/// Private API utilities and implementation details
///
/// This module contains internal implementation details that should not be
/// exposed to library users. These APIs may change without notice and should
/// not be relied upon by external code.
///
/// # Design Principles
///
/// - **Stability**: Items in this module may change without warning
/// - **Internal Use**: These APIs are only intended for use within sklears-core
/// - **No Guarantees**: No backward compatibility guarantees are provided
///
/// # Module Organization
///
/// Private APIs are organized into logical groups:
/// - Internal utilities
/// - Implementation helpers
/// - Testing internals
/// - Debug utilities
use crate::error::{Result, SklearsError};
use crate::types::FloatBounds;
use std::fmt::Debug;

// =============================================================================
// Internal Utilities (Private)
// =============================================================================

/// Internal trait for objects that can be validated
///
/// This is an implementation detail and should not be used directly.
/// Use the public `Validate` trait instead.
#[allow(dead_code)]
pub(crate) trait InternalValidate {
    /// Internal validation method
    fn internal_validate(&self) -> Result<()>;

    /// Get validation context for debugging
    fn validation_context(&self) -> ValidationDebugInfo;
}

/// Debug information for validation (internal use only)
#[derive(Debug, Clone)]
#[allow(dead_code)]
pub(crate) struct ValidationDebugInfo {
    pub type_name: &'static str,
    pub rules_applied: Vec<String>,
    pub validation_time_ns: u64,
    pub memory_used_bytes: usize,
}

impl Default for ValidationDebugInfo {
    fn default() -> Self {
        Self {
            type_name: "Unknown",
            rules_applied: Vec::new(),
            validation_time_ns: 0,
            memory_used_bytes: 0,
        }
    }
}

/// Internal memory management utilities
pub(crate) mod memory {
    use super::*;

    /// Memory pool for internal allocations (private)
    #[allow(dead_code)]
    pub(crate) struct InternalMemoryPool {
        allocated_bytes: usize,
        max_allocation: usize,
        allocation_count: usize,
    }

    #[allow(dead_code)]
    impl InternalMemoryPool {
        /// Create a new memory pool (internal)
        pub(crate) fn new(max_allocation: usize) -> Self {
            Self {
                allocated_bytes: 0,
                max_allocation,
                allocation_count: 0,
            }
        }

        /// Try to allocate memory (internal)
        pub(crate) fn try_allocate(&mut self, bytes: usize) -> Result<AllocationHandle> {
            if self.allocated_bytes + bytes > self.max_allocation {
                return Err(SklearsError::InvalidOperation(
                    "Memory pool exhausted".to_string(),
                ));
            }

            self.allocated_bytes += bytes;
            self.allocation_count += 1;

            Ok(AllocationHandle {
                id: self.allocation_count,
                size: bytes,
            })
        }

        /// Deallocate memory (internal)
        pub(crate) fn deallocate(&mut self, handle: AllocationHandle) {
            self.allocated_bytes = self.allocated_bytes.saturating_sub(handle.size);
        }

        /// Get memory statistics (internal)
        pub(crate) fn stats(&self) -> MemoryStats {
            MemoryStats {
                allocated_bytes: self.allocated_bytes,
                max_allocation: self.max_allocation,
                allocation_count: self.allocation_count,
            }
        }
    }

    /// Handle for memory allocation (internal)
    #[derive(Debug, Clone, Copy)]
    #[allow(dead_code)]
    pub(crate) struct AllocationHandle {
        pub(crate) id: usize,
        pub(crate) size: usize,
    }

    /// Memory statistics (internal)
    #[derive(Debug, Clone)]
    #[allow(dead_code)]
    pub(crate) struct MemoryStats {
        pub allocated_bytes: usize,
        pub max_allocation: usize,
        pub allocation_count: usize,
    }
}

/// Internal algorithm implementation helpers
pub(crate) mod algorithm_internals {
    use super::*;

    /// Internal trait for algorithm state management
    #[allow(dead_code)]
    pub(crate) trait AlgorithmState {
        /// Get the current state of the algorithm
        fn current_state(&self) -> InternalState;

        /// Transition to a new state
        fn transition_to(&mut self, new_state: InternalState) -> Result<()>;

        /// Validate state transition
        fn can_transition_to(&self, new_state: InternalState) -> bool;
    }

    /// Internal algorithm states
    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    #[allow(dead_code)]
    pub(crate) enum InternalState {
        /// Algorithm is uninitialized
        Uninitialized,
        /// Algorithm is configured but not fitted
        Configured,
        /// Algorithm is currently fitting
        Fitting,
        /// Algorithm is fitted and ready for prediction
        Fitted,
        /// Algorithm is in an error state
        Error,
    }

    /// Internal convergence checking utilities
    #[allow(dead_code)]
    pub(crate) struct ConvergenceChecker<T: FloatBounds> {
        tolerance: T,
        max_iterations: usize,
        current_iteration: usize,
        last_value: Option<T>,
        history: Vec<T>,
    }

    impl<T: FloatBounds> ConvergenceChecker<T> {
        /// Create a new convergence checker (internal)
        #[allow(dead_code)]
        pub(crate) fn new(tolerance: T, max_iterations: usize) -> Self {
            Self {
                tolerance,
                max_iterations,
                current_iteration: 0,
                last_value: None,
                history: Vec::new(),
            }
        }

