rustoku_lib/core/

mod.rs

//! Core module for the Rustoku solver and generator.
//!
//! This module includes the `Rustoku` struct for representing and solving Sudoku puzzles using
//! backtracking and Minimum Remaining Values (MRV). It provides functionality for solving
//! puzzles and checking solutions.
//!
//! This module also includes a function to generate new Sudoku puzzles with a specified
//! number of clues, ensuring that the generated puzzle has a unique solution.

mod board;
mod candidates;
mod masks;
mod solution;
mod techniques;

pub use board::Board;
pub use candidates::Candidates;
pub use masks::Masks;
pub use solution::{Solution, SolvePath, SolveStep};
pub use techniques::flags::TechniqueFlags;

use crate::error::RustokuError;
use rand::prelude::SliceRandom;
use rand::rng;
use techniques::TechniquePropagator;

/// Solver primitive that uses backtracking and bitmasking for constraints.
///
/// This struct supports the ability to:
/// - Initialize from a 2D array, a flat byte array, or a string representation
/// - Solve a Sudoku puzzle using backtracking with Minimum Remaining Values (MRV)
/// - Generate a Sudoku puzzle with a unique solution based on the number of clues specified
/// - Check if a Sudoku puzzle is solved correctly
///
/// # Examples
///
/// Solve a Sudoku puzzle:
/// ```
/// use rustoku_lib::Rustoku;
/// let puzzle = "530070000600195000098000060800060003400803001700020006060000280000419005000080079";
/// let mut rustoku = Rustoku::new_from_str(puzzle).unwrap();
/// assert!(rustoku.solve_any().is_some());
/// ```
///
/// Generate a Sudoku puzzle:
/// ```
/// use rustoku_lib::{Rustoku, generate_board};
/// let board = generate_board(30).unwrap();
/// let solution = Rustoku::new(board).unwrap().solve_all();
/// assert_eq!(solution.len(), 1);
/// ```
///
/// Check if a Sudoku puzzle is solved:
/// ```
/// use rustoku_lib::Rustoku;
/// let puzzle = "534678912672195348198342567859761423426853791713924856961537284287419635345286179";
/// let rustoku = Rustoku::new_from_str(puzzle).unwrap();
/// assert!(rustoku.is_solved());
/// ```
#[derive(Debug, Copy, Clone)]
pub struct Rustoku {
    /// The current state of the Sudoku board.
    pub board: Board,
    /// Bitmasks that check if a cell is safe in a row, column and box.
    pub masks: Masks,
    /// Candidate cache from computing the bitmasks.
    pub candidates: Candidates,
    /// Techniques used during the initial phase of solving.
    pub techniques: TechniqueFlags,
}

impl Rustoku {
    /// Constructs a new `Rustoku` instance from an initial `Board`.
    pub fn new(initial_board: Board) -> Result<Self, RustokuError> {
        let board = initial_board; // Now takes a Board directly
        let mut masks = Masks::new();
        let mut candidates = Candidates::new();

        // Initialize masks and check for duplicates based on the provided board
        for r in 0..9 {
            for c in 0..9 {
                let num = board.get(r, c);
                if num != 0 {
                    if !masks.is_safe(r, c, num) {
                        return Err(RustokuError::DuplicateValues);
                    }
                    masks.add_number(r, c, num);
                }
            }
        }

        // Initialize the candidates cache for empty cells based on initial masks and board
        for r in 0..9 {
            for c in 0..9 {
                if board.is_empty(r, c) {
                    candidates.set(r, c, masks.compute_candidates_mask_for_cell(r, c));
                }
            }
        }

        Ok(Self {
            board,
            masks,
            candidates,
            techniques: TechniqueFlags::EASY, // Default
        })
    }

    /// Start building a configured `Rustoku` via a builder pattern.
    pub fn builder() -> RustokuBuilder {
        RustokuBuilder::new()
    }

    /// Constructs a new `Rustoku` instance from a string representation of the board.
    pub fn new_from_str(s: &str) -> Result<Self, RustokuError> {
        let board = Board::try_from(s)?;
        Self::new(board)
    }

    /// Returns the existing Rustoku instance, with modified techniques.
    pub fn with_techniques(mut self, techniques: TechniqueFlags) -> Self {
        self.techniques = techniques;
        self
    }

    /// Extracts candidate numbers (1-9) from a bitmask into a Vec.
    fn candidates_from_mask(mask: u16) -> Vec<u8> {
        let mut nums = Vec::with_capacity(mask.count_ones() as usize);
        for v in 1..=9u8 {
            if mask & (1 << (v - 1)) != 0 {
                nums.push(v);
            }
        }
        nums
    }

    /// Helper for solver to find the next empty cell (MRV).
    fn find_next_empty_cell(&self) -> Option<(usize, usize)> {
        let mut min = (10, None); // Min candidates, (r, c)
        for (r, c) in self.board.iter_empty_cells() {
            let count = self.candidates.get(r, c).count_ones() as u8;
            if count < min.0 {
                min = (count, Some((r, c)));
                if count == 1 {
                    return min.1;
                }
            }
        }
        min.1
    }

