using System.Collections.Immutable;
using System.Diagnostics;

namespace StellaOps.Policy.Engine.DeterminismGuard;

/// <summary>
/// Service that enforces determinism constraints during policy evaluation.
/// Combines static analysis and runtime monitoring.
/// </summary>
public sealed class DeterminismGuardService
{
    private readonly ProhibitedPatternAnalyzer _analyzer;
    private readonly DeterminismGuardOptions _options;
    private readonly RuntimeDeterminismMonitor _runtimeMonitor;

    public DeterminismGuardService(DeterminismGuardOptions? options = null)
    {
        _options = options ?? DeterminismGuardOptions.Default;
        _analyzer = new ProhibitedPatternAnalyzer();
        _runtimeMonitor = new RuntimeDeterminismMonitor(_options);
    }

    /// <summary>
    /// Analyzes source code for determinism violations.
    /// </summary>
    public DeterminismAnalysisResult AnalyzeSource(string sourceCode, string? fileName = null)
    {
        return _analyzer.AnalyzeSource(sourceCode, fileName, _options);
    }

    /// <summary>
    /// Creates a guarded execution scope for policy evaluation.
    /// </summary>
    public EvaluationScope CreateScope(string scopeId, DateTimeOffset evaluationTimestamp)
    {
        return new EvaluationScope(scopeId, evaluationTimestamp, _options, _runtimeMonitor);
    }

    /// <summary>
    /// Validates that a policy evaluation context is deterministic.
    /// </summary>
    public DeterminismAnalysisResult ValidateContext<TContext>(TContext context, string contextName)
    {
        var stopwatch = Stopwatch.StartNew();
        var violations = new List<DeterminismViolation>();

        // Check for null
        if (context is null)
        {
            violations.Add(new DeterminismViolation
            {
                Category = DeterminismViolationCategory.Other,
                ViolationType = "NullContext",
                Message = $"Evaluation context '{contextName}' is null",
                Severity = DeterminismViolationSeverity.Error,
                Remediation = "Provide a valid evaluation context"
            });
        }

        stopwatch.Stop();

        var countBySeverity = violations
            .GroupBy(v => v.Severity)
            .ToImmutableDictionary(g => g.Key, g => g.Count());

        var hasBlockingViolation = violations.Any(v => v.Severity >= _options.FailOnSeverity);
        var passed = !_options.EnforcementEnabled || !hasBlockingViolation;

        return new DeterminismAnalysisResult
        {
            Passed = passed,
            Violations = violations.ToImmutableArray(),
            CountBySeverity = countBySeverity,
            AnalysisDurationMs = stopwatch.ElapsedMilliseconds,
            EnforcementEnabled = _options.EnforcementEnabled
        };
    }

    /// <summary>
    /// Gets a determinism-safe time provider that only returns injected timestamps.
    /// </summary>
    public DeterministicTimeProvider GetTimeProvider(DateTimeOffset fixedTimestamp)
    {
        return new DeterministicTimeProvider(fixedTimestamp);
    }
}

/// <summary>
/// A guarded scope for policy evaluation that tracks determinism violations.
/// </summary>
public sealed class EvaluationScope : IDisposable
{
    private readonly string _scopeId;
    private readonly DateTimeOffset _evaluationTimestamp;
    private readonly DeterminismGuardOptions _options;
    private readonly RuntimeDeterminismMonitor _monitor;
    private readonly Stopwatch _stopwatch;
    private readonly List<DeterminismViolation> _violations;
    private bool _disposed;

    internal EvaluationScope(
        string scopeId,
        DateTimeOffset evaluationTimestamp,
        DeterminismGuardOptions options,
        RuntimeDeterminismMonitor monitor)
    {
        _scopeId = scopeId ?? throw new ArgumentNullException(nameof(scopeId));
        _evaluationTimestamp = evaluationTimestamp;
        _options = options;
        _monitor = monitor;
        _stopwatch = Stopwatch.StartNew();
        _violations = new List<DeterminismViolation>();

        if (_options.EnableRuntimeMonitoring)
        {
            _monitor.EnterScope(scopeId);
        }
    }

    /// <summary>
    /// Scope identifier for tracing.
    /// </summary>
    public string ScopeId => _scopeId;

    /// <summary>
    /// The fixed evaluation timestamp for this scope.
    /// </summary>
    public DateTimeOffset EvaluationTimestamp => _evaluationTimestamp;

    /// <summary>
    /// Reports a runtime violation detected during evaluation.
    /// </summary>
    public void ReportViolation(DeterminismViolation violation)
    {
        ArgumentNullException.ThrowIfNull(violation);

        lock (_violations)
        {
            _violations.Add(violation);
        }

        if (_options.EnforcementEnabled && violation.Severity >= _options.FailOnSeverity)
        {
            throw new DeterminismViolationException(violation);
        }
    }

    /// <summary>
    /// Gets the current timestamp (always returns the fixed evaluation timestamp).
    /// </summary>
    public DateTimeOffset GetTimestamp() => _evaluationTimestamp;

    /// <summary>
    /// Gets all violations recorded in this scope.
    /// </summary>
    public IReadOnlyList<DeterminismViolation> GetViolations()
    {
        lock (_violations)
        {
            return _violations.ToList();
        }
    }

