Add support for setting max difficulty.

TODO is to support for min difficulty.

src/bin/generator.rs

@@ -3,6 +3,59 @@ use rand::prelude::*;
 use sudoku_solver::grid::{Grid, CellValue};
 use std::error::Error;
 use std::io::Write;
+use sudoku_solver::solver::SolveController;
+use std::str::FromStr;
+
+#[derive(Clone)] // Needed for argparse
+enum Difficulty {
+    Hard,
+    Medium,
+    Easy
+}
+
+impl Difficulty {
+    fn map_to_solve_controller(&self) -> SolveController {
+        let mut controller = SolveController{
+            determine_uniqueness: true,
+            search_singles: true,
+            search_hidden_singles: true,
+            find_possibility_groups: true,
+            search_useful_constraint: true,
+            make_guesses: true
+        };
+
+        match self {
+            Difficulty::Hard => {} // Do nothing, already hard
+            Difficulty::Medium => {
+                controller.make_guesses = false;
+            },
+            Difficulty::Easy => {
+                controller.make_guesses = false;
+                controller.search_useful_constraint = false;
+                controller.find_possibility_groups = false;
+            }
+        }
+
+        controller
+    }
+}
+
+impl FromStr for Difficulty { // Needed for argparse
+    type Err = String;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+
+        if s.eq_ignore_ascii_case("EASY"){
+            return Ok(Difficulty::Easy);
+        } else if s.eq_ignore_ascii_case("MEDIUM"){
+            return Ok(Difficulty::Medium);
+        } else if s.eq_ignore_ascii_case("HARD"){
+            return Ok(Difficulty::Hard);
+        }
+
+        return Err(format!("{} is not a valid difficulty", s));
+    }
+}
 
 fn main() {
 
@@ -13,6 +66,7 @@ fn main() {
     let mut max_hints = 81;
     let mut max_attempts = 100;
     let mut filename : Option<String> = None;
+    let mut difficulty = Difficulty::Hard;
 
     { // this block limits scope of borrows by ap.refer() method
         let mut ap = argparse::ArgumentParser::new();
@@ -21,7 +75,7 @@ fn main() {
             .add_option(&["--debug"], argparse::StoreTrue, "Run in debug mode");
 
         ap.refer(&mut seed)
-            .add_option(&["--seed"], argparse::Store, "Provide seed for puzzle generation");
+            .add_option(&["-s", "--seed"], argparse::Store, "Provide seed for puzzle generation");
 
         ap.refer(&mut max_hints)
             .add_option(&["--hints"], argparse::Store, "Only return a puzzle with less than or equal to this number of hints");
@@ -32,6 +86,9 @@ fn main() {
         ap.refer(&mut filename)
             .add_argument("filename", argparse::StoreOption, "Optional filename to store puzzle in as a CSV");
 
+        ap.refer(&mut difficulty)
+            .add_option(&["-d", "--difficulty"], argparse::Store, "Max difficulty setting; values are EASY, MEDIUM, or HARD");
+
         ap.parse_args_or_exit();
     }
 
@@ -46,6 +103,10 @@ fn main() {
     if debug {
         println!("Using seed {}", seed);
     }
+
+    let solve_controller = difficulty.map_to_solve_controller();
+
+
     let mut rng = ChaCha8Rng::seed_from_u64(seed);
 
     let mut num_attempts = 0;
@@ -56,7 +117,7 @@ fn main() {
             return;
         }
 
-        let (grid, num_hints) = sudoku_solver::generator::generate_grid(&mut rng);
+        let (grid, num_hints) = sudoku_solver::generator::generate_grid(&mut rng, &solve_controller);
         num_attempts = num_attempts + 1;
 
         if num_hints <= max_hints {

src/generator.rs

@@ -1,56 +1,11 @@
 use crate::grid::{Cell, Grid, CellValue, Line};
-use crate::solver::{solve_grid_no_guess, SolveStatus, find_smallest_cell};
+use crate::solver::{SolveStatus, SolveController, Uniqueness, evaluate_grid_with_solve_controller};
 use std::rc::Rc;
 use rand::prelude::*;
 use rand_chacha::ChaCha8Rng;
 
 pub static mut DEBUG : bool = false;
 