    /// Place and remove operations for the solver, updated to use the new structs.
    fn place_number(&mut self, r: usize, c: usize, num: u8) {
        self.board.set(r, c, num);
        self.masks.add_number(r, c, num);
        self.candidates
            .update_affected_cells_for(r, c, &self.masks, &self.board, Some(num));
    }

    /// Remove a number from the board and update masks and candidates.
    fn remove_number(&mut self, r: usize, c: usize, num: u8) {
        self.board.set(r, c, 0); // Set back to empty
        self.masks.remove_number(r, c, num);
        self.candidates
            .update_affected_cells(r, c, &self.masks, &self.board);
        // Note: `update_affected_cells` will recalculate candidates for the removed cell.
    }

    /// Recursive function to solve the Sudoku puzzle with backtracking.
    fn solve_until_recursive(
        &mut self,
        solutions: &mut Vec<Solution>,
        path: &mut SolvePath,
        bound: usize,
    ) -> usize {
        // Early return for base case: no empty cells means puzzle is solved
        let Some((r, c)) = self.find_next_empty_cell() else {
            solutions.push(Solution {
                board: self.board,
                solve_path: path.clone(),
            });
            return 1;
        };

        let mut count = 0;
        // Use the candidate cache to only iterate valid candidates
        let mask = self.candidates.get(r, c);
        let mut nums = Self::candidates_from_mask(mask);
        nums.shuffle(&mut rng());

        for &num in &nums {
            if !self.masks.is_safe(r, c, num) {
                continue;
            }

            self.place_number(r, c, num);
            let step_number = path.steps.len() as u32;
            path.steps.push(SolveStep::Placement {
                row: r,
                col: c,
                value: num,
                flags: TechniqueFlags::empty(),
                step_number,
                candidates_eliminated: 0,
                related_cell_count: 0,
                difficulty_point: 0,
            });

            count += self.solve_until_recursive(solutions, path, bound);
            path.steps.pop();
            self.remove_number(r, c, num);

            // Early return if we've found enough solutions
            if bound > 0 && solutions.len() >= bound {
                return count;
            }
        }

        count
    }

    /// Run techniques and check if they make valid changes.
    fn techniques_make_valid_changes(&mut self, path: &mut SolvePath) -> bool {
        let mut propagator = TechniquePropagator::new(
            &mut self.board,
            &mut self.masks,
            &mut self.candidates,
            self.techniques,
        );
        propagator.propagate_constraints(path, 0)
    }

    /// Solves the Sudoku puzzle up to a certain bound, returning solutions with their solve paths.
    pub fn solve_until(&mut self, bound: usize) -> Vec<Solution> {
        let mut solutions = Vec::new();
        let mut path = SolvePath::default();

        if !self.techniques_make_valid_changes(&mut path) {
            return solutions;
        }

        self.solve_until_recursive(&mut solutions, &mut path, bound);
        solutions
    }

    /// Attempts to solve the Sudoku puzzle using backtracking with MRV (Minimum Remaining Values).
    pub fn solve_any(&mut self) -> Option<Solution> {
        self.solve_until(1).into_iter().next()
    }

    /// Finds all possible solutions for the Sudoku puzzle.
    pub fn solve_all(&mut self) -> Vec<Solution> {
        use rayon::prelude::*;

        // Run technique propagation once on the current solver state.
        let mut path = SolvePath::default();
        if !self.techniques_make_valid_changes(&mut path) {
            return Vec::new();
        }

        // If there is at least one empty cell, split work by the first MRV cell's candidates.
        if let Some((r, c)) = self.find_next_empty_cell() {
            let mask = self.candidates.get(r, c);
            let nums = Self::candidates_from_mask(mask);

            let initial_path = path.clone();

            // Parallelize each top-level candidate branch.
            let chunks: Vec<Vec<Solution>> = nums
                .par_iter()
                .map(|&num| {
                    let mut cloned = *self; // Rustoku is Copy/Clone
                    let mut local_solutions: Vec<Solution> = Vec::new();
                    let mut local_path = initial_path.clone();

                    // Place the candidate and record the placement in the path.
                    cloned.place_number(r, c, num);
                    let step_number = local_path.steps.len() as u32;
                    local_path.steps.push(SolveStep::Placement {
                        row: r,
                        col: c,
                        value: num,
                        flags: TechniqueFlags::empty(),
                        step_number,
                        candidates_eliminated: 0,
                        related_cell_count: 0,
                        difficulty_point: 0,
                    });

                    // Continue DFS from this state without re-running the propagator.
                    cloned.solve_until_recursive(&mut local_solutions, &mut local_path, 0);
                    local_solutions
                })
                .collect();

            // Flatten results
            let mut solutions = Vec::new();
            for mut s in chunks {
                solutions.append(&mut s);
            }
            solutions
        } else {
            // Already solved after propagation
            vec![Solution {
                board: self.board,
                solve_path: path,
            }]
        }
    }

