From 49d14859e96647a8dfa5f11f3eebe6c6c036c396 Mon Sep 17 00:00:00 2001
From: dalance <dalance@gmail.com>
Date: Tue, 25 Jun 2024 10:08:03 +0900
Subject: [PATCH] Add hreflang

---
 book/theme/head.hbs | 38 +++++++++++++++++++++++++++++---------
 1 file changed, 29 insertions(+), 9 deletions(-)

diff --git a/book/theme/head.hbs b/book/theme/head.hbs
index 92cbc9d7..03750751 100644
--- a/book/theme/head.hbs
+++ b/book/theme/head.hbs
@@ -13,23 +13,43 @@
         // See these pages for details:
         // https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls
         // https://developers.google.com/search/docs/crawling-indexing/javascript/javascript-seo-basics
-        let base = "https://doc.veryl-lang.org/book";
+
+        function gen_canonical_href(lang) {
+            let base = "https://doc.veryl-lang.org/book";
+            let canonical_href;
+            if (lang == "en") {
+                canonical_href = `${base}/{{ path }}`;
+            } else {
+                canonical_href = `${base}/${lang}/{{ path }}`;
+            }
+            canonical_href = canonical_href.slice(0, -"md".length) + "html";
+            if (canonical_href.endsWith("/index.html")) {
+                canonical_href = canonical_href.slice(0, -"index.html".length);
+            }
+            return canonical_href;
+        }
+
         {{#if (eq language "en")}}
-        let canonical_href = `${base}/{{ path }}`;
+        const canonical_href = gen_canonical_href("en");
         {{else}}
-        let canonical_href = `${base}/{{ language }}/{{ path }}`;
+        const canonical_href = gen_canonical_href("{{ language }}");
         {{/if}}
 
-        // mdbook gives us a string ending in ".md", we replace it with ".html":
-        canonical_href = canonical_href.slice(0, -"md".length) + "html";
-        if (canonical_href.endsWith("/index.html")) {
-            canonical_href = canonical_href.slice(0, -"index.html".length);
-        }
-
         let link = document.createElement("link");
         link.rel = "canonical";
         link.href = canonical_href;
         document.head.appendChild(link);
+
+        const langs = ["en", "ja"];
+        for (const lang of langs) {
+            const canonical_href = gen_canonical_href(lang);
+
+            let link = document.createElement("link");
+            link.rel = "alternate";
+            link.hreflang = lang;
+            link.href = canonical_href;
+            document.head.appendChild(link);
+        }
     })()
 </script>