diff --git a/src/cell.rs b/src/cell.rs
new file mode 100644
index 0000000..d93c51b
--- /dev/null
+++ b/src/cell.rs
@@ -0,0 +1,132 @@
+use std::fmt;
+use std::collections::HashSet;
+use rand::thread_rng;
+use rand::Rng;
+
+const STATE_DISPLAY: &str = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+pub enum CellError {
+	StateNotAllowed,
+	NoAllowedState,
+	StateAlreadySet,
+}
+
+pub enum CollapseOption {
+	Random,
+	First,
+	Set(usize),
+}
+
+pub enum RemoveResult {
+	NumAllowed(usize),
+	Collapsed(usize),
+	Filled,
+}
+
+impl fmt::Display for CellError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+        	CellError::StateNotAllowed => write!(f, "Cell Error : Tried to set a state not allowed."),
+        	CellError::NoAllowedState => write!(f, "Cell Error : The cell has no allowed states."),
+        	CellError::StateAlreadySet => write!(f, "Cell Error : Tried to overwrite an existing state"),
+        }
+    }
+}
+
+#[derive(Clone)]
+pub struct Cell {
+	state: Option<usize>,
+	all_states: Vec<usize>,
+	allowed_states: Vec<usize>,
+}
+
+impl Cell {
+	pub fn new(states: Vec<usize>) -> Self {
+		Self {
+			state: None,
+			all_states: states.clone(),
+			allowed_states: states,
+		}
+	}
+
+	pub fn get(&self) -> Option<usize> {
+		return self.state
+	}
+
+	pub fn get_allowed(&self) -> Vec<usize> {
+		return self.allowed_states.clone()
+	}
+
+	pub fn get_num_allowed(&self) -> usize {
+		return self.allowed_states.len()
+	}
+
+	pub fn is_none(&self) -> bool {
+		return self.state.is_none()
+	}
+
+	pub fn collapse(&mut self, option: &CollapseOption) -> Result<usize, CellError> {
+		if !self.state.is_none() {
+			return Err(CellError::StateAlreadySet)
+		}
+		if self.allowed_states.len() == 0 {
+			return Err(CellError::NoAllowedState)
+		}
+		if let CollapseOption::Set(state) = option {
+			if !self.allowed_states.contains(&state) {
+				return Err(CellError::StateNotAllowed)
+			}
+			self.state = Some(*state);
+			return Ok(*state)
+		}
+		let choice: usize = if let CollapseOption::Random = option {
+			thread_rng().gen_range(0..self.allowed_states.len())
+		} else {
+			0
+		};
+		self.state = Some(self.allowed_states[choice]);
+		return Ok(self.allowed_states[choice])
+	}
+
+	pub fn remove_allowed(&mut self, states: &HashSet<usize>) -> Result<RemoveResult, CellError> {
+		if !self.state.is_none() {
+			return Ok(RemoveResult::Filled)
+		}
+
+		self.allowed_states.retain(|&x| !states.contains(&x));
+
+		if self.allowed_states.len() == 0 {
+			return Err(CellError::NoAllowedState)
+		}
+		
+		if self.allowed_states.len() == 1 {
+			self.state = Some(self.allowed_states[0]);
+			return Ok(RemoveResult::Collapsed(self.allowed_states[0]))
+		}
+		return Ok(RemoveResult::NumAllowed(self.allowed_states.len()))
+	}
+	
+	pub fn reset_allowed(&mut self) {
+		self.allowed_states = self.all_states.clone();
+	}
+
+	pub fn reset(&mut self) {
+		self.state = None;
+		self.reset_allowed();
+	}
+}
+
+impl fmt::Display for Cell {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Some(state) = self.state else {
+        	return write!(f, " ")
+        };
+        
+        let display_state = match STATE_DISPLAY.chars().nth(state) {
+       		Some(state) => state.to_string(),
+       		None => "#".to_string(),
+       	};
+
+		write!(f, "{}", display_state)
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index ee6cde8..be498df 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,24 +1,18 @@
 use std::fmt;
 use std::collections::HashSet;
-use rand::seq::SliceRandom;
 use std::time::Instant;
 use std::env;
 use std::process;
-use text_io::read;
-use std::cmp::min;
 
