helper-design.org for the API contract,
+;; consumer migration shape, and rationale for the deliberate "nil
+;; cached value reads as invalid" decision.
+;;
+;; Out of scope: buffer-local key-based caches like the modeline VC
+;; cache (different lifecycle).
+
+;;; Code:
+
+(require 'cl-lib)
+
+(defun cj/cache-make (&rest plist)
+  "Return a fresh cache state.
+PLIST keywords:
+- =:ttl= seconds to retain a built value (default 3600)."
+  (let ((ttl (or (plist-get plist :ttl) 3600)))
+    (list :value nil :time nil :ttl ttl :building nil)))
+
+(defun cj/cache-valid-p (cache)
+  "Return non-nil when CACHE has a fresh, non-nil value within its TTL.
+A nil cached value reads as invalid by design -- a build that legitimately
+returns nil rebuilds on the next request, matching the prior agenda/refile
+contract."
+  (let ((value (plist-get cache :value))
+        (time (plist-get cache :time))
+        (ttl (plist-get cache :ttl)))
+    (and value
+         time
+         (< (- (float-time) time) ttl))))
+
+(defun cj/cache-building-p (cache)
+  "Return non-nil when a build is currently in progress on CACHE."
+  (plist-get cache :building))
+
+(defun cj/cache-invalidate (cache)
+  "Clear CACHE's value and timestamp.  TTL is preserved."
+  (plist-put cache :value nil)
+  (plist-put cache :time nil))
+
+(cl-defun cj/cache-value-or-rebuild (cache build-fn
+                                           &key force-rebuild
+                                           on-hit
+                                           on-build-start
+                                           on-build-success
+                                           on-build-error)
+  "Return CACHE's value, calling BUILD-FN to rebuild when invalid.
+
+When CACHE is valid and FORCE-REBUILD is nil, return the stored value
+and call ON-HIT (if given) with the value.  Otherwise call BUILD-FN,
+store its result, and return it.
+
+The four callbacks let the consumer log without this helper printing on
+its behalf:
+- ON-HIT (value)
+- ON-BUILD-START ()
+- ON-BUILD-SUCCESS (value)
+- ON-BUILD-ERROR (err)
+
+The :building flag is set before BUILD-FN runs and cleared inside an
+`unwind-protect' regardless of outcome.  Errors from BUILD-FN are
+rethrown after ON-BUILD-ERROR fires."
+  (cond
+   ((and (not force-rebuild) (cj/cache-valid-p cache))
+    (let ((value (plist-get cache :value)))
+      (when on-hit (funcall on-hit value))
+      value))
+   (t
+    (when on-build-start (funcall on-build-start))
+    (plist-put cache :building t)
+    (unwind-protect
+        (condition-case err
+            (let ((value (funcall build-fn)))
+              (plist-put cache :value value)
+              (plist-put cache :time (float-time))
+              (when on-build-success (funcall on-build-success value))
+              value)
+          (error
+           (when on-build-error (funcall on-build-error err))
+           (signal (car err) (cdr err))))
+      (plist-put cache :building nil)))))
+
+(provide 'cj-cache-lib)
+;;; cj-cache-lib.el ends here