docs: add a version switch.

.github/workflows/main.yaml

@@ -193,6 +193,38 @@ jobs:
           branch: main
           folder: docs/
           target-folder: docs/amaranth/${{ github.ref_name }}/
+      - name: Generate list of versions
+        if: ${{ github.event_name == 'push' && (github.event.ref == 'refs/heads/main' || startsWith(github.event.ref, 'refs/tags/v') && !contains(github.event.ref, 'dev')) }}
+        run: |
+          git clone --depth=1 [email protected]:amaranth-lang/amaranth-lang.github.io.git ~/amaranth-lang.github.io
+          cd ~/amaranth-lang.github.io/docs/amaranth/
+
+          (
+          venv_path=$(mktemp -d)
+          python3 -m venv "$venv_path"
+          source "$venv_path"/bin/activate
+          pip install packaging
+          python >versions.json <<EOF
+          import json
+          from pathlib import Path
+          from packaging.version import Version
+
+          versions = Path('.').glob('v*')
+          versions = (path.name[1:] for path in versions if path.is_dir())
+          versions = sorted(versions, key=Version, reverse=True)
+          versions = ({"name": version, "root_url": f"/docs/amaranth/v{version}/"} for version in versions)
+          print(json.dumps(list(versions), indent=2))
+          EOF
+          )
+
+          git add versions.json
+          if ! git diff-index --quiet --cached HEAD; then
+            git \
+              -c user.name="$GITHUB_ACTOR" \
+              -c user.email="[email protected]" \
+              commit -m "Update docs/amaranth/versions.json"
+            git push
+          fi
 
   publish-docs-dev:
     needs: document

docs/_static/version-switch.js

@@ -0,0 +1,111 @@
+document.addEventListener('DOMContentLoaded', () => {
+  let contentRoot = new URL(
+    document.documentElement.dataset.content_root ?? DOCUMENTATION_OPTIONS.URL_ROOT,
+    window.location.href,
+  );
+
+  function insertVersionSwitch(versions) {
+    let versionElement = document.querySelector('.wy-side-nav-search > .version');
+    if (!versionElement) return;
+
+    let root = document.createElement('div');
+    root.innerHTML = `
+      <div class="switch-menus">
+        <div class="version-switch"><select></select></div>
+      </div>
+    `;
+
+    let switchMenus = root.firstElementChild;
+    let versionSwitch = switchMenus.firstElementChild;
+    let versionSwitchSelect = versionSwitch.firstElementChild;
+
+    let versionElementStyles = getComputedStyle(versionElement);
+    let cssStyleSheet = new CSSStyleSheet();
+    cssStyleSheet.replaceSync(String.raw`
+      .wy-side-nav-search > div.switch-menus {
+        margin-top: ${versionElementStyles.marginTop};
+        margin-bottom: ${versionElementStyles.marginBottom};
+        color: ${versionElementStyles.color};
+
+        div.version-switch {
+          select {
+            display: inline-block;
+            margin-right: -2rem;
+            padding-right: 2rem;
+            text-align-last: center;
+            background: none;
+            border: none;
+            border-radius: 0em;
+            box-shadow: none;
+            font-family: inherit;
+            font-size: 1em;
+            font-weight: normal;
+            color: inherit;
+            cursor: pointer;
+            appearance: none;
+
+            &:hover, &:active, &:focus {
+              background: rgba(255, 255, 255, .1);
+              color: rgba(255, 255, 255, .5);
+            }
+
+            option {
+              color: black;
+            }
+          }
+
+          &:has(> select):after {
+            display: inline-block;
+            width: 1.5em;
+            height: 100%;
+            padding: .1em;
+            content: "\f0d7";
+            font-size: 1em;
+            line-height: 1.2em;
+            font-family: FontAwesome;
+            text-align: center;
+            pointer-events: none;
+            box-sizing: border-box;
+          }
+        }
+      }
+    `);
+    document.adoptedStyleSheets.push(cssStyleSheet);
+
+    let currentVersion = DOCUMENTATION_OPTIONS.VERSION;
+    if (!versions.includes(currentVersion)) {
+      versions = [
+        { name: currentVersion, root_url: contentRoot },
+        ...versions,
+      ];
+    }
+
+    for (let { name, root_url: rootURL } of versions) {
+      rootURL = new URL(rootURL, window.location.href);
+
+      let versionOptionElement = document.createElement('option');
+      versionOptionElement.textContent = name;
+      versionOptionElement.dataset.url = rootURL;
+
+      if (name === currentVersion) {
+        versionOptionElement.selected = true;
+      }
+
+      versionSwitchSelect.appendChild(versionOptionElement);
+    }
+
+    versionSwitchSelect.addEventListener('change', (event) => {
+      let option = event.target.selectedIndex;
+      let item = event.target.options[option];
+      window.location.href = item.dataset.url;
+    });
+
+    versionElement.replaceWith(switchMenus);
+  }
+
+  fetch(new URL('../versions.json', contentRoot)).then(async (response) => {
+    if (response.status !== 200) return;
+    let versions = await response.json();
+    insertVersionSwitch(versions);
+  });
+});

docs/conf.py

@@ -57,6 +57,7 @@
 html_theme = "sphinx_rtd_theme"
 html_static_path = ["_static"]
 html_css_files = ["custom.css"]
+html_js_files = ["version-switch.js"]
 html_logo = "_static/logo.png"
 
 rst_prolog = """