[doc] Separate latest-stable from latest release in selectversions.js

Now latest-stable will always point to the latest *stable* release, not
just the latest release.
Latest stable is calculated as (version % 4 == 0), which is true for all
releases after 632 (which are the only ones we're concerned with, since
the previous are not "latest" anymore :-)

Author: silverweed

doc/selectversion.js

@@ -27,25 +27,37 @@ function url2version(patharr) {
 }
 
 ///=============================================================================
-const LATEST_VERSION = 636; // CHANGE THIS WHEN A NEW VERSION IS RELEASED
+
+// The latest release. NOT necessarily equal to latest-stable, as stable releases are only
+// once every two versions.
+// >>> CHANGE THIS WHEN A NEW VERSION IS RELEASED! (stable or not) <<<
+const LATEST_VERSION = 638; 
+const LATEST_VERSION_IS_STABLE = LATEST_VERSION % 4 === 0;
+const LATEST_STABLE = LATEST_VERSION - 2 * !LATEST_VERSION_IS_STABLE;
 const FIRST_VERSION = 610;
 
 // Redirect from latest-stable to the actual latest release
 let patharr = window.location.pathname.replace(/\/+/g, '/').split('/');
 if (url2version(patharr) === 'latest-stable') {
-   patharr[patharr.length - urlrootdirs] = `v${LATEST_VERSION}`;
+   patharr[patharr.length - urlrootdirs] = `v${LATEST_STABLE}`;
    let newLocation = patharr.join('/');
    if (window.location.hash)
       newLocation += `#${window.location.hash}`;
    window.location.replace(newLocation);
 }
 
-const LATEST_STABLE_IDX = 1;
 let versions = ["master", "latest-stable"];
 for (let i = LATEST_VERSION; i >= FIRST_VERSION; i -= 2) {
    versions.push(`v${i}`);
 }
 
+// Index of the "latest-stable" element in `versions`. Never changes.
+const LATEST_STABLE_IDX = 1;
+// Index of the actual version that latest-stable redirects to in `versions`.
+// It's either the one immediately after latest-stable or 2 places after it (depending
+// whether the latest release is stable or not).
+const LATEST_STABLE_REDIRECT_IDX = LATEST_STABLE_IDX + 1 + !LATEST_VERSION_IS_STABLE;
+
 let thisvers = url2version(patharr);
 $('.dropbtn').html("Version " + url2label(thisvers));
 
@@ -59,21 +71,26 @@ for (let i = 0; i < versions.length; ++i) {
    a.innerText = url2label(version);
    dropdown.append(a);
 
-   // latest-stable redirects to the actual latest version
+   // latest-stable redirects to the actual latest stable version
    if (i === LATEST_STABLE_IDX) {
       // this gets checked already by the actual latest version, so don't waste
       // an AJAX request for it.
       latestStableLink = a;
       continue;
    }
 
+   // Enable all links that are reachable by a HEAD request.
    let url = `${baseUrl}/${version}/${patharr[patharr.length-1]}`;
    let request = new XMLHttpRequest();
    request.open('HEAD', url, true);
    request.onreadystatechange = () => {
       let linksToChange = [a];
-      if (i === LATEST_STABLE_IDX + 1)
+
+      // Normally we only enable the link relative to the HEAD request we just did, but in case this is
+      // the latest stable version we also enable the "latest-stable" link.
+      if (i === LATEST_STABLE_REDIRECT_IDX)
          linksToChange.push(latestStableLink);
+
       for (let link of linksToChange) {
          if (request.readyState === 4 && request.status === 404) {
             link.style['color'] = "gray";