        /// Check if the algorithm has converged (internal)
        #[allow(dead_code)]
        pub(crate) fn check_convergence(&mut self, current_value: T) -> ConvergenceStatus {
            self.current_iteration += 1;
            self.history.push(current_value);

            if self.current_iteration >= self.max_iterations {
                return ConvergenceStatus::MaxIterationsReached;
            }

            if let Some(last) = self.last_value {
                let diff = if current_value > last {
                    current_value - last
                } else {
                    last - current_value
                };

                if diff < self.tolerance {
                    return ConvergenceStatus::Converged;
                }
            }

            self.last_value = Some(current_value);
            ConvergenceStatus::Continuing
        }

        /// Get convergence statistics (internal)
        #[allow(dead_code)]
        pub(crate) fn stats(&self) -> ConvergenceStats<T> {
            ConvergenceStats {
                iterations: self.current_iteration,
                tolerance: self.tolerance,
                history: self.history.clone(),
            }
        }
    }

    /// Convergence status (internal)
    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    #[allow(dead_code)]
    pub(crate) enum ConvergenceStatus {
        /// Algorithm is still converging
        Continuing,
        /// Algorithm has converged
        Converged,
        /// Maximum iterations reached without convergence
        MaxIterationsReached,
    }

    /// Convergence statistics (internal)
    #[derive(Debug, Clone)]
    #[allow(dead_code)]
    pub(crate) struct ConvergenceStats<T> {
        pub iterations: usize,
        pub tolerance: T,
        pub history: Vec<T>,
    }
}

/// Internal testing utilities
#[allow(non_snake_case)]
#[cfg(test)]
pub(crate) mod testing {
    use super::*;

    /// Generate test data for internal testing
    pub(crate) fn generate_test_matrix(rows: usize, cols: usize) -> Vec<Vec<f64>> {
        (0..rows)
            .map(|i| (0..cols).map(|j| (i * cols + j) as f64).collect())
            .collect()
    }

    /// Internal test assertion utilities
    pub(crate) fn assert_matrices_close(a: &[Vec<f64>], b: &[Vec<f64>], tolerance: f64) {
        assert_eq!(a.len(), b.len(), "Matrix row count mismatch");
        for (i, (row_a, row_b)) in a.iter().zip(b.iter()).enumerate() {
            assert_eq!(
                row_a.len(),
                row_b.len(),
                "Matrix column count mismatch at row {}",
                i
            );
            for (j, (&val_a, &val_b)) in row_a.iter().zip(row_b.iter()).enumerate() {
                let diff = (val_a - val_b).abs();
                assert!(
                    diff < tolerance,
                    "Values differ at position ({}, {}): {} vs {} (diff: {})",
                    i,
                    j,
                    val_a,
                    val_b,
                    diff
                );
            }
        }
    }

    /// Mock algorithm for testing (internal)
    pub(crate) struct MockAlgorithm {
        state: algorithm_internals::InternalState,
        fitted_data: Option<Vec<f64>>,
    }

    impl MockAlgorithm {
        pub(crate) fn new() -> Self {
            Self {
                state: algorithm_internals::InternalState::Uninitialized,
                fitted_data: None,
            }
        }

        pub(crate) fn fit(&mut self, data: Vec<f64>) -> Result<()> {
            self.state = algorithm_internals::InternalState::Fitting;
            self.fitted_data = Some(data);
            self.state = algorithm_internals::InternalState::Fitted;
            Ok(())
        }

        pub(crate) fn predict(&self, input: &[f64]) -> Result<Vec<f64>> {
            if self.state != algorithm_internals::InternalState::Fitted {
                return Err(SklearsError::InvalidOperation(
                    "Algorithm must be fitted before prediction".to_string(),
                ));
            }

            Ok(input.iter().map(|&x| x * 2.0).collect())
        }
    }

    impl algorithm_internals::AlgorithmState for MockAlgorithm {
        fn current_state(&self) -> algorithm_internals::InternalState {
            self.state
        }

        fn transition_to(&mut self, new_state: algorithm_internals::InternalState) -> Result<()> {
            if !self.can_transition_to(new_state) {
                return Err(SklearsError::InvalidOperation(format!(
                    "Invalid state transition from {:?} to {:?}",
                    self.state, new_state
                )));
            }
            self.state = new_state;
            Ok(())
        }

        fn can_transition_to(&self, new_state: algorithm_internals::InternalState) -> bool {
            use algorithm_internals::InternalState::*;
            matches!(
                (self.state, new_state),
                (Uninitialized, Configured)
                    | (Configured, Fitting)
                    | (Fitting, Fitted)
                    | (Fitting, Error)
                    | (_, Error)
            )
        }
    }
}

/// Internal debug utilities
pub(crate) mod debug {
    use super::*;

    /// Debug information collector (internal)
    #[allow(dead_code)]
    pub(crate) struct DebugCollector {
        enabled: bool,
        entries: Vec<DebugEntry>,
    }