-const CHAR_SET: &str = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+mod cell;
+use cell::Cell;
+use cell::CollapseOption;
 
-enum LineType {
-	Top,
-	Middle,
-	Bottom,
-}
+mod ui;
+use ui::DisplayMode;
 
 enum WaveError {
 	Contradiction,
-	NoEmptyCell,
-	NoPossibility,
 	NoHistory,
 }
 
@@ -26,135 +20,43 @@ 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::NoEmptyCell => write!(f, "Error: No empty cells left to fill."),
-            WaveError::NoPossibility => write!(f, "Error: The cell has no possibility."),
             WaveError::NoHistory => write!(f, "Error: Tried to backtrack but the History is empty."),
         }
     }
 }
 
-enum DisplayMode {
-	Full,
-	Focus(usize),
-}
-
-#[derive(Clone)]
-struct Cell {
-	value: Option<usize>,
-	max: usize,
-	possibilities: Vec<usize>
-}
-
-impl Cell {
-	fn set(&mut self, value: usize) {
-		self.value = if value > 0 && value <= self.max {
-			Some(value)
-		} else {
-			None
-		};
-	}
-
-	fn reset_possibilities(&mut self) {
-		self.possibilities = (1..=self.max).collect();
-	}
-
-	fn remove_possibilities(&mut self, possibilities: &HashSet<usize>) {
-		self.possibilities.retain(|&x| !possibilities.contains(&x))
-	}
-
-	fn new(max: usize) -> Self {
-		Self {
-			value: None,
-			max,
-			possibilities: (1..=max).collect(),
-		}
-	}
-}
-
-impl fmt::Display for Cell {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let Some(value) = self.value else {
-        	return write!(f, " ")
-        };
-        
-        let display_value = match CHAR_SET.chars().nth(value) {
-       		Some(value) => value.to_string(),
-       		None => "#".to_string(),
-       	};
-
-		write!(f, "{}", display_value)
-    }
-}
-
-struct Choice {
+struct Step {
 	cell_selected: [usize; 2],
-	possibilities: Vec<usize>,
-	selected_value: usize,
+	state_selected: usize,
+	num_allowed_states: usize,
 }
 
 struct Sudoku {
 	grid: Vec<Vec<Cell>>,
-	rng: rand::rngs::ThreadRng,
-	history: Vec<Choice>,
+	history: Vec<Step>,
+	last_history: usize,
 	size: usize,
 	square_size: usize,
+	debug_display: bool,
+	collapse_option: CollapseOption,
 }
 
 impl Sudoku {
 	fn new(order: usize) -> Self {
 		let size = order*order;
-		let sudoku_grid: Vec<Vec<Cell>> = vec![vec![Cell::new(size); size]; size];
+		let states = (1..=size).collect();
+		let sudoku_grid: Vec<Vec<Cell>> = vec![vec![Cell::new(states); size]; size];
 		Self {
 			grid: sudoku_grid,
-			rng: rand::thread_rng(),
 			history: vec![],
+			last_history: 0,
 			size,
 			square_size: order,
+			debug_display: false,
+			collapse_option: CollapseOption::Random,
 		}
 	}
 