-// Extension of SolveStatus
-#[derive(Debug, Eq, PartialEq)]
-pub enum GenerateStatus {
-    UniqueSolution,
-    Unfinished,
-    NoSolution,
-    NotUniqueSolution
-}
-
-impl GenerateStatus {
-    fn increment(self, new_status : GenerateStatus) -> GenerateStatus {
-        match self {
-            GenerateStatus::UniqueSolution => {
-                match new_status {
-                    GenerateStatus::UniqueSolution => GenerateStatus::NotUniqueSolution, // We now have two completes, so the solutions are not unique
-                    GenerateStatus::NoSolution => GenerateStatus::UniqueSolution, // We already have a complete, so no issue with another guess being invalid
-                    GenerateStatus::Unfinished => {panic!("Should not have encountered an UNFINISHED status")},
-                    GenerateStatus::NotUniqueSolution => GenerateStatus::NotUniqueSolution // That solver found multiple solutions so no need to keep checking
-                }
-            },
-            GenerateStatus::Unfinished => {
-                match new_status {
-                    GenerateStatus::UniqueSolution => GenerateStatus::UniqueSolution,
-                    GenerateStatus::NoSolution => GenerateStatus::Unfinished,
-                    GenerateStatus::Unfinished => {panic!("Should not have encountered an UNFINISHED status")},
-                    GenerateStatus::NotUniqueSolution => GenerateStatus::NotUniqueSolution // That solver found multiple solutions so no need to keep checking
-                }
-            },
-            GenerateStatus::NotUniqueSolution => GenerateStatus::NotUniqueSolution,
-            GenerateStatus::NoSolution => GenerateStatus::NoSolution // This guess didn't pan out
-        }
-
-    }
-}
-
-impl SolveStatus {
-    fn map_to_generate_status(self) -> GenerateStatus {
-        match self {
-            SolveStatus::Complete => {GenerateStatus::UniqueSolution }
-            SolveStatus::Unfinished => {GenerateStatus::Unfinished }
-            SolveStatus::Invalid => {GenerateStatus::NoSolution }
-        }
-    }
-}
-
 impl Grid {
     fn get_random_empty_cell(&self, rng : &mut ChaCha8Rng) -> Result<Rc<Cell>, &str> {
         // Idea - put all empty cells into a vector and choose one at random
@@ -153,94 +108,17 @@ impl Line {
     }
 }
 
-pub fn generate_grid(rng: &mut ChaCha8Rng) -> (Grid, i32) {
+pub fn generate_grid(rng: &mut ChaCha8Rng, solve_controller: &SolveController) -> (Grid, i32) {
 
-    let mut num_hints;
+    let mut grid = generate_completed_grid(rng);
-    let mut grid : Grid = loop {
+    let mut num_hints = 81;
-        // First step; randomly assign 8 different digits to different empty cells and see if there's a possible solution
-        // We have to ensure that 8 of the digits appear at least once, otherwise the solution can't be unique because you could interchange the two missing digits throughout the puzzle
-        // We do this in a loop so that if we are really unlucky and our guesses stop there from being any solution, we can easily re-run it
-        let mut grid = Grid::new();
-        num_hints = 0;
 
-        let digit_excluded = rng.gen_range(1, 10);
+    // We now trim down cells; first going to put them in a vector and shuffle them
-
-        for digit in 1..10 {
-            if digit != digit_excluded {
-                let cell = grid.get_random_empty_cell(rng);
-                cell.unwrap().set(digit);
-                num_hints = num_hints + 1;
-            }
-        }
-
-        let status = solve_grid(&mut grid);
-        match status {
-            GenerateStatus::UniqueSolution => { // very surprising result, given that the smallest puzzles found have 14 guesses
-                eprintln!("Wow! A puzzle with only 8 guesses have been found");
-                return (grid, num_hints);
-            }
-            GenerateStatus::Unfinished => {panic!("solve_grid should never return UNFINISHED")}
-            GenerateStatus::NoSolution => {continue;} // unlucky; try again
-            GenerateStatus::NotUniqueSolution => {break grid;}
-        };
-    };
-
-    // Alright, we now have a grid that we can start adding more guesses onto until we find a unique solution
-    grid =
-    'outer: loop {
-        num_hints = num_hints + 1;
-        let cell = grid.get_random_empty_cell(rng).unwrap(); // We unwrap because if somehow we're filled each cell without finding a solution, that's reason for a panic
-        let cell = &*cell;
-        let mut cell_possibilities = cell.get_value_possibilities().expect("An empty cell has no possibilities");
-
-        // Let's scramble the order
-        cell_possibilities.shuffle(rng);
-
-        for (_index, digit) in cell_possibilities.iter().enumerate() {
-
-            let grid_clone = grid.clone();
-            let cell = &*grid_clone.get(cell.x, cell.y).unwrap();
-
-            cell.set(*digit);
-
-            let status = solve_grid(&grid_clone);
-            match status {
-                GenerateStatus::UniqueSolution => { // We're done!
-                    break 'outer grid_clone;
-                }
-                GenerateStatus::Unfinished => {
-                    panic!("solve_grid should never return UNFINISHED")
-                }
-                GenerateStatus::NoSolution => { // Try another guess
-                    continue;
-                }
-                GenerateStatus::NotUniqueSolution => { // We need more guesses
-                    grid = grid_clone;
-                    continue 'outer;
-                }
-            }
-
-        };
-
-        // If we reach this point in the loop, then none of the possibilities for cell provided any solution
-        // Which means something serious happened before in the solving process - reason for panic
-        eprint!("No valid hints were found for puzzle\n{} at cell ({}, {})", grid, cell.x, cell.y);
-        panic!("Unable to continue as puzzle is invalid");
-
-
-    };
-
-    // At this point we have a valid puzzle, but from experience it has way too many guesses, and many of them
-    // are likely not needed. Let's now try removing a bunch.
     let mut non_empty_cells = Vec::new();
     for x in 0..9 {
         for y in 0..9 {
             let cell = grid.get(x, y).unwrap();
-            let value = &*cell.value.borrow();
+            non_empty_cells.push(Rc::clone(&cell));
-            match value {
-                CellValue::Fixed(_) => {non_empty_cells.push(Rc::clone(&cell))}
-                CellValue::Unknown(_) => {}
-            }
         }
     }
     // Need to randomly reorder non_empty_cells
@@ -253,92 +131,132 @@ pub fn generate_grid(rng: &mut ChaCha8Rng) -> (Grid, i32) {
 
         cell_clone.delete_value();
 
