using System.Security.Cryptography;
using System.Text;
using System.Text.Json;
using System.Text.RegularExpressions;
using StellaOps.Policy.Registry.Contracts;
using StellaOps.Policy.Registry.Storage;

namespace StellaOps.Policy.Registry.Services;

/// <summary>
/// Default implementation of quick policy simulation service.
/// Evaluates policy rules against provided input and returns violations.
/// </summary>
public sealed partial class PolicySimulationService : IPolicySimulationService
{
    private readonly IPolicyPackStore _packStore;
    private readonly TimeProvider _timeProvider;

    // Regex patterns for input reference extraction
    [GeneratedRegex(@"input\.(\w+(?:\.\w+)*)", RegexOptions.None)]
    private static partial Regex InputReferenceRegex();

    [GeneratedRegex(@"input\[""([^""]+)""\]", RegexOptions.None)]
    private static partial Regex InputBracketReferenceRegex();

    public PolicySimulationService(IPolicyPackStore packStore, TimeProvider? timeProvider = null)
    {
        _packStore = packStore ?? throw new ArgumentNullException(nameof(packStore));
        _timeProvider = timeProvider ?? TimeProvider.System;
    }

    public async Task<PolicySimulationResponse> SimulateAsync(
        Guid tenantId,
        Guid packId,
        SimulationRequest request,
        CancellationToken cancellationToken = default)
    {
        var start = _timeProvider.GetTimestamp();
        var executedAt = _timeProvider.GetUtcNow();
        var simulationId = GenerateSimulationId(tenantId, packId, executedAt);

        var pack = await _packStore.GetByIdAsync(tenantId, packId, cancellationToken);
        if (pack is null)
        {
            return new PolicySimulationResponse
            {
                SimulationId = simulationId,
                Success = false,
                ExecutedAt = executedAt,
                DurationMilliseconds = GetElapsedMs(start),
                Errors = [new SimulationError { Code = "PACK_NOT_FOUND", Message = $"Policy pack {packId} not found" }]
            };
        }

        return await SimulateRulesInternalAsync(
            simulationId,
            pack.Rules ?? [],
            request,
            start,
            executedAt,
            cancellationToken);
    }

    public async Task<PolicySimulationResponse> SimulateRulesAsync(
        Guid tenantId,
        IReadOnlyList<PolicyRule> rules,
        SimulationRequest request,
        CancellationToken cancellationToken = default)
    {
        var start = _timeProvider.GetTimestamp();
        var executedAt = _timeProvider.GetUtcNow();
        var simulationId = GenerateSimulationId(tenantId, Guid.Empty, executedAt);

        return await SimulateRulesInternalAsync(
            simulationId,
            rules,
            request,
            start,
            executedAt,
            cancellationToken);
    }

    public Task<InputValidationResult> ValidateInputAsync(
        IReadOnlyDictionary<string, object> input,
        CancellationToken cancellationToken = default)
    {
        var errors = new List<InputValidationError>();

        if (input.Count == 0)
        {
            errors.Add(new InputValidationError
            {
                Path = "$",
                Message = "Input must contain at least one property"
            });
        }

        // Check for common required fields
        var commonFields = new[] { "subject", "resource", "action", "context" };
        var missingFields = commonFields.Where(f => !input.ContainsKey(f)).ToList();

        if (missingFields.Count == commonFields.Length)
        {
            // Warn if none of the common fields are present
            errors.Add(new InputValidationError
            {
                Path = "$",
                Message = $"Input should contain at least one of: {string.Join(", ", commonFields)}"
            });
        }

        return Task.FromResult(errors.Count > 0
            ? InputValidationResult.Invalid(errors)
            : InputValidationResult.Valid());
    }

    private async Task<PolicySimulationResponse> SimulateRulesInternalAsync(
        string simulationId,
        IReadOnlyList<PolicyRule> rules,
        SimulationRequest request,
        long startTimestamp,
        DateTimeOffset executedAt,
        CancellationToken cancellationToken)
    {
        var violations = new List<SimulatedViolation>();
        var errors = new List<SimulationError>();
        var trace = new List<string>();
        int rulesMatched = 0;