-	fn display(&self, display_mode: DisplayMode) {
-		self.draw_line(LineType::Top);
-        for (row_index, row) in self.grid.iter().enumerate() {
-        	if row_index % self.square_size == 0 && row_index != 0 {
-        		self.draw_line(LineType::Middle);
-        	}
-        	
-        	let mut line: String = "║ ".to_string();
-        	for (cell_index, cell) in row.iter().enumerate() {
-        	
-        		line += &format!("{} ",cell);
-
-        		if cell_index % self.square_size == self.square_size - 1 {
-					line+= "║ ";
-        		}
-        	}
-        	if let DisplayMode::Focus(focus_row) = display_mode {
-        		if row_index == focus_row {
-        			line += "<";
-        		}
-        	}
-        	println!("{}",line);
-        }
-   		self.draw_line(LineType::Bottom);
-    }
-
-    fn draw_line(&self, line_type: LineType) {
-    	let (left_corner, intersection, right_corner) = match line_type {
-    		LineType::Top => ("╔", "╦", "╗"),
-    		LineType::Middle => ("╠", "╬", "╣"),
-    		LineType::Bottom => ("╚", "╩", "╝"),
-    	};
-    	let mut line = left_corner.to_string();
-         for i in 0..(self.square_size) {
-         	line += &"═".repeat(self.square_size*2+1);
-         	if i < self.square_size - 1 {
-         		line += intersection;
-         	} 
-         }
-         println!("{}{}", line, right_corner);
-    }
-
     fn update_possibilities(&mut self) -> Result<(), WaveError> {
     	let mut row_used_values: Vec<HashSet<usize>> = vec![HashSet::new(); self.size];
     	let mut column_used_values: Vec<HashSet<usize>> = vec![HashSet::new(); self.size];
@@ -162,7 +64,7 @@ impl Sudoku {
     	
 		for row_index in 0..self.size {
 			for column_index in 0..self.size {
-				let Some(value) = self.grid[row_index][column_index].value else {
+				let Some(value) = self.grid[row_index][column_index].get() else {
 					continue
 				};
 				if row_used_values[row_index].contains(&value) || column_used_values[column_index].contains(&value) || column_used_values[column_index].contains(&value) {
@@ -173,53 +75,128 @@ impl Sudoku {
 				square_used_values[row_index/self.square_size][column_index/self.square_size].insert(value);
 			}
 		}
-		
+
 		for row_index in 0..self.size {
 			for column_index in 0..self.size {
 				let mut used_values = row_used_values[row_index].clone();
 	            used_values.extend(&column_used_values[column_index]);
 	            used_values.extend(&square_used_values[row_index/self.square_size][column_index/self.square_size]);
 	            
-				self.grid[row_index][column_index].remove_possibilities(&used_values);
+				self.remove_allowed(row_index, column_index, &used_values)?;
 			}
 		}
 		Ok(())
     }
 
-    fn propagate_collapse(&mut self, _debug_display: bool) -> Result<(), WaveError> {
-    	let Some(last_choice) = self.history.last() else {
-    		return self.update_possibilities()
-    	};
-    	
-    	let collapsed_row = last_choice.cell_selected[0];
-    	let collapsed_column = last_choice.cell_selected[1];
-    	
-		let Some(collapsed_value) = self.grid[collapsed_row][collapsed_column].value else {
+    fn propagate_collapse(&mut self) -> Result<(), WaveError> {
+		if self.last_history >= self.history.len() {
+			if self.debug_display {
+				println!("x nothing to propagate")
+			}
 			return Ok(())
-		};
+		}
+		
+	   	let collapsed_row = self.history[self.last_history].cell_selected[0];
+	   	let collapsed_column = self.history[self.last_history].cell_selected[1];
+	   	let collapsed_state = self.history[self.last_history].state_selected;
+		self.last_history += 1;
 
-    	let mut collapsed_possibility = HashSet::new();
-    	collapsed_possibility.insert(collapsed_value);
-    	
-    	for column_index in 0..self.size {
-    		self.grid[collapsed_row][column_index].remove_possibilities(&collapsed_possibility);
+		if self.debug_display {
+	   		println!("- propagating {}", collapsed_state);
+		}
+		
+	   	let mut collapsed_possibility = HashSet::new();
+		collapsed_possibility.insert(collapsed_state);
+
+		for column_index in 0..self.size {
+			if column_index == collapsed_column {
+				continue;
+			}
+			if self.grid[collapsed_row][column_index].get() == Some(collapsed_state) {
+				return Err(WaveError::Contradiction)
+			}
+    		self.remove_allowed(collapsed_row, column_index, &collapsed_possibility)?;
     	}
-
     	for row_index in 0..self.size {
-    		self.grid[row_index][collapsed_column].remove_possibilities(&collapsed_possibility);
+    		if row_index == collapsed_row {
+    			continue;
+    		}
+    		if self.grid[row_index][collapsed_column].get() == Some(collapsed_state) {
+				return Err(WaveError::Contradiction)
+			}
+    		self.remove_allowed(row_index, collapsed_column, &collapsed_possibility)?;
     	}
-
     	for row_index in 0..self.square_size {
     		for column_index in 0..self.square_size {
     			let row = (collapsed_row/self.square_size)*self.square_size + row_index;
     			let column = (collapsed_column/self.square_size)*self.square_size + column_index;
-    			self.grid[row][column].remove_possibilities(&collapsed_possibility);
+				if row == collapsed_row && column == collapsed_column {
+					continue;
+				}
+				if self.grid[row][column].get() == Some(collapsed_state) {
+					return Err(WaveError::Contradiction)
+				}
+    			self.remove_allowed(row, column, &collapsed_possibility)?;
     		}
     	}
-    	Ok(())
-    }
 
