tell user they might downgrade in update popup

src/singletons/Updates.cpp

@@ -13,6 +13,7 @@
 #include <QDesktopServices>
 #include <QMessageBox>
 #include <QProcess>
+#include <QRegularExpression>
 
 namespace chatterino {
 namespace {
@@ -20,6 +21,38 @@ namespace {
     {
         return getSettings()->betaUpdates ? "beta" : "stable";
     }
+
+    /// Checks if the online version is newer or older than the current version.
+    bool isDowngradeOf(const QString &online, const QString &current)
+    {
+        static auto matchVersion =
+            QRegularExpression(R"((\d+)(?:\.(\d+))?(?:\.(\d+))?(?:\.(\d+))?)");
+
+        // Versions are just strings, they don't need to follow a specific
+        // format so we can only assume if one version is newer than another
+        // one.
+
+        // We match x.x.x.x with each version level being optional.
+
+        auto onlineMatch = matchVersion.match(online);
+        auto currentMatch = matchVersion.match(current);
+
+        for (int i = 1; i <= 4; i++)
+        {
+            if (onlineMatch.captured(i).toInt() <
+                currentMatch.captured(i).toInt())
+            {
+                return true;
+            }
+            if (onlineMatch.captured(i).toInt() >
+                currentMatch.captured(i).toInt())
+            {
+                break;
+            }
+        }
+
+        return false;
+    }
 }  // namespace
 
 Updates::Updates()
@@ -263,6 +296,8 @@ void Updates::checkForUpdates()
             if (this->currentVersion_ != this->onlineVersion_)
             {
                 this->setStatus_(UpdateAvailable);
+                this->isDowngrade_ =
+                    isDowngradeOf(this->onlineVersion_, this->currentVersion_);
             }
             else
             {
@@ -309,6 +344,11 @@ bool Updates::isError() const
     }
 }
 
+bool Updates::isDowngrade() const
+{
+    return this->isDowngrade_;
+}
+
 void Updates::setStatus_(Status status)
 {
     if (this->status_ != status)