    /// <summary>
    /// Completes the scope and returns analysis results.
    /// </summary>
    public DeterminismAnalysisResult Complete()
    {
        _stopwatch.Stop();

        IReadOnlyList<DeterminismViolation> allViolations;
        lock (_violations)
        {
            allViolations = _violations.ToList();
        }

        var countBySeverity = allViolations
            .GroupBy(v => v.Severity)
            .ToImmutableDictionary(g => g.Key, g => g.Count());

        var hasBlockingViolation = allViolations.Any(v => v.Severity >= _options.FailOnSeverity);
        var passed = !_options.EnforcementEnabled || !hasBlockingViolation;

        return new DeterminismAnalysisResult
        {
            Passed = passed,
            Violations = allViolations.ToImmutableArray(),
            CountBySeverity = countBySeverity,
            AnalysisDurationMs = _stopwatch.ElapsedMilliseconds,
            EnforcementEnabled = _options.EnforcementEnabled
        };
    }

    public void Dispose()
    {
        if (_disposed)
        {
            return;
        }

        _disposed = true;

        if (_options.EnableRuntimeMonitoring)
        {
            _monitor.ExitScope(_scopeId);
        }
    }
}

/// <summary>
/// Exception thrown when a determinism violation is detected with enforcement enabled.
/// </summary>
public sealed class DeterminismViolationException : Exception
{
    public DeterminismViolationException(DeterminismViolation violation)
        : base($"Determinism violation: {violation.Message}")
    {
        Violation = violation;
    }

    public DeterminismViolation Violation { get; }
}

/// <summary>
/// Time provider that always returns a fixed timestamp.
/// </summary>
public sealed class DeterministicTimeProvider : TimeProvider
{
    private readonly DateTimeOffset _fixedTimestamp;

    public DeterministicTimeProvider(DateTimeOffset fixedTimestamp)
    {
        _fixedTimestamp = fixedTimestamp;
    }

    public override DateTimeOffset GetUtcNow() => _fixedTimestamp;

    public override TimeZoneInfo LocalTimeZone => TimeZoneInfo.Utc;
}

/// <summary>
/// Runtime monitor for detecting non-deterministic operations.
/// </summary>
internal sealed class RuntimeDeterminismMonitor
{
    private readonly DeterminismGuardOptions _options;
    private readonly HashSet<string> _activeScopes = new(StringComparer.Ordinal);
    private readonly object _lock = new();

    public RuntimeDeterminismMonitor(DeterminismGuardOptions options)
    {
        _options = options;
    }

    public void EnterScope(string scopeId)
    {
        lock (_lock)
        {
            _activeScopes.Add(scopeId);
        }
    }

    public void ExitScope(string scopeId)
    {
        lock (_lock)
        {
            _activeScopes.Remove(scopeId);
        }
    }

    public bool IsInScope => _activeScopes.Count > 0;

    /// <summary>
    /// Checks if we're in a guarded scope and should intercept operations.
    /// </summary>
    public bool ShouldIntercept()
    {
        return _options.EnableRuntimeMonitoring && IsInScope;
    }
}

/// <summary>
/// Extension methods for integrating determinism guard with evaluation.
/// </summary>
public static class DeterminismGuardExtensions
{
    /// <summary>
    /// Executes an evaluation function within a determinism-guarded scope.
    /// </summary>
    public static TResult ExecuteGuarded<TResult>(
        this DeterminismGuardService guard,
        string scopeId,
        DateTimeOffset evaluationTimestamp,
        Func<EvaluationScope, TResult> evaluation)
    {
        ArgumentNullException.ThrowIfNull(guard);
        ArgumentNullException.ThrowIfNull(evaluation);

        using var scope = guard.CreateScope(scopeId, evaluationTimestamp);

        try
        {
            return evaluation(scope);
        }
        finally
        {
            var result = scope.Complete();
            if (!result.Passed)
            {
                // Log violations even if not throwing
                foreach (var violation in result.Violations)
                {
                    // In production, this would log to structured logging
                    System.Diagnostics.Debug.WriteLine(
                        $"[DeterminismGuard] {violation.Severity}: {violation.Message}");
                }
            }
        }
    }

    /// <summary>
    /// Executes an async evaluation function within a determinism-guarded scope.
    /// </summary>
    public static async Task<TResult> ExecuteGuardedAsync<TResult>(
        this DeterminismGuardService guard,
        string scopeId,
        DateTimeOffset evaluationTimestamp,
        Func<EvaluationScope, Task<TResult>> evaluation)
    {
        ArgumentNullException.ThrowIfNull(guard);
        ArgumentNullException.ThrowIfNull(evaluation);

        using var scope = guard.CreateScope(scopeId, evaluationTimestamp);

        try
        {
            return await evaluation(scope).ConfigureAwait(false);
        }
        finally
        {
            var result = scope.Complete();
            if (!result.Passed)
            {
                foreach (var violation in result.Violations)
                {
                    System.Diagnostics.Debug.WriteLine(
                        $"[DeterminismGuard] {violation.Severity}: {violation.Message}");
                }
            }
        }
    }
}