Add language percentages to repo pages

GitHub no longer server-renders the repository sidebar; its React UI
lazily fetches /:owner/:repo/_sidebar as JSON, which carries the
language breakdown (name, percentage, color). getGitHubLanguages() uses
that endpoint, and a new /api/:owner/:repo/languages fragment route
renders the bar and legend the same way refs and latest-commit already
work, so repo page loads still cost a single scrape.

The latest-commit placeholder loader in refs.js is generalized over
.placeholder[data-src] so both placeholders share it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

8 files changed, 139 insertions, 13 deletions

diff --git a/public/static/refs.js b/public/static/refs.js
index 28b8373..ffff505 100644
--- a/public/static/refs.js
+++ b/public/static/refs.js
@@ -1,8 +1,8 @@
-// Progressive enhancement for two bits of data GitHub itself loads lazily:
-// the branch/tag dropdown and the latest-commit bar above a file listing.
-// Both degrade gracefully without JS: the button's command/commandfor still
-// opens the (empty) dialog natively; the commit placeholder just never fills
-// in.
+// Progressive enhancement for the bits of data GitHub itself loads lazily:
+// the branch/tag dropdown, the latest-commit bar above a file listing, and the
+// language breakdown. All degrade gracefully without JS: the button's
+// command/commandfor still opens the (empty) dialog natively; the placeholders
+// just never fill in.
 document.addEventListener("DOMContentLoaded", () => {
   const dialog = document.querySelector(".ref-selector-menu");
   if (dialog) {
@@ -44,17 +44,16 @@ document.addEventListener("DOMContentLoaded", () => {
     }
   }
 
-  const commitPlaceholder = document.querySelector(".latest-commit-placeholder");
-  if (commitPlaceholder) {
-    fetch(commitPlaceholder.dataset.src)
+  for (const placeholder of document.querySelectorAll(".placeholder[data-src]")) {
+    fetch(placeholder.dataset.src)
       .then((res) => (res.ok ? res.text() : ""))
       .then((html) => {
         if (html.trim()) {
-          commitPlaceholder.outerHTML = html;
+          placeholder.outerHTML = html;
         } else {
-          commitPlaceholder.remove();
+          placeholder.remove();
         }
       })
-      .catch(() => commitPlaceholder.remove());
+      .catch(() => placeholder.remove());
   }
 });
diff --git a/public/static/style.css b/public/static/style.css
index 983cb86..ed64bc6 100644
--- a/public/static/style.css
+++ b/public/static/style.css
@@ -218,10 +218,45 @@ body > footer {
   font-size: 0.85rem;
 }
 
-.latest-commit-placeholder {
+.placeholder {
   opacity: 0.7;
 }
 
+.languages {
+  .language-bar {
+    display: flex;
+    height: .5rem;
+    border-radius: .25rem;
+    overflow: hidden;
+
+    span {
+      /* Keep slivers of tiny languages visible. */
+      min-width: 1px;
+    }
+  }
+
+  .language-list {
+    display: flex;
+    flex-wrap: wrap;
+    gap: .2rem 1rem;
+    list-style: none;
+    margin: .5rem 0 0;
+    padding: 0;
+    font-size: 0.85rem;
+  }
+
+  .language-dot {
+    display: inline-block;
+    width: .6rem;
+    height: .6rem;
+    border-radius: 50%;
+  }
+
+  .language-percentage {
+    opacity: 0.7;
+  }
+}
+
 .repo-info {
   .description {
     margin-bottom: .2rem;
diff --git a/src/app.ts b/src/app.ts
index ca79b1f..e34c7b1 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -5,6 +5,7 @@ import {
   getGitHubBlob,
   getGitHubCommits,
   getGitHubIssues,
+  getGitHubLanguages,
   getGitHubLatestCommit,
   getGitHubOwner,
   getGitHubPulls,
@@ -158,6 +159,11 @@ export function createApp(eta: Eta) {
     );
   });
 
+  app.get("/api/:owner/:repo/languages", async (c) => {
+    const { owner, repo } = c.req.param();
+    return tryRenderFragment(c, "_languages.eta", getGitHubLanguages(owner, repo));
+  });
+
   app.get("/:owner/:repo/releases/tag/:tag", async (c) => {
     const { owner, repo, tag } = c.req.param();
     return tryRender(c, "release.eta", getGitHubRelease(owner, repo, tag));
diff --git a/src/scraper.test.ts b/src/scraper.test.ts
index 56f14b0..5b5e78b 100644
--- a/src/scraper.test.ts
+++ b/src/scraper.test.ts
@@ -4,6 +4,7 @@ import {
   getGitHubBlob,
   getGitHubCommits,
   getGitHubIssues,
+  getGitHubLanguages,
   getGitHubLatestCommit,
   getGitHubPulls,
   getGitHubRefs,
@@ -219,6 +220,33 @@ describe("GitHub scraper", () => {
       assert.ok(data.commit.authors.length > 0);
     });
 