-        let status = solve_grid(&mut grid_clone);
+
+        let status = evaluate_grid_with_solve_controller(&mut grid_clone, solve_controller);
         match status {
-            GenerateStatus::UniqueSolution => { // great; that cell value was not needed
+            SolveStatus::Complete(uniqueness) => {
+                let uniqueness = uniqueness.unwrap();
+                match uniqueness {
+                    Uniqueness::Unique => {
                         num_hints = num_hints - 1;
                         grid = grid_clone;
-
                     }
-            GenerateStatus::Unfinished => {panic!("solve_grid should never return UNFINISHED")}
+                    Uniqueness::NotUnique => continue // We can't remove this cell; continue onto the next one (note that grid hasn't been modified because of solve_controller)
-            GenerateStatus::NoSolution => {panic!("Removing constraints should not have set the # of solutions to zero")}
+                }
-            GenerateStatus::NotUniqueSolution => {continue;} // We can't remove this cell; continue onto the next one (note that grid hasn't been modified)
+            }
-        };
+            SolveStatus::Unfinished => panic!("evaluate_grid_with_solve_controller should never return UNFINISHED"),
+            SolveStatus::Invalid => panic!("Removing constraints should not have set the # of solutions to zero")
+        }
     }
 
     return (grid, num_hints);
 
 }
 
-fn solve_grid(grid: &Grid) -> GenerateStatus{
+// We generate a completed grid with no mind for difficulty; afterward generate_puzzle will take out as many fields as it can with regards to the difficulty
-    // Code is kind of messy so here it goes - solve_grid first tries to solve without any guesses
+fn generate_completed_grid(rng: &mut ChaCha8Rng) -> Grid {
-    // If that's not enough and a guess is required, then solve_grid_guess is called
+    let solve_controller = SolveController{
-    // solve_grid_guess runs through all the possibilities for the smallest cell, trying to solve them
+        determine_uniqueness: true,
-    // through calling this function.
+        search_singles: true,
-    // solve_grid_no_guess tries to solve without any guesses.
+        search_hidden_singles: true,
-
+        find_possibility_groups: true,
-    let mut grid = grid.clone(); // We're generating a result and don't want to make changes to our input
+        search_useful_constraint: true,
-
+        make_guesses: true
-    let mut status = solve_grid_no_guess(&mut grid).map_to_generate_status();
-    status = match status {
-        GenerateStatus::Unfinished => {
-            solve_grid_guess(&mut grid)
-        },
-        _ => {status}
     };
 
+    let mut grid : Grid = loop {
+        // First step; randomly assign 8 different digits to different empty cells and see if there's a possible solution
+        // We have to ensure that 8 of the digits appear at least once, otherwise the solution can't be unique because you could interchange the two missing digits throughout the puzzle
+        // We do this in a loop so that if we are really unlucky and our guesses stop there from being any solution, we can easily re-run it
+        let grid = Grid::new();
+
+        let digit_excluded = rng.gen_range(1, 10);
+
+        for digit in 1..10 {
+            if digit != digit_excluded {
+                let cell = grid.get_random_empty_cell(rng);
+                cell.unwrap().set(digit);
+            }
+        }
+
+        let status = evaluate_grid_with_solve_controller(&grid, &solve_controller);
         match status {
-        GenerateStatus::Unfinished => panic!("solve_grid_guess should never return UNFINISHED"),
+            SolveStatus::Complete(uniqueness) => {
-        _ => return status
+                let uniqueness = uniqueness.unwrap();
+                match uniqueness {
+                    Uniqueness::Unique => {
+                        eprintln!("Wow! A puzzle with only 8 guesses have been found");
+                        return grid;
+                    }
+                    Uniqueness::NotUnique => {break grid;} // What we expect
                 }
             }
-
+            SolveStatus::Unfinished => {panic!("evaluate_grid_with_solve_controller should never return UNFINISHED if we are making guesses")}
-fn solve_grid_guess(grid: &Grid) -> GenerateStatus{
+            SolveStatus::Invalid => {continue;} // unlucky; try again
-    let smallest_cell = find_smallest_cell(grid);
+        }
-    let smallest_cell = match smallest_cell {
-        Some(cell) => cell,
-        None => return GenerateStatus::NoSolution
     };
 