-    fn collapse(&mut self, debug_display: bool) -> Result<(), WaveError> {
+		let mut missing_states: HashSet<usize> = (1..=self.size).collect(); 
+    	for column_index in 0..self.size {
+    		if let Some(state) = self.grid[collapsed_row][column_index].get() {
+				missing_states.remove(&state);
+    			continue
+    		}
+    		for allowed_state in self.grid[collapsed_row][column_index].get_allowed() {
+    			missing_states.remove(&allowed_state);
+    		}
+    	}
+    	if !missing_states.is_empty() {
+    		if self.debug_display {
+    			println!("xxxxxxxx missing row state : {:?}", missing_states);
+    		}
+    		return Err(WaveError::Contradiction)
+    	}
+    	missing_states = (1..=self.size).collect(); 
+    	for row_index in 0..self.size {
+    		if let Some(state) = self.grid[row_index][collapsed_column].get() {
+				missing_states.remove(&state);
+    			continue
+    		}
+    		for allowed_state in self.grid[row_index][collapsed_column].get_allowed() {
+    			missing_states.remove(&allowed_state);
+    		}
+   		}
+   		if !missing_states.is_empty() {
+   			if self.debug_display {
+	   			println!("xxxxxxxxx missing column state");
+	   		}
+	   		return Err(WaveError::Contradiction)
+	   	}
+   		missing_states = (1..=self.size).collect();
+   		for row_index in 0..self.square_size {
+    		for column_index in 0..self.square_size {
+    			let row = (collapsed_row/self.square_size)*self.square_size + row_index;
+    			let column = (collapsed_column/self.square_size)*self.square_size + column_index;
+	    		if let Some(state) = self.grid[row][column].get() {
+					missing_states.remove(&state);
+	    			continue
+	    		}
+				for allowed_state in self.grid[row][column].get_allowed() {
+	    			missing_states.remove(&allowed_state);
+	    		}
+			}
+		}
+		if !missing_states.is_empty() {
+			if self.debug_display {
+    			println!("xxxxxxxxxx missing square state");
+    		}
+    		return Err(WaveError::Contradiction)
+    	}
+
+		Ok(())
+    }
+    
+    fn collapse(&mut self) -> Result<(), WaveError> {
 		let mut min_row_index: usize = 0;
 		let mut min_column_index: usize = 0;
 		let mut min_len: usize = self.size;
@@ -228,11 +205,11 @@ impl Sudoku {
 		
     	for row_index in 0..self.size {
     		for column_index in 0..self.size {
-    			if !self.grid[row_index][column_index].value.is_none() {
+    			if !self.grid[row_index][column_index].is_none() {
     				continue;
     			}
     			grid_has_empty_cell = true;
-    			let possibilities_len = self.grid[row_index][column_index].possibilities.len();
+    			let possibilities_len = self.grid[row_index][column_index].get_num_allowed();
 				if possibilities_len < min_len {
 					min_row_index = row_index;
 					min_column_index = column_index;
@@ -242,169 +219,252 @@ impl Sudoku {
     	}
 
     	if !grid_has_empty_cell {
-    		if debug_display {
+    		if self.debug_display {
     			println!("x no empty cells");
     		}
-    		return Err(WaveError::NoEmptyCell)
-    	}
-
-    	let Err(reason) = self.collapse_cell(min_row_index, min_column_index, debug_display) else {
     		return Ok(())
-    	};
-
-    	if let WaveError::NoPossibility = reason {
-    		return self.backtrack(debug_display)
-    	} else {
-    		return Err(reason)
     	}
-	}
 
