diff --git a/src/solver/cell.rs b/src/solver/cell.rs
index 9977e8e..6ff510c 100644
--- a/src/solver/cell.rs
+++ b/src/solver/cell.rs
@@ -57,10 +57,6 @@ impl Cell {
 		}
 	}
 
-	pub fn get_state(&self) -> Option<usize> {
-		self.state
-	}
-
 	pub fn get_allowed(&self) -> Vec<usize> {
 		self.blocking_states
 	        .iter()
diff --git a/src/solver/mod.rs b/src/solver/mod.rs
index d281525..6a236a0 100644
--- a/src/solver/mod.rs
+++ b/src/solver/mod.rs
@@ -72,23 +72,15 @@ impl Solver {
 	pub fn solve(&mut self, solver_limit: Option<usize>) -> Result<(), WaveError> {
 		let now = Instant::now();
 		self.display(ui::DisplayMode::Full);
-	    let mut number_cell_init: usize = 0;
-	    for row in &self.grid {
-	    	for cell in row {
-	    		if !cell.is_none() {
-	    			number_cell_init += 1;
-	    		}
-	    	}
-	    }
+	    		
 	    let mut propagation_counter: usize = 0;
 	    let mut collapse_counter: usize = 0;
 		println!("# started");
 		self.debug("--------");
-		self.update_possibilities()?;
 
-		while number_cell_init + self.history.len() < self.size * self.size {
+		while self.history.len() < self.size * self.size {
 			self.debug(&format!("\n## while, h={}/{}", self.last_move_index, self.history.len()));
-			while self.last_move_index < self.history.len() && number_cell_init + self.history.len() < self.size * self.size {
+			while self.last_move_index < self.history.len() && self.history.len() < self.size * self.size {
 				let mut backtrack = 0;
 				match self.propagate_collapse() {
 					Ok(_) => {},
@@ -119,7 +111,7 @@ impl Solver {
 			collapse_counter += 1;
 			
 			if !self.debug_display && !self.grid_display {
-				self.progress_bar(number_cell_init);
+				self.progress_bar();
 		    }
 		    if let Some(limit) = solver_limit {
 		    	if collapse_counter >= limit {
@@ -182,6 +174,10 @@ impl Solver {
 	fn backtrack(&mut self) -> Result<(), WaveError> {
 		let mut fork: Option<Step> = None;
         while let Some(step) = self.history.pop() {
+        	if self.last_move_index == 0 {
+        		self.debug("there is no last move");
+        		return Err(WaveError::NoHistory)
+        	}
         	self.last_move_index -= 1;
         	self.grid[step.position.0][step.position.1].reset_state();
 
diff --git a/src/solver/sudoku.rs b/src/solver/sudoku.rs
index 9d00298..1ab579d 100644
--- a/src/solver/sudoku.rs
+++ b/src/solver/sudoku.rs
@@ -5,39 +5,6 @@ use super::WaveError;
 use super::cell;
 
 impl Solver {
-    pub fn update_possibilities(&mut self) -> Result<(), WaveError> {
-    	let mut row_used_states: Vec<Vec<cell::BlockingCell>> = vec![vec![]; self.size];
-    	let mut column_used_states: Vec<Vec<cell::BlockingCell>> = vec![vec![]; self.size];
-    	let mut square_used_states: Vec<Vec<Vec<cell::BlockingCell>>> = vec![vec![vec![]; self.square_size]; self.square_size];
-    	
-		for row_index in 0..self.size {
-			for column_index in 0..self.size {
-				let Some(state) = self.grid[row_index][column_index].get_state() else {
-					continue
-				};
-				let blocking_cell = cell::BlockingCell {
-					state,
-					position: (row_index, column_index),
-				};
-				row_used_states[row_index].push(blocking_cell.clone());
-				column_used_states[column_index].push(blocking_cell.clone());
-				square_used_states[row_index/self.square_size][column_index/self.square_size].push(blocking_cell);
-			}
-		}
-
-		for row_index in 0..self.size {
-			for column_index in 0..self.size {	            
-				self.remove_allowed((row_index, column_index), &row_used_states[row_index]
-				    .iter()
-				    .chain(&column_used_states[column_index])
-				    .chain(&square_used_states[row_index / self.square_size][column_index / self.square_size])
-				    .cloned()
-				    .collect())?;
-			}
-		}
-		Ok(())
-    }
-
     pub fn propagate_collapse(&mut self) -> Result<(), WaveError> {
 		if self.last_move_index >= self.history.len() {
 			self.debug(&format!("x nothing to propagate"));
diff --git a/src/solver/ui.rs b/src/solver/ui.rs
index 25b7915..00bf550 100644
--- a/src/solver/ui.rs
+++ b/src/solver/ui.rs
@@ -2,6 +2,7 @@ use text_io::read;
 
 use super::Solver;
 use super::cell;
+use super::Step;
 
 
 const CHAR_SET: &str = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
@@ -80,6 +81,11 @@ impl Solver {
 			        Some(value) => {
 			            if value <= self.size {
 			                self.grid[row_index][column_index].collapse(&cell::CollapseOption::Set(value))?;
+			                self.history.push(Step {
+               					position: (row_index, column_index),
+               					state_set: value,
+               					num_allowed_states: 1,
+               				});
 			            }
 			        }
 			        None => {}
@@ -95,9 +101,9 @@ impl Solver {
 		Ok(())
 	}
 
-	pub fn progress_bar(&self, number_cell_init: usize) {
+	pub fn progress_bar(&self) {
 		let bar_size = (self.size + self.square_size - 1)*2 + 1;
-    	let progress = self.history.len()*bar_size/(self.size*self.size - number_cell_init);
+    	let progress = self.history.len()*bar_size/(self.size*self.size);
     	let to_do = bar_size - progress;
     	print!("\r[{}{}]", "#".repeat(progress), "-".repeat(to_do));
 	}