-    let possibilities = smallest_cell.get_value_possibilities().unwrap();
+    // Alright, we now have a grid that we can start adding more guesses onto until we find a unique solution
+    grid =
+        'outer: loop {
+            let cell = grid.get_random_empty_cell(rng).unwrap(); // We unwrap because if somehow we're filled each cell without finding a solution, that's reason for a panic
+            let cell = &*cell;
+            let mut cell_possibilities = cell.get_value_possibilities().expect("An empty cell has no possibilities");
 
-    let mut current_status = GenerateStatus::Unfinished;
+            // Let's scramble the order
+            cell_possibilities.shuffle(rng);
 
-    for (_index, &digit) in possibilities.iter().enumerate() {
+            for (_index, digit) in cell_possibilities.iter().enumerate() {
-        let mut grid_copy = grid.clone();
-        grid_copy.get(smallest_cell.x, smallest_cell.y).unwrap().set(digit);
-        let status = solve_grid(&mut grid_copy);
-        current_status = current_status.increment(status);
 
-        match current_status {
+                let mut grid_clone = grid.clone();
-            GenerateStatus::NotUniqueSolution => return GenerateStatus::NotUniqueSolution, // We have our answer; return it
+                let cell = &*grid_clone.get(cell.x, cell.y).unwrap();
-            GenerateStatus::UniqueSolution => {continue}, // Still looking to see if solution is unique
+
-            GenerateStatus::NoSolution => {panic!("current_status should not be NO_SOLUTION at this point")},
+                cell.set(*digit);
-            GenerateStatus::Unfinished => {continue} // Still looking for a solution
+
+                let status = evaluate_grid_with_solve_controller(&mut grid_clone, &solve_controller);
+                match status {
+                    SolveStatus::Complete(uniqueness) => {
+                        let uniqueness = uniqueness.unwrap();
+                        match uniqueness {
+                            Uniqueness::Unique => {break 'outer grid_clone;} // We're done!
+                            Uniqueness::NotUnique => {// We need more guesses
+                                grid = grid_clone;
+                                continue 'outer;
                             }
                         }
-
+                    }
-    // We've tried all the possibilities for this guess
+                    SolveStatus::Unfinished => panic!("evaluate_grid_with_solve_controller should never return UNFINISHED if making guesses"),
-    match current_status {
+                    SolveStatus::Invalid => continue // Try another guess
-        GenerateStatus::NotUniqueSolution => return current_status,
-        GenerateStatus::Unfinished => return GenerateStatus::NoSolution, // nothing panned out; last guess is a bust
-        GenerateStatus::UniqueSolution => return current_status, // Hey! Looks good!
-        GenerateStatus::NoSolution => {panic!("current_status should not be NO_SOLUTION at this point")}
                 }
 
+            };
+
+            // If we reach this point in the loop, then none of the possibilities for cell provided any solution
+            // Which means something serious happened before in the solving process - reason for panic
+            eprint!("No valid hints were found for puzzle\n{} at cell ({}, {})", grid, cell.x, cell.y);
+            panic!("Unable to continue as puzzle is invalid");
+
+        };
+
+    crate::solver::solve_grid(&mut grid);
+
+    return grid;
 }
 
 
 #[cfg(test)]
 mod tests {
     use crate::grid::*;
-    use crate::generator::{solve_grid, GenerateStatus};
+    use crate::solver::{solve_grid_with_solve_controller, SolveController, Uniqueness, SolveStatus};
+    use crate::generator::generate_grid;
+    use rand_chacha::ChaCha8Rng;
+    use rand_chacha::rand_core::SeedableRng;
 
     #[test]
     fn test_unique_detection() {
         // A puzzle was generated that didn't actually have a unique solution; this is to make sure that the
         // modified solving code can actually detect this case
-        let grid = Grid::new();
+        let mut grid = Grid::new();
 
         grid.get(0, 0).unwrap().set(9);
         grid.get(0, 7).unwrap().set(4);
@@ -372,9 +290,51 @@ mod tests {
 
         grid.get(8, 2).unwrap().set(6);
 
-        let status = solve_grid(&grid);
+        let status = solve_grid_with_solve_controller(&mut grid, &SolveController{
+            determine_uniqueness: true,
+            search_singles: true,
+            search_hidden_singles: true,
+            find_possibility_groups: true,
+            search_useful_constraint: true,
+            make_guesses: true
+        });
 
-        assert_eq!(status, GenerateStatus::NotUniqueSolution);
+        assert_eq!(status, SolveStatus::Complete(Some(Uniqueness::NotUnique)));
+
+    }
+
+    // There was a bug where even though mutate_grid was set to false, the end result was still solved
+    #[test]
+    fn ensure_grid_not_complete(){
+        let solve_controller = SolveController{
+            determine_uniqueness: true,
+            search_singles: true,
+            search_hidden_singles: true,
+            find_possibility_groups: true,
+            search_useful_constraint: true,
+            make_guesses: true
+        };
+
+        // Note that the puzzle itself doesn't matter
+        let (grid, _num_hints) = generate_grid(&mut ChaCha8Rng::seed_from_u64(123), &solve_controller);
+
+        let mut observed_empty_cell = false;
+        'outer : for x in 0..9 {
+            for y in 0..9 {
+                let cell = grid.get(x, y).unwrap();
+                let value = cell.get_value_copy();
+
+                match value {
+                    CellValue::Fixed(_) => {}
+                    CellValue::Unknown(_) => {
+                        observed_empty_cell = true;
+                        break 'outer;
+                    }
+                }
+            }
+        }
+
+        assert!(observed_empty_cell);
 