        var enabledRules = rules.Where(r => r.Enabled).ToList();

        foreach (var rule in enabledRules)
        {
            cancellationToken.ThrowIfCancellationRequested();

            try
            {
                var (matched, violation, traceEntry) = EvaluateRule(rule, request.Input, request.Options);

                if (request.Options?.Trace == true && traceEntry is not null)
                {
                    trace.Add(traceEntry);
                }

                if (matched)
                {
                    rulesMatched++;
                    if (violation is not null)
                    {
                        violations.Add(violation);
                    }
                }
            }
            catch (Exception ex)
            {
                errors.Add(new SimulationError
                {
                    RuleId = rule.RuleId,
                    Code = "EVALUATION_ERROR",
                    Message = ex.Message
                });
            }
        }

        var elapsed = GetElapsedMs(startTimestamp);
        var severityCounts = violations
            .GroupBy(v => v.Severity.ToLowerInvariant())
            .ToDictionary(g => g.Key, g => g.Count());

        var summary = new SimulationSummary
        {
            TotalRulesEvaluated = enabledRules.Count,
            RulesMatched = rulesMatched,
            ViolationsFound = violations.Count,
            ViolationsBySeverity = severityCounts
        };

        var result = new SimulationResult
        {
            Result = new Dictionary<string, object>
            {
                ["allow"] = violations.Count == 0,
                ["violations_count"] = violations.Count
            },
            Violations = violations.Count > 0 ? violations : null,
            Trace = request.Options?.Trace == true && trace.Count > 0 ? trace : null,
            Explain = request.Options?.Explain == true ? BuildExplainTrace(enabledRules, request.Input) : null
        };

        return new PolicySimulationResponse
        {
            SimulationId = simulationId,
            Success = errors.Count == 0,
            ExecutedAt = executedAt,
            DurationMilliseconds = elapsed,
            Result = result,
            Summary = summary,
            Errors = errors.Count > 0 ? errors : null
        };
    }

    private (bool matched, SimulatedViolation? violation, string? trace) EvaluateRule(
        PolicyRule rule,
        IReadOnlyDictionary<string, object> input,
        SimulationOptions? options)
    {
        // If no Rego code, use basic rule matching based on severity and name
        if (string.IsNullOrWhiteSpace(rule.Rego))
        {
            // Without Rego, we do pattern-based matching on rule name/description
            var matched = MatchRuleByName(rule, input);
            var trace = options?.Trace == true
                ? $"Rule {rule.RuleId}: matched={matched} (no Rego, name-based)"
                : null;

            if (matched)
            {
                var violation = new SimulatedViolation
                {
                    RuleId = rule.RuleId,
                    Severity = rule.Severity.ToString().ToLowerInvariant(),
                    Message = rule.Description ?? $"Violation of rule {rule.Name}"
                };
                return (true, violation, trace);
            }

            return (false, null, trace);
        }

        // Evaluate Rego-based rule
        var regoResult = EvaluateRegoRule(rule, input);
        var regoTrace = options?.Trace == true
            ? $"Rule {rule.RuleId}: matched={regoResult.matched}, inputs_used={string.Join(",", regoResult.inputsUsed)}"
            : null;

        if (regoResult.matched)
        {
            var violation = new SimulatedViolation
            {
                RuleId = rule.RuleId,
                Severity = rule.Severity.ToString().ToLowerInvariant(),
                Message = rule.Description ?? $"Violation of rule {rule.Name}",
                Context = regoResult.context
            };
            return (true, violation, regoTrace);
        }

        return (false, null, regoTrace);
    }

    private static bool MatchRuleByName(PolicyRule rule, IReadOnlyDictionary<string, object> input)
    {
        // Simple heuristic matching for rules without Rego
        var ruleName = rule.Name.ToLowerInvariant();
        var ruleDesc = rule.Description?.ToLowerInvariant() ?? "";

