//! REPL Replay Testing System
//! 
//! Provides deterministic replay capabilities for testing and educational assessment.
//! Based on docs/specifications/repl-replay-testing-spec.md

use anyhow::Result;
use serde::{Serialize, Deserialize};
use std::collections::{HashMap, BTreeMap};
use std::time::Instant;
use crate::runtime::repl::Value;

// ============================================================================
// Core Data Structures
// ============================================================================

/// Unique identifier for events in a session
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize)]
pub struct EventId(pub u64);

/// Semantic version for compatibility checking
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct SemVer {
    major: u32,
    minor: u32,
    patch: u32,
}

impl SemVer {
    pub fn new(major: u32, minor: u32, patch: u32) -> Self {
        Self { major, minor, patch }
    }
}

/// Complete REPL session recording
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ReplSession {
    pub version: SemVer,
    pub metadata: SessionMetadata,
    pub environment: Environment,
    pub timeline: Vec<TimestampedEvent>,
    pub checkpoints: BTreeMap<EventId, StateCheckpoint>,
}

/// Session metadata for tracking and analysis
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SessionMetadata {
    pub session_id: String,
    pub created_at: String,
    pub ruchy_version: String,
    pub student_id: Option<String>,
    pub assignment_id: Option<String>,
    pub tags: Vec<String>,
}

/// Environment configuration for deterministic replay
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Environment {
    pub seed: u64,
    pub feature_flags: Vec<String>,
    pub resource_limits: ResourceLimits,
}

/// Resource limits for bounded execution
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ResourceLimits {
    pub heap_mb: usize,
    pub stack_kb: usize,
    pub cpu_ms: u64,
}

/// Timestamped event with causality tracking
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct TimestampedEvent {
    pub id: EventId,
    pub timestamp_ns: u64,
    pub event: Event,
    pub causality: Vec<EventId>, // Lamport clock for distributed replay
}

/// Event types that can occur in a REPL session
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum Event {
    Input {
        text: String,
        mode: InputMode,
    },
    Output {
        result: EvalResult,
        stdout: Vec<u8>,
        stderr: Vec<u8>,
    },
    StateChange {
        bindings_delta: HashMap<String, String>, // Simplified for now
        state_hash: String,
    },
    ResourceUsage {
        heap_bytes: usize,
        stack_depth: usize,
        cpu_ns: u64,
    },
}

/// Input modes for different interaction patterns
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum InputMode {
    Interactive,
    Paste,
    File,
    Script,
}

/// Result of evaluating an expression
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum EvalResult {
    Success { value: String },
    Error { message: String },
    Unit,
}

/// Complete state checkpoint for rollback
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct StateCheckpoint {
    pub bindings: HashMap<String, String>,
    pub type_environment: HashMap<String, String>,
    pub state_hash: String,
    pub resource_usage: ResourceUsage,
}

/// Resource usage snapshot
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ResourceUsage {
    pub heap_bytes: usize,
    pub stack_depth: usize,
    pub cpu_ns: u64,
}

// ============================================================================
// Deterministic Execution
// ============================================================================

/// Result of deterministic replay execution
#[derive(Debug)]
pub struct ReplayResult {
    pub output: Result<Value>,
    pub state_hash: String,
    pub resource_usage: ResourceUsage,
}

/// Trait for deterministic REPL execution
pub trait DeterministicRepl {
    /// Execute with a fixed seed for deterministic behavior
    fn execute_with_seed(&mut self, input: &str, seed: u64) -> ReplayResult;
    
    /// Create a state checkpoint
    fn checkpoint(&self) -> StateCheckpoint;
    
    /// Restore from a checkpoint
    fn restore(&mut self, checkpoint: &StateCheckpoint) -> Result<()>;
    
    /// Validate determinism against another instance
    fn validate_determinism(&self, other: &Self) -> ValidationResult;
}

/// Result of determinism validation
#[derive(Debug)]
pub struct ValidationResult {
    pub is_deterministic: bool,
    pub divergences: Vec<Divergence>,
}

/// Types of divergence in replay
#[derive(Debug, Clone)]
pub enum Divergence {
    Output { expected: String, actual: String },
    State { expected_hash: String, actual_hash: String },
    Resources { expected: ResourceUsage, actual: ResourceUsage },
}

// ============================================================================
// Replay Validation Engine
// ============================================================================

/// Validates replay sessions for correctness
pub struct ReplayValidator {
    pub strict_mode: bool,
    pub tolerance: ResourceTolerance,
}

/// Tolerance for resource usage variations
#[derive(Debug, Clone)]
pub struct ResourceTolerance {
    pub heap_bytes_percent: f64,
    pub cpu_ns_percent: f64,
}

impl Default for ResourceTolerance {
    fn default() -> Self {
        Self {
            heap_bytes_percent: 10.0,  // Allow 10% variation
            cpu_ns_percent: 20.0,       // Allow 20% CPU variation
        }
    }
}

/// Report from validation
#[derive(Debug, Default)]
pub struct ValidationReport {
    pub passed: bool,
    pub total_events: usize,
    pub successful_events: usize,
    pub divergences: Vec<(EventId, Divergence)>,
}

impl ValidationReport {
    pub fn new() -> Self {
        Self::default()
    }
    
    pub fn add_divergence(&mut self, event_id: EventId, divergence: Divergence) {
        self.divergences.push((event_id, divergence));
        self.passed = false;
    }
}

impl ReplayValidator {
    pub fn new(strict_mode: bool) -> Self {
        Self {
            strict_mode,
            tolerance: ResourceTolerance::default(),
        }
    }
    
    pub fn validate_session(
        &self,
        recorded: &ReplSession,
        implementation: &mut impl DeterministicRepl,
    ) -> ValidationReport {
        let mut report = ValidationReport::new();
        report.total_events = recorded.timeline.len();
        
        for event in &recorded.timeline {
            if let Event::Input { text, .. } = &event.event {
                let result = implementation.execute_with_seed(text, recorded.environment.seed);
                