-	fn backtrack(&mut self, debug_display: bool) -> Result<(), WaveError> {
-		let mut fork: Option<Choice> = None;
+		return self.collapse_cell(min_row_index, min_column_index)
+//     	let Err(reason) = self.collapse_cell(min_row_index, min_column_index) else {
+//     		return Ok(())
+//     	};
+// 
+//     	if let WaveError::Contradiction = reason {
+//     		return self.backtrack()
+//     	} else {
+//     		return Err(reason)
+//     	}
+	}
+// 
+	fn backtrack(&mut self) -> Result<(), WaveError> {
+		let mut fork: Option<Step> = None;
 		
-        while let Some(choice) = self.history.pop() {
-        	if debug_display {
-        		println!("* backtracking");
-        	}
-        	self.grid[choice.cell_selected[0]][choice.cell_selected[1]].value = None;
-        	if choice.possibilities.len() > 1 {
-        		fork = Some(choice);
+        while let Some(step) = self.history.pop() {
+        	self.last_history -= 1;
+        	self.grid[step.cell_selected[0]][step.cell_selected[1]].reset();
+        	if step.num_allowed_states > 1 {
+        		fork = Some(step);
         		break;
         	}
+			if self.debug_display {
+        		println!("* backtracking [{}][{}] : {}", step.cell_selected[0], step.cell_selected[1], step.state_selected);
+        	}
         }
 
-        let Some(choice) = fork else {
-        	if debug_display {
+        let Some(step) = fork else {
+        	if self.debug_display {
         		println!("x backtracked to start");
         	}
         	return Err(WaveError::NoHistory)
         };
+        if self.debug_display {
+       		println!("* fork [{}][{}] : {}", step.cell_selected[0], step.cell_selected[1], step.state_selected);
+       	}
         
-       	self.reset_possibilities()?;
+       	self.reset_allowed();
 
-       	let mut selected_value = HashSet::new();
-       	selected_value.insert(choice.selected_value);
-       	
-		self.grid[choice.cell_selected[0]][choice.cell_selected[1]].remove_possibilities(&selected_value);
-       	return self.collapse_cell(choice.cell_selected[0], choice.cell_selected[1], debug_display)
+       	let mut state_selected_set = HashSet::new();
+       	state_selected_set.insert(step.state_selected);
+
+       	self.remove_allowed(step.cell_selected[0], step.cell_selected[1], &state_selected_set)?;
+       	Ok(())
 	}
-
-    fn reset_possibilities(&mut self) -> Result<(), WaveError> {
+ 
+    fn reset_allowed(&mut self) {
     	for row in &mut self.grid {
     		for cell in row {
-    			cell.reset_possibilities();
+    			cell.reset_allowed();
     		}
     	}
-    	
-        return self.update_possibilities();
     }
 