     }
 }

src/solver.rs

@@ -1,12 +1,128 @@
 
-use std::rc::{Rc, Weak};
+use std::rc::Rc;
-use std::cell::{RefCell};
+use crate::grid::{Cell, Line, Grid, CellValue};
-use std::collections::HashSet;
-
-use crate::grid::{Cell, Line, Grid, CellValue, LineType};
 
 pub static mut DEBUG: bool = false;
 
+#[derive(Copy, Clone, Eq, PartialEq, Debug)]
+pub enum Uniqueness {
+    Unique,
+    NotUnique
+}
+
+#[derive(Eq, PartialEq, Debug)]
+pub enum SolveStatus {
+    Complete(Option<Uniqueness>),
+    Unfinished,
+    Invalid
+}
+
+impl SolveStatus {
+
+    fn increment(self, additional_status : SolveStatus) -> SolveStatus {
+        match self {
+            SolveStatus::Complete(uniqueness_option) => {
+                if uniqueness_option.is_none() {
+                    return SolveStatus::Complete(None);
+                } else {
+                    match uniqueness_option.unwrap() {
+                        Uniqueness::NotUnique => SolveStatus::Complete(Some(Uniqueness::NotUnique)),
+                        Uniqueness::Unique => match additional_status {
+                            SolveStatus::Complete(_) => SolveStatus::Complete(Some(Uniqueness::NotUnique)),
+                            SolveStatus::Unfinished => SolveStatus::Complete(Some(Uniqueness::Unique)),
+                            SolveStatus::Invalid => SolveStatus::Complete(Some(Uniqueness::Unique))
+                        }
+                    }
+                }
+
+            }
+            SolveStatus::Unfinished => match additional_status {
+                SolveStatus::Invalid => SolveStatus::Unfinished,
+                _ => additional_status
+            },
+            SolveStatus::Invalid => panic!("increment() shouldn't be called on SolveStatus::Invalid")
+        }
+    }
+}
+
+pub struct SolveController {
+    pub determine_uniqueness: bool,
+    pub search_singles: bool,
+    pub search_hidden_singles: bool,
+    pub find_possibility_groups: bool,
+    pub search_useful_constraint: bool,
+    pub make_guesses: bool,
+}
+
+impl SolveController {
+    fn determine_uniqueness(&self) -> bool {
+        self.determine_uniqueness
+    }
+
+    fn search_singles(&self) -> bool {
+        self.search_singles
+    }
+
+    fn search_hidden_singles(&self) -> bool {
+        // search_hidden_singles is a special case of find_possibility_groups, so if find_possibility_groups
+        // is enabled then it's a waste of resources to keep this on
+        self.search_hidden_singles && !self.find_possibility_groups
+    }
+
+    fn find_possibility_groups(&self) -> bool {
+        self.find_possibility_groups
+    }
+
+    fn search_useful_constraint(&self) -> bool {
+        self.search_useful_constraint
+    }
+
+    fn make_guesses(&self) -> bool {
+        self.make_guesses
+    }
+}
+
+
+pub fn find_smallest_cell(grid: &Grid) -> Option<Rc<Cell>>{
+    // Find a cell of smallest size (in terms of possibilities) and make a guess
+    // Can assume that no cells of only possibility 1 exist
+
+    let mut smallest_cell : Option<Rc<Cell>> = None;
+    let mut smallest_size = usize::MAX;
+
+    'outer: for x in 0..9 {
+        for y in 0..9 {
+            let cell_rc = grid.get(x, y).unwrap();
+            let cell = &*grid.get(x, y).unwrap();
+            let cell_value = &*cell.value.borrow();
+
+            match cell_value {
+                CellValue::Unknown(possibilities) => {
+                    if (possibilities.len() < smallest_size) && (possibilities.len() > 0){
+                        smallest_size = possibilities.len();
+                        smallest_cell = Some(cell_rc);
+                    }
+                },
+                _ => {}
+            }
+
+            if smallest_size <= 2 {
+                break 'outer; // We aren't going to get smaller
+            }
+
+        }
+    }
+    smallest_cell
+
+}
+
+
+// Code for identify_and_process_possibility_groups (it uses it's own structs)
+mod process_possibility_groups {
+    use crate::grid::{Line, CellValue};
+    use std::collections::HashSet;
+    use std::rc::Rc;
+
     struct FauxCell{
         index: usize,
         possibilities: HashSet<u8>,
@@ -38,9 +154,9 @@ impl FauxLine {
 
