Enhanced the related posts.

If the number of related posts is less than 3, use the latest posts to supplement.

_includes/related-posts.html

@@ -1,21 +1,34 @@
 <!--
- The related posts of current post.
- Placed in the bottom of every single post.
+ Recommend the other 3 posts according to the tags and categories of the current post,
+ if the number is not enough, use the other latest posts to supplement.
+
  v2.0
  https://github.com/cotes2020/jekyll-theme-chirpy
  © 2019 Cotes Chung
  Published under the MIT License
 -->
 
-{% assign MAX_SIZE = 3 %}
+<!-- The total size of related posts  -->
+{% assign TOTAL_SIZE = 3 %}
+
+<!-- An random integer that bigger than 0  -->
 {% assign TAG_SCORE = 1 %}
+
+<!-- Equals to TAG_SCORE / {max_categories_hierarchy}  -->
 {% assign CATEGORY_SCORE = 0.5 %}
 
-{% assign score_list = "" | split: "" %}
-{% assign post_index = 0 %}
+{% assign SEPARATOR = ":" %}
+
+{% assign score_list = "" | split: "" %}
+{% assign last_index = site.posts.size | minus: 1 %}
+
+{% for i in (0..last_index) %}
+  {% assign post = site.posts[i] %}
+
+  {% if post.url == page.url %}
+    {% continue %}
+  {% endif %}
 
-{% for post in site.posts %}
-  {% if post.url != page.url %}
   {% assign score = 0 %}
 
   {% for tag in post.tags %}
@@ -31,23 +44,51 @@
   {% endfor %}
 
   {% if score > 0 %}
-      {% capture score_item %}{{ score }}:{{ post_index }}{% endcapture %}
+    {% capture score_item %}{{ score }}{{ SEPARATOR }}{{ i }}{% endcapture %}
     {% assign score_list = score_list | push: score_item %}
   {% endif %}
 
-  {% endif %}
-  {% assign post_index = post_index | plus: 1 %}
 {% endfor %}
 
+
+{% assign index_list = "" | split: "" %}
+
 {% if score_list.size > 0 %}
   {% assign score_list = score_list | sort | reverse %}
-  {% assign count = 0 %}
+  {% for entry in score_list limit: TOTAL_SIZE %}
+    {% assign index = entry | split: SEPARATOR | last %}
+    {% assign index_list = index_list | push: index %}
+  {% endfor %}
+{% endif %}
+
+<!-- Fill with the other newlest posts  -->
+{% assign less = TOTAL_SIZE | minus: index_list.size %}
+
+{% if less > 0 %}
+
+  {% for i in (0..last_index) %}
+    {% assign post = site.posts[i] %}
+    {% if post.url != page.url  %}
+      {% capture cur_index %}{{ i }}{% endcapture %}
+      {% unless index_list contains cur_index %}
+        {% assign index_list = index_list | push: cur_index %}
+        {% assign less = less | minus: 1 %}
+        {% if less <= 0 %}
+          {% break %}
+        {% endif %}
+      {% endunless %}
+    {% endif %}
+  {% endfor %}
+
+{% endif %}
+
+
+{% if index_list.size > 0 %}
   <div id="related-posts" class="mt-4 mb-2 mb-sm-4 pb-2">
     <h3 class="pt-2 mt-1 mb-4 ml-1" data-toc-skip>{{ site.data.label.post.relate_posts }}</h3>
     <div class="card-deck mb-4">
-    {% for score_item in score_list %}
-      {% assign data = score_item | split: ":" %}
-      {% assign index = data[1] | plus: 0 %}
+    {% for entry in index_list %}
+      {% assign index = entry | plus: 0 %}
       {% assign post = site.posts[index] %}
       <div class="card">
         <a href="{{ post.url | relative_url }}">
@@ -65,11 +106,7 @@
           </div>
         </a>
       </div>
-      {% assign count = count | plus: 1 %}
-      {% if count >= MAX_SIZE %}
-        {% break %}
-      {% endif %}
     {% endfor %}
     </div> <!-- .card-deck -->
-  </div>
+  </div> <!-- #related-posts -->
 {% endif %}