libdeng2|App|Config: Remember the old version when detecting an upgrade

The old version is available as Version.OLD_VERSION.

Also, de::Value now has the as<> template for easy casting.

doomsday/libdeng2/include/de/core/config.h

@@ -23,6 +23,7 @@
 #include "../Process"
 #include "../String"
 #include "../Path"
+#include "../Version"
 
 namespace de {
 
@@ -164,6 +165,12 @@ class DENG2_PUBLIC Config
      */
     Variable const &operator [] (String const &name) const;
 
+    /**
+     * Returns the old version, when a new installed version has been detected.
+     * If no upgrade has occurred, returns the current version.
+     */
+    Version upgradedFromVersion() const;
+
 private:
     DENG2_PRIVATE(d)
 };

doomsday/libdeng2/include/de/core/version.h

@@ -87,6 +87,10 @@ class Version
 
     bool operator == (Version const &other) const;
 
+    bool operator != (Version const &other) const {
+        return !(*this == other);
+    }
+
     bool operator > (Version const &other) const;
 
     /**

doomsday/libdeng2/include/de/data/value.h

@@ -85,6 +85,20 @@ class DENG2_PUBLIC Value : public String::IPatternArg, public ISerializable
      */
     virtual Text asText() const = 0;
 
+    template <typename ValueType>
+    ValueType &as() {
+        ValueType *t = dynamic_cast<ValueType *>(this);
+        if(!t) throw ConversionError("Value::as<>", "Illegal type conversion");
+        return *t;
+    }
+
+    template <typename ValueType>
+    ValueType const &as() const {
+        ValueType const *t = dynamic_cast<ValueType const *>(this);
+        if(!t) throw ConversionError("Value::as<>", "Illegal const type conversion");
+        return *t;
+    }
+
     /**
      * Determine the size of the value.  The meaning of this
      * depends on the type of the value.

doomsday/libdeng2/src/core/app.cpp

@@ -350,6 +350,17 @@ void App::initSubsystems(SubsystemInitFlags flags)
 
     d->config->read();
 
+    // Immediately after upgrading, OLD_VERSION is also present in the Version module.
+    Version oldVer = d->config->upgradedFromVersion();
+    if(oldVer != Version())
+    {
+        ArrayValue *old = new ArrayValue;
+        *old << NumberValue(oldVer.major) << NumberValue(oldVer.minor)
+             << NumberValue(oldVer.patch) << NumberValue(oldVer.build);
+        d->scriptSys.nativeModule("Version").addArray("OLD_VERSION", old).setReadOnly();
+    }
+
+    // Set up the log buffer.
     LogBuffer &logBuf = LogBuffer::appBuffer();
 
     // Update the log buffer max entry count: number of items to hold in memory.

doomsday/libdeng2/src/core/config.cpp

@@ -42,10 +42,27 @@ DENG2_PIMPL_NOREF(Config)
     /// The configuration namespace.
     Process config;
 
+    /// Previous installed version (__version__ in the read persistent Config).
+    Version oldVersion;
+
     Instance(Path const &path)
         : configPath(path),
           persistentPath("modules/Config")
     {}
+
+    void setOldVersion(Value const &old)
+    {
+        try
+        {
+            ArrayValue const &vers = old.as<ArrayValue>();
+            oldVersion.major = int(vers.at(0).asNumber());
+            oldVersion.minor = int(vers.at(1).asNumber());
+            oldVersion.patch = int(vers.at(2).asNumber());
+            oldVersion.build = int(vers.at(3).asNumber());
+        }
+        catch(...)
+        {}
+    }
 };
 
 Config::Config(Path const &path) : d(new Instance(path))
@@ -89,6 +106,7 @@ void Config::read()
 
         // If the saved config is from a different version, rerun the script.
         Value const &oldVersion = names()["__version__"].value();
+        d->setOldVersion(oldVersion);
         if(oldVersion.compare(*version))
         {
             // Version mismatch: store the old version in a separate variable.
@@ -162,6 +180,11 @@ Variable const &Config::operator [] (String const &name) const
     return names()[name];
 }
 
+Version Config::upgradedFromVersion() const
+{
+    return d->oldVersion;
+}
+
 Value &Config::get(String const &name) const
 {
     return d->config.globals()[name].value();