     // See if there's a set of cells with possibilities that exclude those possibilities from other cells.
 // Runs recursively on each group to identify all groups in case there's more than 2.
-fn identify_and_process_possibility_groups(line: &Line){
+    pub fn identify_and_process_possibility_groups(line: &Line){
         unsafe {
-        if DEBUG {
+            if super::DEBUG {
                 println!("Looking for possibility groups on line {:?} {}", line.line_type, line.index);
             }
         }
@@ -216,6 +332,7 @@ fn bisect_possibility_groups(line: &Line, cells_of_interest: Vec<usize>){
             bisect_possibility_groups(line, out_group_indices);
         }
     }
+}
 
 // Search for a cell with only one possibility so that we can set it to FIXED
 fn search_single_possibility(line: &Line){
@@ -238,6 +355,56 @@ fn search_single_possibility(line: &Line){
     }
 }
 
+// Count up how many times each possibility occurs in the Line. If it only occurs once, that's a hidden single that we can set
+fn search_hidden_single(line: &Line){
+    enum Count {
+        None,
+        One(Rc<Cell>),
+        Many
+    };
+
+    impl Count {
+        fn increment(&self, cell: Rc<Cell>) -> Count{
+            match self {
+                Count::None => {Count::One(cell)}
+                Count::One(_) => {Count::Many}
+                Count::Many => {Count::Many}
+            }
+        }
+    }
+
+    let mut counts = [Count::None, Count::None, Count::None, Count::None, Count::None, Count::None, Count::None, Count::None, Count::None];
+
+    for (_index, cell) in line.vec.iter().enumerate() {
+        let value = &*cell.value.borrow();
+        match value {
+            CellValue::Unknown(possibilities) => {
+                for digit in 1..10 {
+                    if possibilities.contains(&(digit as u8)){
+                        counts[digit-1] = counts[digit-1].increment(Rc::clone(cell));
+                    }
+                }
+            },
+            CellValue::Fixed(_) => {} // do nothing
+        }
+    }
+
+    for (digit, count) in counts.iter().enumerate() {
+        match count {
+            Count::One(cell) => {
+                cell.set((digit + 1) as u8);
+            },
+            _ => {}
+        }
+
+    }
+}
+
+mod search_useful_constraint{
+    use crate::grid::{Grid, Line, LineType, CellValue};
+    use std::rc::{Rc, Weak};
+    use std::cell::RefCell;
+
     enum PossibilityLines {
         Unique(usize),
         Invalid,
@@ -257,9 +424,9 @@ impl PossibilityLines {
 // I.e. If possibility '1' only occurs in the first row for section 0, then you can remove that possibility
 // from row 0 across the other sections. Conversely, if the possibility only occurs in the first section
 // for row 0, then you can remove the possibility from the rest of section 0.
-fn search_useful_constraint(grid: &Grid, line: &Line){
+    pub fn search_useful_constraint(grid: &Grid, line: &Line){
         unsafe {
-        if DEBUG {
+            if super::DEBUG {
                 println!("Searching for a useful constraint on line {:?} {}", line.line_type, line.index);
             }
         }
@@ -375,7 +542,7 @@ fn remove_possibilities_line(line: &Rc<RefCell<Line>>, digit_to_remove: u8, init
         }
     }
 
-// We detected
+    // We detected a useful constraint
     fn process_possibility_line(possibility_line: PossibilityLines, line: &Weak<RefCell<Line>>) -> PossibilityLines {
         let line = line.upgrade().unwrap();
         let line = &*(&*line).borrow();
@@ -393,8 +560,10 @@ fn process_possibility_line(possibility_line: PossibilityLines, line: &Weak<RefC
         }
     }
 
+}
 
-fn solve_line(grid: &Grid, line: &Line){
+
+fn solve_line(grid: &Grid, line: &Line, solve_controller: &SolveController){
     unsafe {
         if DEBUG {
             println!("Solving {:?} {}", line.line_type, line.index);
@@ -403,91 +572,89 @@ fn solve_line(grid: &Grid, line: &Line){
 
     line.do_update.replace(false);
 
+    if solve_controller.search_singles() {
+        unsafe {
+            if DEBUG {
+                println!("Searching for singles on line {:?} of {}\n{}", line.line_type, line.index, grid);
+            }
+        }
         search_single_possibility(line);
+    }
+
+    if solve_controller.search_hidden_singles() {
         unsafe {
             if DEBUG {
-            println!("{}", grid);
+                println!("Searching for hidden singles on line {:?} of {}\n{}", line.line_type, line.index, grid);
             }
         }
+        search_hidden_single(line);
+    }
 
-    identify_and_process_possibility_groups(line);
+    if solve_controller.find_possibility_groups() {
         unsafe {
             if DEBUG {
-            println!("{}", grid);
+                println!("Searching for possibility groups on line {:?} of {}\n{}", line.line_type, line.index, grid);
             }
         }
+        process_possibility_groups::identify_and_process_possibility_groups(line);
+    }
 
-    search_useful_constraint(grid, line);
+    if solve_controller.search_useful_constraint() {
         unsafe {
             if DEBUG {
-            println!("{}", grid);
+                println!("Searching for useful constraints on line {:?} of {}\n{}", line.line_type, line.index, grid);
             }
         }
-
+        search_useful_constraint::search_useful_constraint(grid, line);
     }
 