-    fn collapse_cell(&mut self, row_index: usize, column_index: usize, debug_display: bool) -> Result<(), WaveError> {
-    	let Some(&selected_value) = self.grid[row_index][column_index].possibilities.choose(&mut self.rng) else {
-			if debug_display {
-   	    		println!("x no possibilities for [{}][{}]", row_index, column_index);
-   	    	}
-			return Err(WaveError::NoPossibility)
-		};
-
-    	self.history.push(Choice {
-			cell_selected: [row_index, column_index],
-			possibilities: self.grid[row_index][column_index].possibilities.clone(),
-			selected_value,
-		});
-
-  	    self.grid[row_index][column_index].set(selected_value);
-  	    if debug_display {
-			println!("# collapsing [{}][{}] ({:?}) to {}", row_index, column_index, self.grid[row_index][column_index].possibilities, selected_value);
+    fn remove_allowed(&mut self, row_index: usize, column_index: usize, set_to_remove: &HashSet<usize>) -> Result<(), WaveError> {
+    	match self.grid[row_index][column_index].remove_allowed(set_to_remove) {
+			Ok(result) => {
+				let cell::RemoveResult::Collapsed(state) = result else {
+					return Ok(())
+				};
+				if self.debug_display {
+					println!("* collapsed by removal [{}][{}] to {}", row_index, column_index, state)
+				}
+				self.history.push(Step {
+					cell_selected: [row_index, column_index],
+					state_selected: state,
+					num_allowed_states: 1,
+				});
+				return Ok(())
+			}
+			Err(reason) => {
+				if self.debug_display {
+					println!("x failed to update states allowed of [{}][{}] : {}", row_index, column_index, reason);
+				}
+				return Err(WaveError::Contradiction)
+			}
+		}
+    }
+
+    fn collapse_cell(&mut self, row_index: usize, column_index: usize) -> Result<(), WaveError> {
+		match self.grid[row_index][column_index].collapse(&self.collapse_option) {
+			Ok(state_selected) => {
+				let num_allowed_states = self.grid[row_index][column_index].get_num_allowed();
+				if self.debug_display {
+					println!("# collapsing [{}][{}] ({}) to {}", row_index, column_index, num_allowed_states, state_selected);
+				}
+				self.history.push(Step {
+					cell_selected: [row_index, column_index],
+					state_selected,
+					num_allowed_states,
+				});
+				return Ok(())
+			}
+			Err(reason) => {
+				if self.debug_display {
+	   	    		println!("x could not collapse [{}][{}] : {}", row_index, column_index, reason);
+	   	    	}
+	   	    	return Err(WaveError::Contradiction)
+			}
 		}
-		Ok(())
 	}
+// 
+// 	fn solve(&mut self) -> Result<(), WaveError> {
+// 		let now = Instant::now();
+// 		
+// 		self.display(DisplayMode::Full);
+// 		if self.debug_display {
+// 	    	println!("--------");
+// 	    }
+// 
+// 
+// 	    let initial_grid = self.grid.clone();
+// 	    
+// 		println!("# started");
+// 	    
+// 	    self.update_possibilities()?;
+// 	    
+// 		let mut step_counter: usize = 0;
+// 		let mut reset_counter: usize = 0;
+// 		
+// 	    loop {
+// 			match self.collapse() {
+// 				Ok(()) => {}
+// 				Err(reason) => {
+// 					if let WaveError::NoEmptyCell = reason {
+// 						break;
+// 					} else {
+// 						return Err(reason)
+// 					}
+// 				}
+// 			}
+// 			self.propagate_collapse()?;
+// 
+// 			
+// 	    	if step_counter%(2*self.size*self.size) == 0  {
+// 	    		self.grid = initial_grid.clone();
+// 	    		self.reset_allowed();
+// 	    		self.history = vec![];
+// 	    		self.update_possibilities()?;
+// 	    		reset_counter += 1;
+// 	    	}
+// 
+// 			if !self.debug_display {
+// 				let bar_size = (self.size + self.square_size - 1)*2 + 1;
+// 		    	let progress = self.history.len()*bar_size/(self.size*self.size - filled_cells_number);
+// 		    	let to_do = bar_size - progress;
+// 		    	print!("\r[{}{}]", "#".repeat(progress), "-".repeat(to_do));
+// 		    }
+// 		    step_counter += 1;
+// 	    }
+// 	    
+// 	    println!();
+// 
+// 		if reset_counter > 0 {
+// 			println!("# {} resets", reset_counter);
+// 		}
+// 	    self.display(DisplayMode::Full);
+// 	    
+// 	    Ok(())
+// 	}
 
