//! Klondike solvability checker using upstream `card_game::Session::solve()`.
//!
//! Used by the engine to back the **Settings → Gameplay → "Winnable deals only"**
//! toggle and by the hint system when it wants the first move on a winning path.

use card_game::{Session, SessionConfig, SolveError, StateSnapshot};
use klondike::{Klondike, KlondikeInstruction, KlondikePile, KlondikePileStack};

use crate::game_state::{DrawMode, GameState};
use crate::klondike_adapter::KlondikeAdapter;

/// Verdict returned by [`try_solve`].
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SolverResult {
    /// The solver found a sequence of moves that wins the deal.
    Winnable,
    /// The solver exhaustively searched and confirmed no win exists.
    Unwinnable,
    /// The move / state budget was exceeded before a verdict could be reached.
    Inconclusive,
}

/// Tunable budgets controlling how long [`try_solve`] is willing to search.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct SolverConfig {
    /// Maximum total moves to consider across the entire search tree.
    pub move_budget: u64,
    /// Maximum unique states to visit.
    pub state_budget: usize,
}

impl Default for SolverConfig {
    fn default() -> Self {
        Self {
            move_budget: 100_000,
            state_budget: 200_000,
        }
    }
}

/// A single move the solver can recommend, expressed in engine-level pile terms.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct SolverMove {
    /// Pile the move originates from.
    pub source: KlondikePile,
    /// Pile the move lands on.
    pub dest: KlondikePile,
    /// Number of cards in the move (1 for non-tableau-to-tableau moves).
    pub count: usize,
}

/// Solver verdict plus, when winnable, the first move on a winning path.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct SolveOutcome {
    /// The high-level verdict (Winnable / Unwinnable / Inconclusive).
    pub result: SolverResult,
    /// First move on the solution path when `result == Winnable`.
    pub first_move: Option<SolverMove>,
}

/// Tries to solve a fresh Classic-mode game from `seed` + `draw_mode`.
pub fn try_solve(seed: u64, draw_mode: DrawMode, config: &SolverConfig) -> SolverResult {
    try_solve_with_first_move(seed, draw_mode, config).result
}

/// Tries to solve a fresh Classic-mode game and, when winnable, returns the
/// first move on a winning path.
///
/// Fresh-deal solving models standard Klondike rules, so the non-standard
/// take-from-foundation house rule stays disabled here.
pub fn try_solve_with_first_move(
    seed: u64,
    draw_mode: DrawMode,
    config: &SolverConfig,
) -> SolveOutcome {
    let mut game = GameState::new(seed, draw_mode);
    game.take_from_foundation = false;
    solve_game_state(&game, config)
}

/// Tries to solve from an existing in-progress [`GameState`].
pub fn try_solve_from_state(state: &GameState, config: &SolverConfig) -> SolveOutcome {
    solve_game_state(state, config)
}

fn solve_game_state(initial: &GameState, config: &SolverConfig) -> SolveOutcome {
    if config.state_budget == 0 {
        return SolveOutcome {
            result: SolverResult::Inconclusive,
            first_move: None,
        };
    }

    // Preserve the historical payload contract: winnable verdicts always carry
    // a first move. An already-won state therefore returns no recommendation.
    if initial.is_won {
        return SolveOutcome {
            result: SolverResult::Unwinnable,
            first_move: None,
        };
    }

    let solver_config = SessionConfig {
        inner: KlondikeAdapter::config_for(initial.draw_mode, initial.take_from_foundation),
        undo_penalty: 0,
        solve_moves_budget: config.move_budget,
        solve_states_budget: config.state_budget as u64,
    };
    let solver_session = Session::new(initial.session().state().state().clone(), solver_config);

    match solver_session.solve() {
        Ok(Some(solution)) => {
            let first_move = solution
                .raw_solution()
                .iter()
                .find_map(snapshot_to_solver_move);
            if let Some(first_move) = first_move {
                SolveOutcome {
                    result: SolverResult::Winnable,
                    first_move: Some(first_move),
                }
            } else {
                SolveOutcome {
                    result: SolverResult::Inconclusive,
                    first_move: None,
                }
            }
        }
        Ok(None) => SolveOutcome {
            result: SolverResult::Unwinnable,
            first_move: None,
        },
        Err(SolveError::MovesBudgetExceeded | SolveError::StatesBudgetExceeded) => SolveOutcome {
            result: SolverResult::Inconclusive,
            first_move: None,
        },
    }
}