    /// Checks if the Sudoku puzzle is solved correctly.
    pub fn is_solved(&self) -> bool {
        self.board.cells.iter().flatten().all(|&val| val != 0) && Rustoku::new(self.board).is_ok()
    }
}

/// A simple builder for constructing `Rustoku` with fluent configuration.
pub struct RustokuBuilder {
    board: Option<Board>,
    techniques: TechniqueFlags,
    max_solutions: Option<usize>,
}

impl RustokuBuilder {
    /// Create a new builder with reasonable defaults.
    pub fn new() -> Self {
        RustokuBuilder {
            board: None,
            techniques: TechniqueFlags::EASY,
            max_solutions: None,
        }
    }
}

impl Default for RustokuBuilder {
    fn default() -> Self {
        Self::new()
    }
}

impl RustokuBuilder {
    /// Provide the initial `Board` for the solver.
    pub fn board(mut self, board: Board) -> Self {
        self.board = Some(board);
        self
    }

    /// Provide the initial board as a string (convenience).
    pub fn board_from_str(mut self, s: &str) -> Result<Self, RustokuError> {
        let board = Board::try_from(s)?;
        self.board = Some(board);
        Ok(self)
    }

    /// Configure which techniques the solver should use.
    pub fn techniques(mut self, techniques: TechniqueFlags) -> Self {
        self.techniques = techniques;
        self
    }

    /// Optionally hint the builder with a maximum number of solutions.
    pub fn max_solutions(mut self, max: usize) -> Self {
        self.max_solutions = Some(max);
        self
    }

    /// Finalize the builder and construct the `Rustoku` instance.
    pub fn build(self) -> Result<Rustoku, RustokuError> {
        let board = self.board.unwrap_or_default();
        let mut r = Rustoku::new(board)?;
        r.techniques = self.techniques;
        // If the user provided a max_solutions hint, we store it in techniques as not applicable
        // for now; the builder primarily configures creation state.
        Ok(r)
    }
}

/// Lazy iterator wrapper for solutions. Uses an explicit DFS stack and yields
/// solutions one-by-one without computing them all up-front.
#[derive(Debug)]
pub struct Solutions {
    solver: Rustoku,
    path: SolvePath,
    stack: Vec<Frame>,
    finished: bool,
}

#[derive(Debug)]
struct Frame {
    r: usize,
    c: usize,
    nums: Vec<u8>,
    idx: usize,
    placed: Option<u8>,
}

impl Solutions {
    /// Construct a `Solutions` iterator from an existing `Rustoku` solver.
    /// This will run the technique propagator once before starting DFS.
    pub fn from_solver(mut solver: Rustoku) -> Self {
        let mut path = SolvePath::default();
        let mut finished = false;

        if !solver.techniques_make_valid_changes(&mut path) {
            finished = true;
        }

        let mut stack = Vec::new();
        if !finished {
            if let Some((r, c)) = solver.find_next_empty_cell() {
                let mask = solver.candidates.get(r, c);
                let mut nums = Rustoku::candidates_from_mask(mask);
                nums.shuffle(&mut rng());
                stack.push(Frame {
                    r,
                    c,
                    nums,
                    idx: 0,
                    placed: None,
                });
            } else {
                // Already solved; leave stack empty and let next() yield the board once
            }
        }

        Solutions {
            solver,
            path,
            stack,
            finished,
        }
    }
}

impl Iterator for Solutions {
    type Item = Solution;

    fn next(&mut self) -> Option<Self::Item> {
        if self.finished {
            return None;
        }

        loop {
            // If stack is empty, check if there are any empty cells left
            if self.stack.is_empty() {
                if let Some((r, c)) = self.solver.find_next_empty_cell() {
                    let mask = self.solver.candidates.get(r, c);
                    let mut nums = Rustoku::candidates_from_mask(mask);
                    nums.shuffle(&mut rng());
                    self.stack.push(Frame {
                        r,
                        c,
                        nums,
                        idx: 0,
                        placed: None,
                    });
                    continue;
                } else {
                    // No empty cells -> current board is a solution
                    let sol = Solution {
                        board: self.solver.board,
                        solve_path: self.path.clone(),
                    };
                    self.finished = true;
                    return Some(sol);
                }
            }

            let last_idx = self.stack.len() - 1;
            let frame = &mut self.stack[last_idx];

            // If we've exhausted candidates for this frame
            if frame.idx >= frame.nums.len() {
                if let Some(num) = frame.placed {
                    // remove the previously placed number
                    self.solver.remove_number(frame.r, frame.c, num);
                    self.path.steps.pop();
                    frame.placed = None;
                } else {
                    // No placement was made for this frame; pop it and continue
                    self.stack.pop();
                }
                continue;
            }

            let num = frame.nums[frame.idx];
            frame.idx += 1;

            if self.solver.masks.is_safe(frame.r, frame.c, num) {
                // place and record
                self.solver.place_number(frame.r, frame.c, num);
                let step_number = self.path.steps.len() as u32;
                self.path.steps.push(SolveStep::Placement {
                    row: frame.r,
                    col: frame.c,
                    value: num,
                    flags: TechniqueFlags::empty(),
                    step_number,
                    candidates_eliminated: 0,
                    related_cell_count: 0,
                    difficulty_point: 0,
                });
                frame.placed = Some(num);

