Abstract time-travel feature in TimeMachine struct

devtools/src/time_machine.rs

@@ -0,0 +1,88 @@
+use crate::Program;
+
+#[cfg(feature = "time-travel")]
+pub struct TimeMachine<P>
+where
+    P: Program,
+{
+    state: Option<P::State>,
+    messages: Vec<P::Message>,
+}
+
+#[cfg(feature = "time-travel")]
+impl<P> TimeMachine<P>
+where
+    P: Program,
+{
+    pub fn new() -> Self {
+        Self {
+            state: None,
+            messages: Vec::new(),
+        }
+    }
+
+    pub fn is_rewinding(&self) -> bool {
+        self.state.is_some()
+    }
+
+    pub fn push(&mut self, message: &P::Message) {
+        self.messages.push(message.clone());
+    }
+
+    pub fn rewind(&mut self, program: &P, message: usize) {
+        let (mut state, _) = program.boot();
+
+        if message < self.messages.len() {
+            // TODO: Run concurrently (?)
+            for message in &self.messages[0..message] {
+                let _ = program.update(&mut state, message.clone());
+            }
+        }
+
+        self.state = Some(state);
+        crate::debug::disable();
+    }
+
+    pub fn go_to_present(&mut self) {
+        self.state = None;
+        crate::debug::enable();
+    }
+
+    pub fn state(&self) -> Option<&P::State> {
+        self.state.as_ref()
+    }
+}
+
+#[cfg(not(feature = "time-travel"))]
+pub struct TimeMachine<P>
+where
+    P: Program,
+{
+    _program: std::marker::PhantomData<P>,
+}
+
+#[cfg(not(feature = "time-travel"))]
+impl<P> TimeMachine<P>
+where
+    P: Program,
+{
+    pub fn new() -> Self {
+        Self {
+            _program: std::marker::PhantomData,
+        }
+    }
+
+    pub fn is_rewinding(&self) -> bool {
+        false
+    }
+
+    pub fn push(&mut self, _message: &P::Message) {}
+
+    pub fn rewind(&mut self, _program: &P, _message: usize) {}
+
+    pub fn go_to_present(&mut self) {}
+
+    pub fn state(&self) -> Option<&P::State> {
+        None
+    }
+}