detect impossible puzzle

src/main.rs

@@ -47,6 +47,8 @@ fn main() {
     
     if let Err(reason) = solver.solve(args.limit) {
         println!("{}", reason);
+        if args.debug {
         	solver.display(solver::ui::DisplayMode::Full);
         }
+    }
 }

src/solver/cell.rs

@@ -57,6 +57,10 @@ impl Cell {
 		}
 	}
 
+	pub fn get_state(&self) -> Option<usize> {
+		self.state
+	}
+
 	pub fn get_allowed(&self) -> Vec<usize> {
 		self.blocking_states
 	        .iter()
@@ -107,16 +111,19 @@ impl Cell {
 	}
 
 	pub fn remove_allowed(&mut self, blocking_cells: &Vec<BlockingCell>) -> Result<RemoveResult, CellError> {
-		if !self.state.is_none() {
-			return Ok(RemoveResult::Filled)
-		}
-
 		for blocking_cell in blocking_cells {
 	        if let Some(blocking) = self.blocking_states.get_mut(&blocking_cell.state) {
 	            blocking.push(blocking_cell.position);
 	        }
 	    }
 
+	    if let Some(state) = self.state {
+	    	if self.blocking_states[&state].len() > 0 {
+	    		return Err(CellError::StateNotAllowed)
+	    	}
+   			return Ok(RemoveResult::Filled)
+   		}
+		
 		let allowed_states = self.get_allowed();
 		
 	    match allowed_states.len() {

src/solver/mod.rs

@@ -15,7 +15,7 @@ impl fmt::Display for WaveError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
         	WaveError::Contradiction => write!(f, "Error: The puzzle contradicts itself."),
-            WaveError::NoHistory => write!(f, "Error: Tried to backtrack but the History is empty."),
+            WaveError::NoHistory => write!(f, "Error: The puzzle is impossible (backtracked to start)."),
         }
     }
 }