-pub fn find_smallest_cell(grid: &Grid) -> Option<Rc<Cell>>{
-    // Find a cell of smallest size (in terms of possibilities) and make a guess
-    // Can assume that no cells of only possibility 1 exist
-
-    let mut smallest_cell : Option<Rc<Cell>> = None;
-    let mut smallest_size = usize::MAX;
-
-    'outer: for x in 0..9 {
-        for y in 0..9 {
-            let cell_rc = grid.get(x, y).unwrap();
-            let cell = &*grid.get(x, y).unwrap();
-            let cell_value = &*cell.value.borrow();
-
-            match cell_value {
-                CellValue::Unknown(possibilities) => {
-                    if (possibilities.len() < smallest_size) && (possibilities.len() > 0){
-                        smallest_size = possibilities.len();
-                        smallest_cell = Some(cell_rc);
-                    }
-                },
-                _ => {}
-            }
-
-            if smallest_size <= 2 {
-                break 'outer; // We aren't going to get smaller
-            }
-
-        }
-    }
-    smallest_cell
-
-}
-
-
-pub enum SolveStatus {
-    Complete,
-    Unfinished,
-    Invalid
 }
 
 pub fn solve_grid(grid: &mut Grid) -> SolveStatus {
+    // By default we enable everything
+    let solve_controller = SolveController {
+        determine_uniqueness: true,
+        search_singles: true,
+        search_hidden_singles: true,
+        find_possibility_groups: true,
+        search_useful_constraint: true,
+        make_guesses: true
+    };
+
+    solve_grid_with_solve_controller(grid, &solve_controller)
+
+}
+
+pub fn solve_grid_with_solve_controller(grid: &mut Grid, solve_controller: &SolveController) -> SolveStatus{
     // Code is kind of messy so here it goes - solve_grid first tries to solve without any guesses
     // If that's not enough and a guess is required, then solve_grid_guess is called
     // solve_grid_guess runs through all the possibilities for the smallest cell, trying to solve them
     // through calling this function.
     // solve_grid_no_guess tries to solve without any guesses.
+    // Of course this is if the solve_controller lets everything be used for solving it
 
-    let mut status = solve_grid_no_guess(grid);
+    let mut status = solve_grid_no_guess(grid, solve_controller);
     status = match status {
         SolveStatus::Unfinished => {
-            solve_grid_guess(grid)
+            if solve_controller.make_guesses() {
+                solve_grid_guess(grid, solve_controller)
+            } else {
+                SolveStatus::Complete(Some(Uniqueness::NotUnique)) // solve_grid_no_guess couldn't finish and we can't make guesses, so it's 'not unique' in the sense that we need more guesses
+            }
         },
         _ => {status}
     };
 
-    match status {
+    return status;
-        SolveStatus::Unfinished => panic!("solve_grid_guess should never return UNFINISHED"),
-        _ => return status
-    }
 }
 
-pub fn solve_grid_no_guess(grid: &mut Grid) -> SolveStatus{
+// Similar to solve_grid_with_solve_controller except that we don't modify the input Grid; we only determine SolveStatus
+pub fn evaluate_grid_with_solve_controller(grid: &Grid, solve_controller: &SolveController) -> SolveStatus{
+    let mut mut_grid = grid.clone();
+    return solve_grid_with_solve_controller(&mut mut_grid, solve_controller);
+}
+
+pub fn solve_grid_no_guess(grid: &mut Grid, solve_controller: &SolveController) -> SolveStatus{
 
     loop {
         let mut ran_something = false;
@@ -495,7 +662,7 @@ pub fn solve_grid_no_guess(grid: &mut Grid) -> SolveStatus{
             //println!("Processing row {}", _index);
             let line_ref = &*(&**line_ref).borrow();
             if line_ref.do_update() {
-                solve_line(&grid, line_ref);
+                solve_line(&grid, line_ref, solve_controller);
                 ran_something = true;
             }
         }
@@ -503,7 +670,7 @@ pub fn solve_grid_no_guess(grid: &mut Grid) -> SolveStatus{
             //println!("Processing column {}", _index);
             let line_ref = &*(&**line_ref).borrow();
             if line_ref.do_update() {
-                solve_line(&grid, line_ref);
+                solve_line(&grid, line_ref, solve_controller);
                 ran_something = true;
             }
         }
@@ -511,7 +678,7 @@ pub fn solve_grid_no_guess(grid: &mut Grid) -> SolveStatus{
             //println!("Processing section {}", _index);
             let line_ref = &*(&**line_ref).borrow();
             if line_ref.do_update() {
-                solve_line(&grid, line_ref);
+                solve_line(&grid, line_ref, solve_controller);
                 ran_something = true;
             }
         }
@@ -542,13 +709,15 @@ pub fn solve_grid_no_guess(grid: &mut Grid) -> SolveStatus{
         }
 
         if appears_complete {
-            return SolveStatus::Complete;
+            // Solving by logic rules only implies Uniqueness;
+            // may be overridden if guesses were made
+            return SolveStatus::Complete(Some(Uniqueness::Unique));
         }
     }
 