                // Find next empty cell after this placement
                if let Some((nr, nc)) = self.solver.find_next_empty_cell() {
                    let mask = self.solver.candidates.get(nr, nc);
                    let mut nums2 = Rustoku::candidates_from_mask(mask);
                    nums2.shuffle(&mut rng());
                    self.stack.push(Frame {
                        r: nr,
                        c: nc,
                        nums: nums2,
                        idx: 0,
                        placed: None,
                    });
                    continue;
                } else {
                    // Found a solution. Capture it, then backtrack one placement so iteration can continue.
                    let solution = Solution {
                        board: self.solver.board,
                        solve_path: self.path.clone(),
                    };
                    // Backtrack the placement we just made on this frame
                    if let Some(pnum) = frame.placed {
                        self.solver.remove_number(frame.r, frame.c, pnum);
                        self.path.steps.pop();
                        frame.placed = None;
                    }
                    return Some(solution);
                }
            }
            // else try next candidate
        }
    }
}

/// Generates a new Sudoku puzzle with a unique solution.
///
/// The `num_clues` parameter specifies the desired number of initially
/// filled cells (clues) in the generated puzzle. Fewer clues generally
/// result in a harder puzzle. The actual number of clues may be slightly
/// more than `num_clues` if it's impossible to remove more numbers
/// while maintaining a unique solution.
///
/// # Example
///
/// Generate a puzzle with 30 clues:
/// ```
/// use rustoku_lib::generate_board;
/// let puzzle = generate_board(30);
/// assert!(puzzle.is_ok());
/// ```
pub fn generate_board(num_clues: usize) -> Result<Board, RustokuError> {
    if !(17..=81).contains(&num_clues) {
        return Err(RustokuError::InvalidClueCount);
    }

    // Start with a fully solved board
    let mut rustoku = Rustoku::new(Board::default())?;
    let solution = rustoku.solve_any().ok_or(RustokuError::DuplicateValues)?;
    let mut board = solution.board;

    // Shuffle all cell coordinates
    let mut cells: Vec<(usize, usize)> = board.iter_cells().collect();
    cells.shuffle(&mut rng());

    let mut clues = 81;

    // Remove numbers while maintaining a unique solution
    for &(r, c) in &cells {
        if clues <= num_clues {
            break;
        }

        let original = board.cells[r][c];
        board.cells[r][c] = 0;

        if Rustoku::new(board)?.solve_until(2).len() != 1 {
            board.cells[r][c] = original; // Restore if not unique
        } else {
            clues -= 1;
        }
    }

    // Check if the generated puzzle has a unique solution
    if Rustoku::new(board)?.solve_until(2).len() != 1 {
        // If not unique, return an error
        return Err(RustokuError::GenerateFailure);
    }

    Ok(board)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::core::board::Board;
    use crate::error::RustokuError;
    use crate::format::format_line;

    const UNIQUE_PUZZLE: &str =
        "530070000600195000098000060800060003400803001700020006060000280000419005000080079";
    const UNIQUE_SOLUTION: &str =
        "534678912672195348198342567859761423426853791713924856961537284287419635345286179";
    const TWO_PUZZLE: &str =
        "295743861431865900876192543387459216612387495549216738763504189928671354154938600";
    const SIX_PUZZLE: &str =
        "295743001431865900876192543387459216612387495549216738763500000000000000000000000";

    #[test]
    fn test_new_with_bytes_and_str() {
        let board = [
            [5, 3, 0, 0, 7, 0, 0, 0, 0],
            [6, 0, 0, 1, 9, 5, 0, 0, 0],
            [0, 9, 8, 0, 0, 0, 0, 6, 0],
            [8, 0, 0, 0, 6, 0, 0, 0, 3],
            [4, 0, 0, 8, 0, 3, 0, 0, 1],
            [7, 0, 0, 0, 2, 0, 0, 0, 6],
            [0, 6, 0, 0, 0, 0, 2, 8, 0],
            [0, 0, 0, 4, 1, 9, 0, 0, 5],
            [0, 0, 0, 0, 8, 0, 0, 7, 9],
        ];

        let flat_bytes: [u8; 81] = board
            .concat()
            .try_into()
            .expect("Concat board to bytes failed");
        let board_str: String = flat_bytes.iter().map(|&b| (b + b'0') as char).collect();

        let board_from_new = Board::new(board);
        let board_from_bytes = Board::try_from(flat_bytes).expect("Board from flat bytes failed");
        let board_from_str = Board::try_from(board_str.as_str()).expect("Board from string failed");

        assert_eq!(board_from_new, board_from_bytes);
        assert_eq!(board_from_new, board_from_str);
        assert_eq!(board_from_bytes, board_from_str);
    }