-	fn solve(&mut self, debug_display: bool) -> Result<(), WaveError> {
+	fn solve(&mut self) -> Result<(), WaveError> {
 		let now = Instant::now();
-		
 		self.display(DisplayMode::Full);
-		if debug_display {
-	    	println!("--------");
-	    }
-
-	    let mut filled_cells_number: usize = 0;
+	    let mut n_start_cells: usize = 0;
 	    for row in &self.grid {
 	    	for cell in row {
-	    		if !cell.value.is_none() {
-	    			filled_cells_number += 1;
+	    		if !cell.is_none() {
+	    			n_start_cells += 1;
 	    		}
 	    	}
 	    }
-
-	    let initial_grid = self.grid.clone();
-	    
+	    let mut propagation_counter: usize = 0;
+	    let mut collapse_counter: usize = 0;
 		println!("# started");
-	    
-	    self.update_possibilities()?;
-	    
-		let mut step_counter: usize = 0;
-		let mut reset_counter: usize = 0;
-		
-	    while let Ok(_) = self.collapse(debug_display) {
-	    	//self.update_possibilities();
-	    	self.propagate_collapse(debug_display)?;
-	    	step_counter += 1;
+		if self.debug_display {
+	    	println!("--------");
+	    }
+		self.update_possibilities()?;
 
-	    	if step_counter%(2*self.size*self.size) == 0 {
-	    		self.grid = initial_grid.clone();
-	    		self.update_possibilities()?;
-	    		self.history = vec![];
-	    		reset_counter += 1;
-	    	}
+		while n_start_cells + self.history.len() < self.size * self.size {
+			if self.debug_display {
+				println!("## while, h={}/{}", self.last_history, self.history.len());
+			}
+			while self.last_history < self.history.len() && n_start_cells + self.history.len() < self.size * self.size {
+				let mut backtrack = 0;
+				match self.propagate_collapse() {
+					Ok(_) => {},
+					Err(reason) => {
+						if let WaveError::Contradiction = reason {
+							backtrack = 1;
+						} else {
+							return Err(reason)
+						}
+					}
+				};
+				while backtrack > 0 {
+					backtrack -=1;
+					self.backtrack()?;
+					match self.update_possibilities() {
+						Ok(_) => {},
+						Err(reason) => {
+							if let WaveError::Contradiction = reason {
+								backtrack += 1;
+							} else {
+								return Err(reason)
+							}
+						}
+					};
+				}
+				propagation_counter += 1;
+			}
+			self.collapse()?;
+			collapse_counter += 1;
 
-			if !debug_display {
+			if !self.debug_display {
 				let bar_size = (self.size + self.square_size - 1)*2 + 1;
-		    	let progress = self.history.len()*bar_size/(self.size*self.size - filled_cells_number);
+		    	let progress = self.history.len()*bar_size/(self.size*self.size - n_start_cells);
 		    	let to_do = bar_size - progress;
 		    	print!("\r[{}{}]", "#".repeat(progress), "-".repeat(to_do));
 		    }
-	    }
-	    
-	    println!();
+		}
+		println!();
 
-	    let elapsed = now.elapsed();
-
-		if debug_display {
+		if self.debug_display {
 	    	println!("--------");
 	    }
-	    println!("# finished in {} steps, {:.2?} ({:.2?}/step)", step_counter, elapsed, elapsed/(step_counter as u32));
-		if reset_counter > 0 {
-			println!("# {} resets", reset_counter);
-		}
-	    self.display(DisplayMode::Full);
-	    
-	    Ok(())
+	    let elapsed = now.elapsed();
+	    println!("# finished in {} propagations ({} forced collapse), {:.2?} ({:.2?}/propagation)", propagation_counter, collapse_counter, elapsed, elapsed/(propagation_counter as u32));
+		self.display(DisplayMode::Full);
+		Ok(())
 	}
 
-	fn ask(&mut self) {
-		for row_index in 0..self.size {
-			self.display(DisplayMode::Focus(row_index));
-			print!("> ");
-			let line: String = read!("{}\n");
-			for (column_index, c) in line.chars().enumerate() {
-			    match CHAR_SET.find(c) {
-			        Some(value) => {
-			            if value <= self.size {
-			                self.grid[row_index][column_index].set(value);
-			            }
-			        }
-			        None => {}
-			    }
-			}
-			let height = self.size + self.square_size + 2;
-			print!("\x1b[{}A", height);
-			for _ in 0..height {
-				println!("{}"," ".repeat((self.size + self.square_size - 1)*2 + 5));
-			}
-			print!("\x1b[{}A", height);
+	fn random_mode(&mut self, collapse_random: bool) {
+		if collapse_random {
+			self.collapse_option = CollapseOption::Random;
+		} else {
+			self.collapse_option = CollapseOption::First
 		}
-	}
+	} 
 }
 
 fn main() {
@@ -419,13 +479,24 @@ fn main() {
         process::exit(1);
     });
 
