O(1) undo

card_game/src/lib.rs

@@ -331,14 +331,32 @@ where
 	config: G::Config,
 	state: SessionState<G>,
 }
-#[derive(Clone, Eq, Hash, PartialEq)]
+#[derive(Clone)]
+pub struct StateSnapshot<G: Game>
+where
+	G::Instruction: Clone,
+{
+	state: G,
+	instruction: G::Instruction,
+}
+impl<G: Game> StateSnapshot<G>
+where
+	G::Instruction: Clone,
+{
+	pub const fn state(&self) -> &G {
+		&self.state
+	}
+	pub const fn instruction(&self) -> &G::Instruction {
+		&self.instruction
+	}
+}
+#[derive(Clone)]
 pub struct SessionState<G: Game>
 where
 	G::Instruction: Clone,
 {
-	seed: G,
 	state: G,
-	history: Vec<G::Instruction>,
+	history: Vec<StateSnapshot<G>>,
 }
 impl<G: Game + Clone> SessionState<G>
 where
@@ -346,7 +364,6 @@ where
 {
 	fn new(state: G) -> Self {
 		Self {
-			seed: state.clone(),
 			state,
 			history: Vec::new(),
 		}
@@ -381,7 +398,7 @@ where
 	pub const fn config(&self) -> &G::Config {
 		&self.config
 	}
-	pub fn history(&self) -> &[G::Instruction] {
+	pub fn history(&self) -> &[StateSnapshot<G>] {
 		&self.state.history
 	}
 	pub fn undo(&mut self) {
@@ -401,7 +418,7 @@ where
 	pub fn is_win(&self) -> bool {
 		self.state.is_win()
 	}
-	pub fn is_winnable(&self) -> Option<Vec<G::Instruction>> {
+	pub fn is_winnable(&self) -> Option<Vec<StateSnapshot<G>>> {
 		let mut state_moves = std::collections::HashMap::new();
 		let mut state = self.clone();
 		while !state.is_win() {
@@ -456,19 +473,16 @@ where
 	) {
 		match instruction {
 			SessionInstruction::Undo => {
-				// replay the entire history of the game except one move
+				if let Some(snapshot) = self.history.pop() {
-				self.history.pop();
+					self.state = snapshot.state;
-				let mut inner_stats = G::Stats::default();
+					stats.increment_undos();
-				let mut state = self.seed.clone();
-				for instruction in &self.history {
-					state.process_instruction(&mut inner_stats, config, instruction.clone());
 				}
-				self.state = state;
-				stats.inner_stats = inner_stats;
-				stats.increment_undos();
 			}
 			SessionInstruction::InnerInstruction(instruction) => {
-				self.history.push(instruction.clone());
+				self.history.push(StateSnapshot {
+					state: self.state.clone(),
+					instruction: instruction.clone(),
+				});
 				self.state
 					.process_instruction(&mut stats.inner_stats, config, instruction);
 			}