    #[test]
    fn test_try_from_with_valid_input() {
        let rustoku = Board::try_from(UNIQUE_PUZZLE);
        assert!(rustoku.is_ok());
    }

    #[test]
    fn test_try_from_with_invalid_length() {
        let s = "530070000"; // Too short
        let rustoku = Board::try_from(s);
        assert!(matches!(rustoku, Err(RustokuError::InvalidInputLength)));
    }

    #[test]
    fn test_try_from_with_invalid_character() {
        let s = "53007000060019500009800006080006000340080300170002000606000028000041900500008007X"; // 'X'
        let rustoku = Board::try_from(s);
        assert!(matches!(rustoku, Err(RustokuError::InvalidInputCharacter)));
    }

    #[test]
    fn test_builder_and_iterator() {
        let board = Board::try_from(UNIQUE_PUZZLE).expect("valid puzzle");
        let solver = Rustoku::builder()
            .board(board)
            .techniques(TechniqueFlags::all())
            .build()
            .expect("builder build");

        // Using the iterator wrapper (eager compute, lazy yield)
        let mut sols = Solutions::from_solver(solver);
        let first = sols.next();
        assert!(first.is_some());
        // For unique puzzle, there should be exactly one solution
        assert!(sols.next().is_none());
    }

    #[test]
    fn test_try_from_with_duplicate_initial_values() {
        let s = "530070000600195000098000060800060003400803001700020006060000280000419005500080079";
        let board = Board::try_from(s).expect("Board parsing failed before duplicate check");
        let rustoku = Rustoku::new(board);
        assert!(matches!(rustoku, Err(RustokuError::DuplicateValues)));
    }

    #[test]
    fn test_solve_any_with_solvable_sudoku() {
        let s = UNIQUE_PUZZLE;
        let mut rustoku =
            Rustoku::new_from_str(s).expect("Rustoku creation failed from puzzle string");
        let solution = rustoku.solve_any().expect("Solving solvable puzzle failed");

        assert_eq!(
            UNIQUE_SOLUTION,
            format_line(&solution.board),
            "Solution does not match the expected result"
        );
    }

    #[test]
    fn test_solve_any_with_unsolvable_sudoku() {
        let s = "078002609030008020002000083000000040043090000007300090200001036001840902050003007";
        let mut rustoku = Rustoku::new_from_str(s).expect("Rustoku creation failed");
        let solution = rustoku.solve_any();
        assert!(
            solution.is_none(),
            "Expected no solution for this unsolvable puzzle"
        );
    }

    #[test]
    fn test_solve_until_with_bound() {
        let s = UNIQUE_PUZZLE;
        let mut rustoku =
            Rustoku::new_from_str(s).expect("Rustoku creation failed from puzzle string");

        let solutions = rustoku.solve_until(1);
        assert_eq!(
            1,
            solutions.len(),
            "Expected exactly one solution with bound = 1"
        );

        let all_solutions = rustoku.solve_until(0);
        assert_eq!(
            1,
            all_solutions.len(),
            "Expected exactly one solution for this board with bound = 0"
        );

        assert_eq!(
            solutions[0].board, all_solutions[0].board,
            "Solution with bound = 1 does not match the solution with bound = 0"
        );
    }

    #[test]
    fn test_solve_all_with_unique_puzzle() {
        let s = UNIQUE_PUZZLE;
        let mut rustoku =
            Rustoku::new_from_str(s).expect("Rustoku creation failed from unique puzzle string");
        let solutions = rustoku.solve_all();
        assert_eq!(
            1,
            solutions.len(),
            "Expected a unique solution for the board"
        );
    }

    #[test]
    fn test_solve_all_with_two_puzzle() {
        let s = TWO_PUZZLE;
        let mut rustoku =
            Rustoku::new_from_str(s).expect("Rustoku creation failed from two puzzle string");
        let solutions = rustoku.solve_all();
        assert_eq!(
            2,
            solutions.len(),
            "Expected two solutions for the given board"
        );
    }

    #[test]
    fn test_solve_all_with_six_puzzle() {
        let s = SIX_PUZZLE;
        let mut rustoku =
            Rustoku::new_from_str(s).expect("Rustoku creation failed from six puzzle string");
        let solutions = rustoku.solve_all();
        assert_eq!(
            6,
            solutions.len(),
            "Expected one solution for the six puzzle"
        );
    }

    #[test]
    fn test_solve_any_with_all_techniques() {
        let s = UNIQUE_PUZZLE;
        let rustoku = Rustoku::new_from_str(s).expect("Rustoku creation failed for technique test");
        let solution = rustoku
            .with_techniques(TechniqueFlags::all())
            .solve_any()
            .expect("Solving with all techniques failed");

        assert_eq!(
            UNIQUE_SOLUTION,
            format_line(&solution.board),
            "Solution does not match the expected result with all techniques"
        );
    }

