From 4e8104f11f81ca2a5e7f800f4601662508f6f885 Mon Sep 17 00:00:00 2001
From: Tom Willemse <tom@ryuslash.org>
Date: Tue, 25 Jul 2023 12:21:16 -0700
Subject: [PATCH] Only generate blog index when necessary

---
 publish.el | 46 ++++++++++++++++++++++++++++++++++++----------
 1 file changed, 36 insertions(+), 10 deletions(-)

diff --git a/publish.el b/publish.el
index 1b3bac2..e06b0f1 100644
--- a/publish.el
+++ b/publish.el
@@ -52,7 +52,9 @@
           time
           (if (= time 1) "" "s")))
 
-(defun publish-get-summary-from-file (file props)
+(defun publish-extract-summary-from-file (file props)
+  "Extract a summary from FILE.
+PROPS is used as an aid in getting the right information from the file."
   (format "* %s\n:PROPERTIES:\n:CUSTOM_ID:  %s\n:PUBDATE:  %s\n:RSS_PERMALINK:  %s\n:END:\n\n%s\n\n[[file:%s][Read More]]\n\n"
           (car (org-publish-find-property file :title props))
           (file-name-nondirectory file)
@@ -66,19 +68,43 @@
                                 (list file)))
           (file-name-nondirectory file)))
 
+(defun publish-get-latest-modified-time (files)
+  "Get the latest modification time of any file from the list FILES."
+  (car
+   (last
+    (sort (mapcar #'org-publish-cache-mtime-of-src files) #'time-less-p))))
+
+(defun publish-time>= (a b)
+  "Check if time A is greater than or equal to time B."
+  (not (time-less-p a b)))
+
+(defun publish-empty-time ()
+  "Get an empty time value."
+  (let ((current-time (current-time)))
+    (time-subtract current-time current-time)))
+
 (defun publish-generate-index (props)
   "Generate an index from my posts.
 Argument PROPS
 ."
-  (let ((files (directory-files "posts/" t (rx bos (= 8 digit) "-" (= 4 digit) "-" (one-or-more nonl) (not "~") eos))))
-    (with-temp-buffer
-      (insert "#+title: ryuslash's blog\n")
-      (insert "#+options: num:nil\n")
-      (insert "\n")
-      (apply 'insert
-             (mapcar (lambda (file) (publish-get-summary-from-file file props))
-                     (take 30 (reverse files))))
-      (write-file "posts/index.org"))))
+  (let* ((index-file (expand-file-name "posts/index.org"))
+         (index-generated-time (or (and (file-exists-p index-file)
+                                        (org-publish-cache-mtime-of-src index-file))
+                                   (publish-empty-time)))
+         (files (directory-files "posts/" t (rx bos (= 8 digit) "-" (= 4 digit) "-" (one-or-more nonl) (not "~") eos)))
+         (latest-modification-time (publish-get-latest-modified-time files)))
+    (if (publish-time>= index-generated-time latest-modification-time)
+        (message "Not generating index...")
+      (progn
+        (message "Generating index...")
+        (with-temp-buffer
+          (insert "#+title: ryuslash's blog\n")
+          (insert "#+options: num:nil\n")
+          (insert "\n")
+          (apply 'insert
+                 (mapcar (lambda (file) (publish-extract-summary-from-file file props))
+                         (take 30 (reverse files))))
+          (write-file "posts/index.org"))))))
 
 (setq org-export-exclude-tags '("noexport" "draft"))