        // Check if any input key matches rule keywords
        foreach (var (key, value) in input)
        {
            var keyLower = key.ToLowerInvariant();
            var valueLower = value?.ToString()?.ToLowerInvariant() ?? "";

            if (ruleName.Contains(keyLower) || ruleDesc.Contains(keyLower))
            {
                return true;
            }

            if (ruleName.Contains(valueLower) || ruleDesc.Contains(valueLower))
            {
                return true;
            }
        }

        return false;
    }

    private (bool matched, HashSet<string> inputsUsed, IReadOnlyDictionary<string, object>? context) EvaluateRegoRule(
        PolicyRule rule,
        IReadOnlyDictionary<string, object> input)
    {
        // Extract input references from Rego code
        var inputRefs = ExtractInputReferences(rule.Rego!);
        var inputsUsed = new HashSet<string>();
        var context = new Dictionary<string, object>();

        // Simple evaluation: check if referenced inputs exist and have values
        bool allInputsPresent = true;
        foreach (var inputRef in inputRefs)
        {
            var value = GetNestedValue(input, inputRef);
            if (value is not null)
            {
                inputsUsed.Add(inputRef);
                context[inputRef] = value;
            }
            else
            {
                allInputsPresent = false;
            }
        }

        // For this simplified simulation:
        // - Rule matches if all referenced inputs are present
        // - This simulates the rule being able to evaluate
        var matched = inputRefs.Count > 0 && allInputsPresent;

        return (matched, inputsUsed, context.Count > 0 ? context : null);
    }

    private static HashSet<string> ExtractInputReferences(string rego)
    {
        var refs = new HashSet<string>(StringComparer.Ordinal);

        // Match input.field.subfield pattern
        foreach (Match match in InputReferenceRegex().Matches(rego))
        {
            refs.Add(match.Groups[1].Value);
        }

        // Match input["field"] pattern
        foreach (Match match in InputBracketReferenceRegex().Matches(rego))
        {
            refs.Add(match.Groups[1].Value);
        }

        return refs;
    }

    private static object? GetNestedValue(IReadOnlyDictionary<string, object> input, string path)
    {
        var parts = path.Split('.');
        object? current = input;

        foreach (var part in parts)
        {
            if (current is IReadOnlyDictionary<string, object> dict)
            {
                if (!dict.TryGetValue(part, out current))
                {
                    return null;
                }
            }
            else if (current is JsonElement jsonElement)
            {
                if (jsonElement.ValueKind == JsonValueKind.Object &&
                    jsonElement.TryGetProperty(part, out var prop))
                {
                    current = prop;
                }
                else
                {
                    return null;
                }
            }
            else
            {
                return null;
            }
        }

        return current;
    }

    private static PolicyExplainTrace BuildExplainTrace(
        IReadOnlyList<PolicyRule> rules,
        IReadOnlyDictionary<string, object> input)
    {
        var steps = new List<object>();

        steps.Add(new { type = "input_received", keys = input.Keys.ToList() });

        foreach (var rule in rules)
        {
            steps.Add(new
            {
                type = "rule_evaluation",
                rule_id = rule.RuleId,
                rule_name = rule.Name,
                severity = rule.Severity.ToString(),
                has_rego = !string.IsNullOrWhiteSpace(rule.Rego)
            });
        }

        steps.Add(new { type = "evaluation_complete", rules_count = rules.Count });

        return new PolicyExplainTrace { Steps = steps };
    }

    private static string GenerateSimulationId(Guid tenantId, Guid packId, DateTimeOffset timestamp)
    {
        var content = $"{tenantId}:{packId}:{timestamp.ToUnixTimeMilliseconds()}";
        var hash = SHA256.HashData(Encoding.UTF8.GetBytes(content));
        return $"sim_{Convert.ToHexString(hash)[..16].ToLowerInvariant()}";
    }

    private long GetElapsedMs(long startTimestamp)
    {
        var elapsed = _timeProvider.GetElapsedTime(startTimestamp, _timeProvider.GetTimestamp());
        return (long)Math.Ceiling(elapsed.TotalMilliseconds);
    }
}