1 files changed, 39 insertions, 0 deletions
diff --git a/src/data/post.ts b/src/data/post.ts
index 85cc0d0..08a6678 100644
--- a/src/data/post.ts
+++ b/src/data/post.ts
@@ -1,4 +1,5 @@
 import { type CollectionEntry, getCollection } from "astro:content";
+import { collectionDateSort } from "@/utils/date";
 
 /** filter out draft posts based on the environment and optionally archived posts */
 export async function getAllPosts(includeArchived = false): Promise<CollectionEntry<"post">[]> {
@@ -62,3 +63,41 @@ export function getUniqueTagsWithCount(posts: CollectionEntry<"post">[]): [strin
 		),
 	].sort((a, b) => b[1] - a[1]);
 }
+
+export type PostCategory = "regular" | "microblog" | "archived";
+
+/** Determine the category of a post based on its tags. "archived" takes precedence over "microblog". */
+export function getPostCategory(post: CollectionEntry<"post">): PostCategory {
+	const tags = post.data.tags;
+	if (tags.includes("archived")) return "archived";
+	if (tags.includes("microblog")) return "microblog";
+	return "regular";
+}
+
+export interface PostNavigation {
+	prevPost: CollectionEntry<"post"> | null;
+	nextPost: CollectionEntry<"post"> | null;
+}
+
+/** Get previous (newer) and next (older) posts within the same category.
+ *  Posts are sorted newest-first. prevPost = newer, nextPost = older.
+ */
+export function getPostNavigation(
+	allPosts: CollectionEntry<"post">[],
+	currentPost: CollectionEntry<"post">,
+): PostNavigation {
+	const category = getPostCategory(currentPost);
+	const sameCategoryPosts = allPosts
+		.filter((p) => getPostCategory(p) === category)
+		.sort(collectionDateSort);
+
+	const currentIndex = sameCategoryPosts.findIndex((p) => p.id === currentPost.id);
+
+	return {
+		prevPost: currentIndex > 0 ? (sameCategoryPosts[currentIndex - 1] ?? null) : null,
+		nextPost:
+			currentIndex < sameCategoryPosts.length - 1
+				? (sameCategoryPosts[currentIndex + 1] ?? null)
+				: null,
+	};
+}