    #[test]
    fn test_solve_all_with_all_techniques() {
        let s = TWO_PUZZLE;
        let rustoku = Rustoku::new_from_str(s)
            .expect("Rustoku creation failed for multi-solution technique test");
        let solutions = rustoku.with_techniques(TechniqueFlags::all()).solve_all();

        assert_eq!(
            2,
            solutions.len(),
            "Expected two solutions for the given board with all techniques"
        );
    }

    #[test]
    fn test_generate_with_enough_clues() {
        (20..=80).step_by(20).for_each(|num_clues| {
            let board = generate_board(num_clues)
                .unwrap_or_else(|_| panic!("Board generation failed for {num_clues} clues"));
            let mut rustoku =
                Rustoku::new(board).expect("Rustoku creation failed from generated board");
            let clues_count = board
                .cells
                .iter()
                .flatten()
                .filter(|&&cell| cell != 0)
                .count();
            assert!(
                clues_count >= num_clues,
                "Expected at least {num_clues} clues, but found {clues_count} clues"
            );

            let solutions = rustoku.solve_all();
            assert_eq!(
                1,
                solutions.len(),
                "Generated puzzle with {num_clues} clues should have a unique solution"
            );
        })
    }

    #[test]
    fn test_generate_with_too_few_clues() {
        let num_clues = 16;
        let result = generate_board(num_clues);
        assert!(matches!(result, Err(RustokuError::InvalidClueCount)));
    }

    #[test]
    fn test_generate_with_too_many_clues() {
        let num_clues = 82;
        let result = generate_board(num_clues);
        assert!(matches!(result, Err(RustokuError::InvalidClueCount)));
    }

    #[test]
    fn test_is_solved_with_valid_solution() {
        let s = UNIQUE_SOLUTION;
        let rustoku = Rustoku::new_from_str(s).expect("Rustoku creation failed for solved check");
        assert!(rustoku.is_solved(), "The Sudoku puzzle should be solved");
    }

    #[test]
    fn test_is_solved_with_unsolved_board() {
        let s = UNIQUE_PUZZLE;
        let rustoku = Rustoku::new_from_str(s).expect("Rustoku creation failed for unsolved check");
        assert!(!rustoku.is_solved(), "The board should not be valid");
    }

    struct TechniqueTestCase<'a> {
        name: &'a str,
        trigger_string: &'a str,
        technique_flag: TechniqueFlags,
    }

    #[test]
    fn test_each_technique_makes_valid_changes() {
        let test_cases = vec![
            // Last digit is empty, no other option exists
            TechniqueTestCase {
                name: "Naked Singles",
                trigger_string: "385421967194756328627983145571892634839645271246137589462579813918364752753218490",
                technique_flag: TechniqueFlags::NAKED_SINGLES,
            },
            // https://hodoku.sourceforge.net/en/show_example.php?file=h101&tech=Hidden+Single
            TechniqueTestCase {
                name: "Hidden Singles",
                trigger_string: "008007000016083000000000051107290000000000000000046307290000000000860140000300700",
                technique_flag: TechniqueFlags::HIDDEN_SINGLES,
            },
            // https://hodoku.sourceforge.net/en/show_example.php?file=n201&tech=Naked+Pair
            TechniqueTestCase {
                name: "Naked Pairs",
                trigger_string: "700009030000105006400260009002083951007000000005600000000000003100000060000004010",
                technique_flag: TechniqueFlags::NAKED_PAIRS,
            },
            // https://hodoku.sourceforge.net/en/show_example.php?file=h201&tech=Hidden+Pair
            TechniqueTestCase {
                name: "Hidden Pairs",
                trigger_string: "000032000000000000007600914096000800005008000030040005050200000700000560904010000",
                // Needs easy techniques to reduce candidates first, then hidden pairs to find the pair
                technique_flag: TechniqueFlags::EASY | TechniqueFlags::HIDDEN_PAIRS,
            },
            // https://hodoku.sourceforge.net/en/show_example.php?file=lc101&tech=Locked+Candidates+Type+1+%28Pointing%29
            TechniqueTestCase {
                name: "Locked Candidates",
                trigger_string: "984000000000500040000000002006097200003002000000000010005060003407051890030009700",
                technique_flag: TechniqueFlags::LOCKED_CANDIDATES,
            },
            // https://hodoku.sourceforge.net/en/show_example.php?file=bf201&tech=X-Wing
            TechniqueTestCase {
                name: "X-Wing",
                trigger_string: "000000000760003002002640009403900070000004903005000020010560000370090041000000060",
                technique_flag: TechniqueFlags::XWING,
            },
        ];

        for test_case in test_cases {
            let rustoku = Rustoku::new_from_str(test_case.trigger_string)
                .unwrap_or_else(|_| panic!("Rustoku creation failed for '{}'", test_case.name));
            let mut path = SolvePath::default();
            assert!(
                rustoku
                    .with_techniques(test_case.technique_flag)
                    .techniques_make_valid_changes(&mut path),
                "Propagation should not contradict for '{}'",
                test_case.name
            );
            assert!(
                !path.steps.is_empty(),
                "Expected at least one placement or elimination for '{}'",
                test_case.name
            )
        }
    }