fn snapshot_to_solver_move(snapshot: &StateSnapshot<Klondike>) -> Option<SolverMove> {
    let source_state = snapshot.state().state();
    match *snapshot.instruction() {
        KlondikeInstruction::RotateStock => Some(SolverMove {
            source: KlondikePile::Stock,
            dest: KlondikePile::Stock,
            count: 1,
        }),
        KlondikeInstruction::DstFoundation(dst_foundation) => {
            let source = match dst_foundation.src {
                KlondikePile::Tableau(tableau) => KlondikePile::Tableau(tableau),
                KlondikePile::Stock => KlondikePile::Stock,
                KlondikePile::Foundation(_) => return None,
            };
            Some(SolverMove {
                source,
                dest: KlondikePile::Foundation(dst_foundation.foundation),
                count: 1,
            })
        }
        KlondikeInstruction::DstTableau(dst_tableau) => {
            let (source, count) = match dst_tableau.src {
                KlondikePileStack::Tableau(tableau_stack) => {
                    let face_up_count = source_state.tableau_face_up_cards(tableau_stack.tableau).len();
                    let count = face_up_count.checked_sub(tableau_stack.skip_cards as usize)?;
                    if count == 0 {
                        return None;
                    }
                    (KlondikePile::Tableau(tableau_stack.tableau), count)
                }
                KlondikePileStack::Stock => (KlondikePile::Stock, 1),
                KlondikePileStack::Foundation(foundation) => {
                    (KlondikePile::Foundation(foundation), 1)
                }
            };

            Some(SolverMove {
                source,
                dest: KlondikePile::Tableau(dst_tableau.tableau),
                count,
            })
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn try_solve_with_first_move_is_deterministic() {
        let config = SolverConfig::default();
        let a = try_solve_with_first_move(7, DrawMode::DrawOne, &config);
        let b = try_solve_with_first_move(7, DrawMode::DrawOne, &config);
        let c = try_solve_with_first_move(7, DrawMode::DrawOne, &config);
        assert_eq!(a, b);
        assert_eq!(b, c);
    }

    #[test]
    fn try_solve_with_first_move_returns_consistent_payload() {
        let config = SolverConfig {
            move_budget: 5_000,
            state_budget: 5_000,
        };
        let outcome = try_solve_with_first_move(7, DrawMode::DrawOne, &config);
        match outcome.result {
            SolverResult::Winnable => assert!(outcome.first_move.is_some()),
            SolverResult::Unwinnable | SolverResult::Inconclusive => {
                assert!(outcome.first_move.is_none())
            }
        }
    }

    #[test]
    fn try_solve_from_state_uses_live_game_state() {
        let mut game = GameState::new(42, DrawMode::DrawOne);
        game.draw().expect("draw must succeed");

        let config = SolverConfig {
            move_budget: 5_000,
            state_budget: 5_000,
        };
        let outcome = try_solve_from_state(&game, &config);
        match outcome.result {
            SolverResult::Winnable => assert!(outcome.first_move.is_some()),
            SolverResult::Unwinnable | SolverResult::Inconclusive => {
                assert!(outcome.first_move.is_none())
            }
        }
    }

    #[test]
    fn zero_state_budget_is_inconclusive() {
        let config = SolverConfig {
            move_budget: 5_000,
            state_budget: 0,
        };
        let outcome = try_solve_with_first_move(7, DrawMode::DrawOne, &config);
        assert_eq!(outcome.result, SolverResult::Inconclusive);
        assert!(outcome.first_move.is_none());
    }

    #[test]
    fn budget_is_passed_through_not_clamped() {
        // 0xD1FF_0000_0000_0012 is a Medium-tier catalog seed: Inconclusive at
        // the Easy budget (1 000 states) but Winnable at Medium (5 000 states).
        // Differing results confirm solve_game_state passes the caller's
        // state_budget unchanged to the underlying solver.
        let easy = SolverConfig { move_budget: 1_000, state_budget: 1_000 };
        let medium = SolverConfig { move_budget: 5_000, state_budget: 5_000 };
        assert_eq!(
            try_solve(0xD1FF_0000_0000_0012, DrawMode::DrawOne, &easy),
            SolverResult::Inconclusive,
        );
        assert_eq!(
            try_solve(0xD1FF_0000_0000_0012, DrawMode::DrawOne, &medium),
            SolverResult::Winnable,
        );
    }

    #[test]
    fn budget_above_five_thousand_is_not_clamped() {
        // 0xD1FF_0000_0000_00DE is a hard catalog seed: Inconclusive at 5 000
        // states but Winnable at 50 000. Before this fix, solve_game_state
        // applied `config.state_budget.min(5_000)` internally, so a 50k config
        // was silently reduced to 5k — making both calls return Inconclusive and
        // preventing the generator from certifying Hard/Expert/Grandmaster seeds.
        // This assertion fails if the cap is re-introduced.
        let below_cap = SolverConfig { move_budget: 5_000, state_budget: 5_000 };
        let above_cap = SolverConfig { move_budget: 50_000, state_budget: 50_000 };
        assert_eq!(
            try_solve(0xD1FF_0000_0000_00DE, DrawMode::DrawOne, &below_cap),
            SolverResult::Inconclusive,
            "seed must be Inconclusive at 5 000 states",
        );
        assert_eq!(
            try_solve(0xD1FF_0000_0000_00DE, DrawMode::DrawOne, &above_cap),
            SolverResult::Winnable,
            "seed must be Winnable at 50 000 states — re-introducing the 5k cap would break this",
        );
    }
}