 }
 
-fn solve_grid_guess(grid: &mut Grid) -> SolveStatus{
+fn solve_grid_guess(grid: &mut Grid, solve_controller: &SolveController) -> SolveStatus{
     let smallest_cell = find_smallest_cell(grid);
     let smallest_cell = match smallest_cell {
         Some(cell) => cell,
@@ -556,26 +725,60 @@ fn solve_grid_guess(grid: &mut Grid) -> SolveStatus{
     };
 
     let possibilities = smallest_cell.get_value_possibilities().unwrap();
+
+    let mut current_status = SolveStatus::Unfinished;
+    let mut grid_solution = None;
+
     for (_index, &digit) in possibilities.iter().enumerate() {
+
         let mut grid_copy = grid.clone();
         grid_copy.get(smallest_cell.x, smallest_cell.y).unwrap().set(digit);
-        let status = solve_grid(&mut grid_copy);
+        let status = solve_grid_with_solve_controller(&mut grid_copy, solve_controller);
 
+        // Keep a copy of grid_copy in case we later mutate grid with it
         match status {
-            SolveStatus::Complete => {
+            SolveStatus::Complete(_) => {
-                grid.clone_from(&grid_copy);
+                grid_solution = Some(grid_copy);
-                return SolveStatus::Complete;
+            },
+            _ => {}
+        }
+
+        current_status = current_status.increment(status);
+
+        match current_status {
+            SolveStatus::Complete(uniqueness) => {
+                if !solve_controller.determine_uniqueness() {
+                    current_status = SolveStatus::Complete(None); // be explicit we don't know
+                    break; // no point in continuing
+                }
+
+                let uniqueness = uniqueness.expect("We're looking for uniqueness and yet an earlier function didn't make a claim");
+                match uniqueness {
+                    Uniqueness::Unique => {continue;} // gotta keep on checking
+                    Uniqueness::NotUnique => {
+                        break; // We can stop looking as we already found at least two solutions
+                    }
+                }
+            }
+            SolveStatus::Unfinished => {continue} // Keep looking for a solution
+            SolveStatus::Invalid => panic!("current_status should not be INVALID at this point")
+        }
+
+
+    }
+
+    // We've finished the for-loop
+    match current_status {
+        SolveStatus::Complete(_) => {
+            grid.clone_from(&grid_solution.expect("grid_solution should have value if we found a solution"));
         },
         SolveStatus::Unfinished => {
-                panic!("solve_grid should never return UNFINISHED")
+            current_status = SolveStatus::Invalid; // We can now say Invalid
         },
-            SolveStatus::Invalid => {
+        SolveStatus::Invalid => {}
-                continue;
-            }
-        }
     }
 
-    return SolveStatus::Invalid;
+    return current_status;
 
 }
 
@@ -619,7 +822,7 @@ mod tests {
         let line = grid.rows.first().unwrap();
         let line = &*(**line).borrow();
 
-        identify_and_process_possibility_groups(line);
+        process_possibility_groups::identify_and_process_possibility_groups(line);
 
         assert_eq!(CellValue::Unknown(vec![1, 2, 3]), grid.get(0, 0).unwrap().get_value_copy());
     }
@@ -643,7 +846,7 @@ mod tests {
         let line = grid.rows.first().unwrap();
         let line = &*(**line).borrow();
 
-        search_useful_constraint(&grid, line);
+        search_useful_constraint::search_useful_constraint(&grid, line);
 
         assert_eq!(CellValue::Unknown(vec![4, 5, 6, 7, 8, 9]), grid.get(2, 0).unwrap().get_value_copy());
     }
@@ -676,11 +879,38 @@ mod tests {
         let line = grid.columns.get(1).unwrap();
         let line = &*(**line).borrow();
 
-        search_useful_constraint(&grid, line);
+        search_useful_constraint::search_useful_constraint(&grid, line);
 
         assert_eq!(CellValue::Unknown(vec![1, 3, 4, 5, 9]), grid.get(1, 0).unwrap().get_value_copy());
 
     }
 
+    #[test]
+    fn test_hidden_single() {
+        let grid = Grid::new();
+
+        // In Row 0 there should be only one spot for 1s and 2s to be set, even though every cell will
+        // have possibilities for values 3 - 9
+
+        grid.get(1, 5).unwrap().set(1);
+        grid.get(2, 6).unwrap().set(1);
+        grid.get(5, 2).unwrap().set(1);
+        grid.get(6, 1).unwrap().set(1);
+
+        grid.get(1, 6).unwrap().set(2);
+        grid.get(2, 5).unwrap().set(2);
+        grid.get(5, 1).unwrap().set(2);
+        grid.get(6, 0).unwrap().set(2);
+
+        let first_row = grid.rows.get(0).unwrap();
+        let first_row = &*(**first_row).borrow();
+
+        search_hidden_single(&first_row);
+
+        assert_eq!(CellValue::Fixed(1), grid.get(0, 0).unwrap().get_value_copy());
+        assert_eq!(CellValue::Fixed(2), grid.get(0, 2).unwrap().get_value_copy());
+
+    }
+
 
 }