    // ── Targeted technique tests ──────────────────────────────────────────

    #[test]
    fn test_naked_singles_places_correct_value() {
        // Board with a single empty cell at (8,8) – only candidate is 6
        let s = "385421967194756328627983145571892634839645271246137589462579813918364752753218490";
        let mut rustoku = Rustoku::new_from_str(s)
            .unwrap()
            .with_techniques(TechniqueFlags::NAKED_SINGLES);
        let mut path = SolvePath::default();
        rustoku.techniques_make_valid_changes(&mut path);

        // Expect exactly one placement at (8,8) = 6
        let placements: Vec<_> = path
            .steps
            .iter()
            .filter_map(|step| match step {
                SolveStep::Placement {
                    row, col, value, ..
                } => Some((*row, *col, *value)),
                _ => None,
            })
            .collect();

        assert!(
            placements.contains(&(8, 8, 6)),
            "Expected placement of 6 at (8,8), got {:?}",
            placements
        );
    }

    #[test]
    fn test_hidden_singles_places_in_correct_cell() {
        // Hodoku hidden single example
        let s = "008007000016083000000000051107290000000000000000046307290000000000860140000300700";
        let mut rustoku = Rustoku::new_from_str(s)
            .unwrap()
            .with_techniques(TechniqueFlags::HIDDEN_SINGLES);
        let mut path = SolvePath::default();
        rustoku.techniques_make_valid_changes(&mut path);

        let placements: Vec<_> = path
            .steps
            .iter()
            .filter_map(|step| match step {
                SolveStep::Placement {
                    row,
                    col,
                    value,
                    flags,
                    ..
                } if flags.contains(TechniqueFlags::HIDDEN_SINGLES) => Some((*row, *col, *value)),
                _ => None,
            })
            .collect();

        assert!(
            !placements.is_empty(),
            "Hidden singles should produce at least one placement"
        );

        // Verify each placement is valid in the final board
        for &(r, c, v) in &placements {
            assert!((1..=9).contains(&v), "Placed value must be 1-9, got {v}");
            assert_eq!(
                rustoku.board.get(r, c),
                v,
                "Board cell ({r},{c}) should be {v} after hidden single"
            );
        }
    }

    #[test]
    fn test_naked_pairs_eliminates_candidates() {
        // Hodoku naked pair example
        let s = "700009030000105006400260009002083951007000000005600000000000003100000060000004010";
        let mut rustoku = Rustoku::new_from_str(s)
            .unwrap()
            .with_techniques(TechniqueFlags::NAKED_PAIRS);
        let mut path = SolvePath::default();
        rustoku.techniques_make_valid_changes(&mut path);

        let eliminations: Vec<_> = path
            .steps
            .iter()
            .filter_map(|step| match step {
                SolveStep::CandidateElimination {
                    row,
                    col,
                    value,
                    flags,
                    ..
                } if flags.contains(TechniqueFlags::NAKED_PAIRS) => Some((*row, *col, *value)),
                _ => None,
            })
            .collect();

        assert!(
            !eliminations.is_empty(),
            "Naked pairs should produce at least one candidate elimination"
        );

        // Verify eliminated candidates are no longer present
        for &(r, c, v) in &eliminations {
            let cand_bit = 1u16 << (v - 1);
            let remaining = rustoku.candidates.get(r, c);
            assert_eq!(
                remaining & cand_bit,
                0,
                "Candidate {v} should be eliminated from ({r},{c})"
            );
        }
    }

    #[test]
    fn test_hidden_pairs_eliminates_non_pair_candidates() {
        // Hodoku hidden pair example – needs EASY techniques to simplify first
        let s = "000032000000000000007600914096000800005008000030040005050200000700000560904010000";
        let mut rustoku = Rustoku::new_from_str(s)
            .unwrap()
            .with_techniques(TechniqueFlags::EASY | TechniqueFlags::HIDDEN_PAIRS);
        let mut path = SolvePath::default();
        rustoku.techniques_make_valid_changes(&mut path);

        let eliminations: Vec<_> = path
            .steps
            .iter()
            .filter_map(|step| match step {
                SolveStep::CandidateElimination {
                    row,
                    col,
                    value,
                    flags,
                    ..
                } if flags.contains(TechniqueFlags::HIDDEN_PAIRS) => Some((*row, *col, *value)),
                _ => None,
            })
            .collect();

        assert!(
            !eliminations.is_empty(),
            "Hidden pairs should produce at least one candidate elimination"
        );

        // Verify eliminated candidates are no longer present
        for &(r, c, v) in &eliminations {
            let cand_bit = 1u16 << (v - 1);
            let remaining = rustoku.candidates.get(r, c);
            assert_eq!(
                remaining & cand_bit,
                0,
                "Candidate {v} should be eliminated from ({r},{c}) by hidden pair"
            );
        }
    }

