gstreamer: Add some API to calculate the next state convert state changes into their component states and back

gstreamer/src/enums.rs

@@ -11,6 +11,8 @@ use thiserror::Error;
 use ClockReturn;
 use FlowReturn;
 use PadLinkReturn;
+use State;
+use StateChange;
 use StateChangeReturn;
 
 use glib::translate::*;
@@ -631,3 +633,55 @@ impl SetValue for MessageType {
         gobject_sys::g_value_set_flags(value.to_glib_none_mut().0, this.to_glib())
     }
 }
+
+impl State {
+    pub fn next(self, pending: Self) -> Self {
+        let current = self.to_glib();
+        let pending = pending.to_glib();
+
+        let sign = (pending - current).signum();
+
+        from_glib(current + sign)
+    }
+}
+
+impl StateChange {
+    pub fn new(current: State, next: State) -> Self {
+        skip_assert_initialized!();
+        let current = current.to_glib();
+        let next = next.to_glib();
+        from_glib((current << 3) | next)
+    }
+
+    pub fn current(self) -> State {
+        match self {
+            StateChange::NullToReady => State::Null,
+            StateChange::ReadyToPaused => State::Ready,
+            StateChange::PausedToPlaying => State::Paused,
+            StateChange::PlayingToPaused => State::Playing,
+            StateChange::PausedToReady => State::Paused,
+            StateChange::ReadyToNull => State::Ready,
+            StateChange::NullToNull => State::Null,
+            StateChange::ReadyToReady => State::Ready,
+            StateChange::PausedToPaused => State::Paused,
+            StateChange::PlayingToPlaying => State::Playing,
+            StateChange::__Unknown(value) => State::__Unknown(value >> 3),
+        }
+    }
+
+    pub fn next(self) -> State {
+        match self {
+            StateChange::NullToReady => State::Ready,
+            StateChange::ReadyToPaused => State::Paused,
+            StateChange::PausedToPlaying => State::Playing,
+            StateChange::PlayingToPaused => State::Paused,
+            StateChange::PausedToReady => State::Ready,
+            StateChange::ReadyToNull => State::Null,
+            StateChange::NullToNull => State::Null,
+            StateChange::ReadyToReady => State::Ready,
+            StateChange::PausedToPaused => State::Paused,
+            StateChange::PlayingToPlaying => State::Playing,
+            StateChange::__Unknown(value) => State::__Unknown(value & 0x7),
+        }
+    }
+}