feat: add optional tag-based RSS feed generation

- Add 'rssTags' option to ContentIndex plugin for flexible tag-based RSS feeds
- Implement RSS feed generation for specific tags
- Add deduplication and slugification of tags
- Output feeds to 'tags/<tag>/index.xml'

Author: suryaanshrai

quartz/plugins/emitters/contentIndex.tsx

@@ -2,7 +2,15 @@ import { Root } from "hast"
 import { GlobalConfiguration } from "../../cfg"
 import { getDate } from "../../components/Date"
 import { escapeHTML } from "../../util/escape"
-import { FilePath, FullSlug, SimpleSlug, joinSegments, simplifySlug } from "../../util/path"
+import {
+  FilePath,
+  FullSlug,
+  SimpleSlug,
+  getAllSegmentPrefixes,
+  joinSegments,
+  simplifySlug,
+  slugTag,
+} from "../../util/path"
 import { QuartzEmitterPlugin } from "../types"
 import { toHtml } from "hast-util-to-html"
 import { write } from "./helpers"
@@ -28,6 +36,9 @@ interface Options {
   rssFullHtml: boolean
   rssSlug: string
   includeEmptyFiles: boolean
+  includeTags: boolean
+  rssTagsLimit: number
+  rssTags: string[]
 }
 
 const defaultOptions: Options = {
@@ -37,6 +48,9 @@ const defaultOptions: Options = {
   rssFullHtml: false,
   rssSlug: "index",
   includeEmptyFiles: true,
+  includeTags: false,
+  rssTagsLimit: 15,
+  rssTags: [],
 }
 
 function generateSiteMap(cfg: GlobalConfiguration, idx: ContentIndexMap): string {
@@ -135,6 +149,62 @@ export const ContentIndex: QuartzEmitterPlugin<Partial<Options>> = (opts) => {
           slug: (opts?.rssSlug ?? "index") as FullSlug,
           ext: ".xml",
         })
+
+        if (opts?.includeTags) {
+          // Optimization: Build a map of tag -> content list once (reverse index)
+          const tagsInput: Map<string, ContentDetails[]> = new Map()
+
+          // Iterate over the content index once to populate the reverse index
+          for (const [_, content] of linkIndex) {
+            const tags = content.tags.flatMap(getAllSegmentPrefixes)
+            for (const tag of new Set(tags)) {
+              // Use Set to avoid double counting per file
+              if (!tagsInput.has(tag)) {
+                tagsInput.set(tag, [])
+              }
+              tagsInput.get(tag)!.push(content)
+            }
+          }
+
+          let sortedTags: string[] = []
+
+          if (opts.rssTags && opts.rssTags.length > 0) {
+            // Deduplicate and slugify user-provided tags
+            const userTags = new Set(opts.rssTags.map((tag) => slugTag(tag)))
+
+            // Filter user tags to only those that exist in the content
+            sortedTags = Array.from(userTags).filter((tag) => tagsInput.has(tag))
+          } else if ((opts.rssTagsLimit ?? 0) > 0) {
+            // Sort available tags by frequency (number of content items)
+            sortedTags = Array.from(tagsInput.entries())
+              .sort((a, b) => b[1].length - a[1].length) // Sort by frequency descending
+              .slice(0, opts.rssTagsLimit)
+              .map(([tag]) => tag)
+          }
+
+          if (sortedTags.length === 0) {
+            console.warn(
+              "[contentIndex] includeTags is enabled, but no tag-based RSS feeds will be generated. " +
+                "Either provide non-empty `rssTags` matching content tags or set `rssTagsLimit` to a positive number.",
+            )
+          }
+
+          for (const tag of sortedTags) {
+            const tagContent = tagsInput.get(tag)
+            if (!tagContent) continue // Should not happen given logic above
+
+            // Reconstruct a map for generateRSSFeed (it expects a ContentIndexMap)
+            // We can optimize this by making generateRSSFeed accept an array, but for now we conform to the interface
+            const tagFilteredIndex = new Map(tagContent.map((content) => [content.slug, content]))
+
+            yield write({
+              ctx,
+              content: generateRSSFeed(cfg, tagFilteredIndex, opts.rssLimit),
+              slug: joinSegments("tags", tag, "index") as FullSlug,
+              ext: ".xml",
+            })
+          }
+        }
       }
 
       const fp = joinSegments("static", "contentIndex") as FullSlug