+    it("should fetch languages", async () => {
+      const data = await getGitHubLanguages("actions", "deploy-pages");
+
+      assert.strictEqual(data.repo.owner, "actions");
+      assert.strictEqual(data.repo.name, "deploy-pages");
+
+      assert.deepStrictEqual(data.languages, [
+        { name: "JavaScript", percentage: 100, color: "#f1e05a" },
+      ]);
+    });
+
+    it("should fetch languages for a multi-language repository", async () => {
+      const data = await getGitHubLanguages("rust-lang", "rust");
+
+      const rust = data.languages.find((language) => language.name === "Rust");
+      assert.ok(rust);
+      assert.ok(rust.percentage > 50);
+      assert.strictEqual(rust.color, "#dea584");
+
+      // The aggregate bucket comes back without a name.
+      const other = data.languages.find((language) => language.name === "Other");
+      assert.ok(other);
+
+      const total = data.languages.reduce((sum, language) => sum + language.percentage, 0);
+      assert.ok(Math.abs(total - 100) < 1);
+    });
+
     it("should fetch branches", async () => {
       const data = await getGitHubRefs("actions", "deploy-pages", "branches");
 
diff --git a/src/scraper.ts b/src/scraper.ts
index 39ad2cd..223839f 100644
--- a/src/scraper.ts
+++ b/src/scraper.ts
@@ -285,6 +285,17 @@ export interface GitHubLatestCommit extends GitHubCommon {
   commit: LatestCommit;
 }
 
+export interface Language {
+  // "Other" for the aggregate bucket GitHub reports without a name.
+  name: string;
+  percentage: number;
+  color: string | null;
+}
+
+export interface GitHubLanguages extends GitHubCommon {
+  languages: Language[];
+}
+
 export interface Ref {
   name: string;
   isDefault?: boolean;
@@ -722,6 +733,34 @@ export async function getGitHubLatestCommit(
   };
 }
 
+interface SidebarPayload {
+  languages?: {
+    languages: {
+      // Absent for the "Other" bucket.
+      name?: string;
+      percentage: number;
+      color?: string;
+    }[];
+  };
+}
+
+// The repository sidebar (languages, contributors, ...) is not rendered into
+// the repo page at all; GitHub's own UI lazily fetches it as JSON from here.
+export async function getGitHubLanguages(owner: string, repo: string): Promise<GitHubLanguages> {
+  const payload = await fetchGitHubJson<SidebarPayload>(`${owner}/${repo}/_sidebar`);
+
+  const languages: Language[] = [];
+  for (const entry of payload.languages?.languages ?? []) {
+    languages.push({
+      name: entry.name ?? "Other",
+      percentage: entry.percentage,
+      color: entry.color ?? null,
+    });
+  }
+
+  return { repo: { owner, name: repo }, languages };
+}
+
 interface BranchRefEntry {
   name: string;
   isDefault: boolean;
diff --git a/views/_languages.eta b/views/_languages.eta
new file mode 100644
index 0000000..8aca447
--- /dev/null
+++ b/views/_languages.eta
@@ -0,0 +1,17 @@
+<% if (it.languages.length > 0) { %>
+  <div class="languages box">
+    <div class="language-bar">
+      <% for (const lang of it.languages) { %>
+        <span style="width: <%= lang.percentage %>%; background: <%= lang.color ?? "#ededed" %>" title="<%= lang.name %> <%= lang.percentage %>%"></span>
+      <% } %>
+    </div>
+    <ul class="language-list">
+      <% for (const lang of it.languages) { %>
+        <li>
+          <span class="language-dot" style="background: <%= lang.color ?? "#ededed" %>"></span>
+          <%= lang.name %> <span class="language-percentage"><%= lang.percentage %>%</span>
+        </li>
+      <% } %>
+    </ul>
+  </div>
+<% } %>
diff --git a/views/_tree.eta b/views/_tree.eta
index a0ecfab..2eb445f 100644
--- a/views/_tree.eta
+++ b/views/_tree.eta
@@ -1,7 +1,7 @@
 <%~ include("./_nav.eta", {...it, isTree: true}) %>
 
 <% const commitPath = it.path && it.path !== "/" ? it.path : ""; %>
-<div class="latest-commit-placeholder box" data-src="/api/<%= it.repo.owner %>/<%= it.repo.name %>/latest-commit/<%= it.branch %>/<%= commitPath %>">Loading latest commit…</div>
+<div class="placeholder box" data-src="/api/<%= it.repo.owner %>/<%= it.repo.name %>/latest-commit/<%= it.branch %>/<%= commitPath %>">Loading latest commit…</div>
 
 <table>
   <% for (const item of it.items) { %>
diff --git a/views/repo.eta b/views/repo.eta
index 683a9ad..9122594 100644
--- a/views/repo.eta
+++ b/views/repo.eta
@@ -29,4 +29,6 @@
   </div>
 <% } %>
 
+<div class="placeholder box" data-src="/api/<%= it.repo.owner %>/<%= it.repo.name %>/languages">Loading languages…</div>
+
 <%~ include("./_tree", it) %>