    /// Debug entry (internal)
    #[derive(Debug, Clone)]
    #[allow(dead_code)]
    pub(crate) struct DebugEntry {
        pub timestamp: std::time::Instant,
        pub category: DebugCategory,
        pub message: String,
        pub data: Option<String>,
    }

    /// Debug categories (internal)
    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    #[allow(dead_code)]
    pub(crate) enum DebugCategory {
        Memory,
        Performance,
        Algorithm,
        Validation,
        Other,
    }

    impl DebugCollector {
        /// Create a new debug collector (internal)
        #[allow(dead_code)]
        pub(crate) fn new(enabled: bool) -> Self {
            Self {
                enabled,
                entries: Vec::new(),
            }
        }

        /// Add a debug entry (internal)
        #[allow(dead_code)]
        pub(crate) fn add_entry(
            &mut self,
            category: DebugCategory,
            message: String,
            data: Option<String>,
        ) {
            if self.enabled {
                self.entries.push(DebugEntry {
                    timestamp: std::time::Instant::now(),
                    category,
                    message,
                    data,
                });
            }
        }

        /// Get all debug entries (internal)
        #[allow(dead_code)]
        pub(crate) fn entries(&self) -> &[DebugEntry] {
            &self.entries
        }

        /// Clear debug entries (internal)
        #[allow(dead_code)]
        pub(crate) fn clear(&mut self) {
            self.entries.clear();
        }
    }
}

// =============================================================================
// Tests for Private APIs
// =============================================================================

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

    #[test]
    fn test_memory_pool() {
        let mut pool = memory::InternalMemoryPool::new(1024);

        // Test allocation
        let handle1 = pool.try_allocate(512).expect("try_allocate should succeed");
        assert_eq!(handle1.size, 512);

        let handle2 = pool.try_allocate(256).expect("try_allocate should succeed");
        assert_eq!(handle2.size, 256);

        // Test overflow
        assert!(pool.try_allocate(512).is_err());

        // Test deallocation
        pool.deallocate(handle1);
        let handle3 = pool.try_allocate(512).expect("try_allocate should succeed");
        assert_eq!(handle3.size, 512);

        // Check stats
        let stats = pool.stats();
        assert_eq!(stats.allocated_bytes, 768); // 256 + 512
        assert_eq!(stats.allocation_count, 3);
    }

    #[test]
    fn test_convergence_checker() {
        let mut checker = algorithm_internals::ConvergenceChecker::new(0.01, 10);

        // Test normal convergence
        assert_eq!(
            checker.check_convergence(1.0),
            algorithm_internals::ConvergenceStatus::Continuing
        );
        assert_eq!(
            checker.check_convergence(1.005),
            algorithm_internals::ConvergenceStatus::Converged
        );

        // Test max iterations
        let mut checker2 = algorithm_internals::ConvergenceChecker::new(0.01, 2);
        assert_eq!(
            checker2.check_convergence(1.0),
            algorithm_internals::ConvergenceStatus::Continuing
        );
        assert_eq!(
            checker2.check_convergence(2.0),
            algorithm_internals::ConvergenceStatus::MaxIterationsReached
        );
    }

    #[test]
    fn test_mock_algorithm() {
        use algorithm_internals::AlgorithmState;

        let mut algo = testing::MockAlgorithm::new();
        assert_eq!(
            algo.current_state(),
            algorithm_internals::InternalState::Uninitialized
        );

        // Test state transitions
        assert!(algo
            .transition_to(algorithm_internals::InternalState::Configured)
            .is_ok());
        assert!(algo
            .transition_to(algorithm_internals::InternalState::Fitting)
            .is_ok());
        assert!(algo
            .transition_to(algorithm_internals::InternalState::Fitted)
            .is_ok());

        // Test invalid transitions
        assert!(algo
            .transition_to(algorithm_internals::InternalState::Uninitialized)
            .is_err());
    }

    #[test]
    fn test_debug_collector() {
        let mut collector = debug::DebugCollector::new(true);

        collector.add_entry(
            debug::DebugCategory::Performance,
            "Test message".to_string(),
            Some("Test data".to_string()),
        );

        let entries = collector.entries();
        assert_eq!(entries.len(), 1);
        assert_eq!(entries[0].category, debug::DebugCategory::Performance);
        assert_eq!(entries[0].message, "Test message");

        collector.clear();
        assert_eq!(collector.entries().len(), 0);
    }

    #[test]
    fn test_testing_utilities() {
        let matrix = testing::generate_test_matrix(2, 3);
        assert_eq!(matrix.len(), 2);
        assert_eq!(matrix[0].len(), 3);
        assert_eq!(matrix[0], vec![0.0, 1.0, 2.0]);
        assert_eq!(matrix[1], vec![3.0, 4.0, 5.0]);

        // Test matrix comparison
        let matrix2 = vec![vec![0.0, 1.0, 2.0], vec![3.0, 4.0, 5.0]];
        testing::assert_matrices_close(&matrix, &matrix2, 1e-10);
    }
}