                // Find corresponding output event
                if let Some(expected_output) = self.find_next_output(recorded, event.id) {
                    if self.outputs_equivalent(&result, expected_output) {
                        report.successful_events += 1;
                    } else {
                        report.add_divergence(
                            event.id,
                            Divergence::Output {
                                expected: format!("{expected_output:?}"),
                                actual: format!("{:?}", result.output),
                            }
                        );
                    }
                }
                
                // Validate resource bounds
                if !self.tolerance_accepts(&result.resource_usage) {
                    report.add_divergence(
                        event.id,
                        Divergence::Resources {
                            expected: ResourceUsage {
                                heap_bytes: 0,
                                stack_depth: 0,
                                cpu_ns: 0,
                            },
                            actual: result.resource_usage.clone(),
                        }
                    );
                }
            }
        }
        
        if report.divergences.is_empty() {
            report.passed = true;
        }
        
        report
    }
    
    fn find_next_output<'a>(&self, session: &'a ReplSession, after_id: EventId) -> Option<&'a Event> {
        session.timeline
            .iter()
            .find(|e| e.id > after_id && matches!(e.event, Event::Output { .. }))
            .map(|e| &e.event)
    }
    
    fn outputs_equivalent(&self, result: &ReplayResult, expected: &Event) -> bool {
        match expected {
            Event::Output { result: expected_result, .. } => {
                match (&result.output, expected_result) {
                    (Ok(value), EvalResult::Success { value: expected }) => {
                        format!("{value:?}") == *expected
                    }
                    (Err(e), EvalResult::Error { message }) => {
                        e.to_string().contains(message)
                    }
                    _ => false
                }
            }
            _ => false
        }
    }
    
    fn tolerance_accepts(&self, _usage: &ResourceUsage) -> bool {
        // For now, always accept - will implement proper bounds checking later
        true
    }
}

// ============================================================================
// Session Recording
// ============================================================================

/// Records REPL sessions for later replay
pub struct SessionRecorder {
    session: ReplSession,
    next_event_id: u64,
    start_time: Instant,
}

impl SessionRecorder {
    pub fn new(metadata: SessionMetadata) -> Self {
        Self {
            session: ReplSession {
                version: SemVer::new(1, 0, 0),
                metadata,
                environment: Environment {
                    seed: 0,
                    feature_flags: vec![],
                    resource_limits: ResourceLimits {
                        heap_mb: 100,
                        stack_kb: 8192,
                        cpu_ms: 5000,
                    },
                },
                timeline: vec![],
                checkpoints: BTreeMap::new(),
            },
            next_event_id: 1,
            start_time: Instant::now(),
        }
    }
    
    pub fn record_input(&mut self, text: String, mode: InputMode) -> EventId {
        let id = EventId(self.next_event_id);
        self.next_event_id += 1;
        
        let event = TimestampedEvent {
            id,
            timestamp_ns: self.elapsed_ns(),
            event: Event::Input { text, mode },
            causality: vec![],
        };
        
        self.session.timeline.push(event);
        id
    }
    
    pub fn record_output(&mut self, result: Result<Value>) -> EventId {
        let id = EventId(self.next_event_id);
        self.next_event_id += 1;
        
        let eval_result = match result {
            Ok(Value::Unit) => EvalResult::Unit,
            Ok(value) => EvalResult::Success { 
                value: format!("{value:?}") 
            },
            Err(e) => EvalResult::Error { 
                message: e.to_string() 
            },
        };
        
        let event = TimestampedEvent {
            id,
            timestamp_ns: self.elapsed_ns(),
            event: Event::Output {
                result: eval_result,
                stdout: vec![],
                stderr: vec![],
            },
            causality: vec![],
        };
        
        self.session.timeline.push(event);
        id
    }
    
    pub fn add_checkpoint(&mut self, event_id: EventId, checkpoint: StateCheckpoint) {
        self.session.checkpoints.insert(event_id, checkpoint);
    }
    
    pub fn get_session(&self) -> &ReplSession {
        &self.session
    }
    
    pub fn into_session(self) -> ReplSession {
        self.session
    }
    
    fn elapsed_ns(&self) -> u64 {
        self.start_time.elapsed().as_nanos() as u64
    }
}

// ============================================================================
// Testing Utilities
// ============================================================================

#[cfg(test)]
mod tests {
    use super::*;
    
    #[test]
    fn test_session_recording() {
        let metadata = SessionMetadata {
            session_id: "test-001".to_string(),
            created_at: "2025-08-28T10:00:00Z".to_string(),
            ruchy_version: "1.23.0".to_string(),
            student_id: None,
            assignment_id: None,
            tags: vec!["test".to_string()],
        };
        
        let mut recorder = SessionRecorder::new(metadata);
        
        let input_id = recorder.record_input("let x = 42".to_string(), InputMode::Interactive);
        assert_eq!(input_id, EventId(1));
        
        let output_id = recorder.record_output(Ok(Value::Unit));
        assert_eq!(output_id, EventId(2));
        
        let session = recorder.get_session();
        assert_eq!(session.timeline.len(), 2);
    }
    
    #[test]
    fn test_replay_validation() {
        // This will be implemented once we have the DeterministicRepl implementation
    }
}