-    let mut sudoku = Sudoku::new(min(size, 5));
+    let mut sudoku = Sudoku::new(size);
 
     if args.contains(&"--ask".to_string()) {
-    	sudoku.ask();
+    	if let Err(reason) = sudoku.ask() {
+   			println!("{}",reason);
+   		}
+    }
+
+    if args.contains(&"--debug".to_string()) {
+    	sudoku.debug_mode(true);
+    }
+
+    if args.contains(&"--norand".to_string()) {
+    	sudoku.random_mode(false);
     }
     
-	if let Err(reason) = sudoku.solve(args.contains(&"--debug".to_string())) {
+	if let Err(reason) = sudoku.solve() {
 		println!("{}",reason);
+		sudoku.display(DisplayMode::Full);
 	}
 }
diff --git a/src/ui.rs b/src/ui.rs
new file mode 100644
index 0000000..ad6b100
--- /dev/null
+++ b/src/ui.rs
@@ -0,0 +1,90 @@
+use text_io::read;
+
+use crate::Sudoku;
+use crate::cell;
+
+const CHAR_SET: &str = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+enum LineType {
+	Top,
+	Middle,
+	Bottom,
+}
+
+pub enum DisplayMode {
+	Full,
+	Focus(usize),
+}
+
+impl Sudoku {
+	pub fn display(&self, display_mode: DisplayMode) {
+		self.draw_line(LineType::Top);
+        for (row_index, row) in self.grid.iter().enumerate() {
+        	if row_index % self.square_size == 0 && row_index != 0 {
+        		self.draw_line(LineType::Middle);
+        	}
+        	
+        	let mut line: String = "║ ".to_string();
+        	for (cell_index, cell) in row.iter().enumerate() {
+        	
+        		line += &format!("{} ",cell);
+
+        		if cell_index % self.square_size == self.square_size - 1 {
+					line+= "║ ";
+        		}
+        	}
+        	if let DisplayMode::Focus(focus_row) = display_mode {
+        		if row_index == focus_row {
+        			line += "<";
+        		}
+        	}
+        	println!("{}",line);
+        }
+   		self.draw_line(LineType::Bottom);
+    }
+
+    fn draw_line(&self, line_type: LineType) {
+    	let (left_corner, intersection, right_corner) = match line_type {
+    		LineType::Top => ("╔", "╦", "╗"),
+    		LineType::Middle => ("╠", "╬", "╣"),
+    		LineType::Bottom => ("╚", "╩", "╝"),
+    	};
+    	let mut line = left_corner.to_string();
+         for i in 0..(self.square_size) {
+         	line += &"═".repeat(self.square_size*2+1);
+         	if i < self.square_size - 1 {
+         		line += intersection;
+         	} 
+         }
+         println!("{}{}", line, right_corner);
+    }
+    
+	pub fn ask(&mut self) -> Result<(), cell::CellError> {
+		for row_index in 0..self.size {
+			self.display(DisplayMode::Focus(row_index));
+			print!("> ");
+			let line: String = read!("{}\n");
+			for (column_index, c) in line.chars().enumerate() {
+			    match CHAR_SET.find(c) {
+			        Some(value) => {
+			            if value <= self.size {
+			                self.grid[row_index][column_index].collapse(&cell::CollapseOption::Set(value))?;
+			            }
+			        }
+			        None => {}
+			    }
+			}
+			let height = self.size + self.square_size + 2;
+			print!("\x1b[{}A", height);
+			for _ in 0..height {
+				println!("{}"," ".repeat((self.size + self.square_size - 1)*2 + 5));
+			}
+			print!("\x1b[{}A", height);
+		}
+		Ok(())
+	}
+
+	pub fn debug_mode(&mut self, debug_display: bool) {
+		self.debug_display = debug_display;
+	}
+}