    #[test]
    fn test_locked_candidates_eliminates_outside_box() {
        // Hodoku locked candidates (pointing) example
        let s = "984000000000500040000000002006097200003002000000000010005060003407051890030009700";
        let mut rustoku = Rustoku::new_from_str(s)
            .unwrap()
            .with_techniques(TechniqueFlags::LOCKED_CANDIDATES);
        let mut path = SolvePath::default();
        rustoku.techniques_make_valid_changes(&mut path);

        let eliminations: Vec<_> = path
            .steps
            .iter()
            .filter_map(|step| match step {
                SolveStep::CandidateElimination {
                    row,
                    col,
                    value,
                    flags,
                    ..
                } if flags.contains(TechniqueFlags::LOCKED_CANDIDATES) => {
                    Some((*row, *col, *value))
                }
                _ => None,
            })
            .collect();

        assert!(
            !eliminations.is_empty(),
            "Locked candidates should produce at least one candidate elimination"
        );

        for &(r, c, v) in &eliminations {
            let cand_bit = 1u16 << (v - 1);
            let remaining = rustoku.candidates.get(r, c);
            assert_eq!(
                remaining & cand_bit,
                0,
                "Candidate {v} should be eliminated from ({r},{c}) by locked candidates"
            );
        }
    }

    #[test]
    fn test_xwing_eliminates_from_correct_lines() {
        // Hodoku X-Wing example
        let s = "000000000760003002002640009403900070000004903005000020010560000370090041000000060";
        let mut rustoku = Rustoku::new_from_str(s)
            .unwrap()
            .with_techniques(TechniqueFlags::XWING);
        let mut path = SolvePath::default();
        rustoku.techniques_make_valid_changes(&mut path);

        let eliminations: Vec<_> = path
            .steps
            .iter()
            .filter_map(|step| match step {
                SolveStep::CandidateElimination {
                    row,
                    col,
                    value,
                    flags,
                    ..
                } if flags.contains(TechniqueFlags::XWING) => Some((*row, *col, *value)),
                _ => None,
            })
            .collect();

        assert!(
            !eliminations.is_empty(),
            "X-Wing should produce at least one candidate elimination"
        );

        for &(r, c, v) in &eliminations {
            let cand_bit = 1u16 << (v - 1);
            let remaining = rustoku.candidates.get(r, c);
            assert_eq!(
                remaining & cand_bit,
                0,
                "Candidate {v} should be eliminated from ({r},{c}) by X-Wing"
            );
        }
    }

    #[test]
    fn test_all_techniques_produce_valid_solution() {
        // Every technique-trigger puzzle must still solve to a valid board
        let puzzles = vec![
            "385421967194756328627983145571892634839645271246137589462579813918364752753218490",
            "008007000016083000000000051107290000000000000000046307290000000000860140000300700",
            "700009030000105006400260009002083951007000000005600000000000003100000060000004010",
            "000032000000000000007600914096000800005008000030040005050200000700000560904010000",
            "984000000000500040000000002006097200003002000000000010005060003407051890030009700",
            "000000000760003002002640009403900070000004903005000020010560000370090041000000060",
        ];

        for puzzle in puzzles {
            let mut rustoku = Rustoku::new_from_str(puzzle)
                .unwrap()
                .with_techniques(TechniqueFlags::all());
            let solution = rustoku.solve_any();
            assert!(
                solution.is_some(),
                "Puzzle should be solvable with all techniques: {puzzle}"
            );
            let solved = solution.unwrap().board;
            let check = Rustoku::new(solved).unwrap();
            assert!(
                check.is_solved(),
                "Solution must be valid for puzzle: {puzzle}"
            );
        }
    }

    #[test]
    fn test_techniques_do_not_alter_given_clues() {
        // Verify that constraint propagation never overwrites already-given clues
        let puzzles = vec![
            (
                "Naked Singles",
                "385421967194756328627983145571892634839645271246137589462579813918364752753218490",
                TechniqueFlags::NAKED_SINGLES,
            ),
            (
                "Hidden Singles",
                "008007000016083000000000051107290000000000000000046307290000000000860140000300700",
                TechniqueFlags::HIDDEN_SINGLES,
            ),
            (
                "Naked Pairs",
                "700009030000105006400260009002083951007000000005600000000000003100000060000004010",
                TechniqueFlags::NAKED_PAIRS,
            ),
        ];

        for (name, puzzle, flag) in puzzles {
            let original = Board::try_from(puzzle).unwrap();
            let mut rustoku = Rustoku::new_from_str(puzzle).unwrap().with_techniques(flag);
            let mut path = SolvePath::default();
            rustoku.techniques_make_valid_changes(&mut path);

            for r in 0..9 {
                for c in 0..9 {
                    let orig_val = original.get(r, c);
                    if orig_val != 0 {
                        assert_eq!(
                            rustoku.board.get(r, c),
                            orig_val,
                            "{name}: clue at ({r},{c}) was overwritten"
                        );
                    }
                }
            }
        }
    }
}