using System;
using System.Collections.Immutable;
using StellaOps.PolicyDsl;

namespace StellaOps.Policy.Engine.Compilation;

/// <summary>
/// Computes deterministic complexity metrics for compiled policies.
/// </summary>
internal sealed class PolicyComplexityAnalyzer
{
    public PolicyComplexityReport Analyze(PolicyIrDocument document)
    {
        ArgumentNullException.ThrowIfNull(document);

        var metrics = new ComplexityMetrics();
        metrics.RuleCount = document.Rules.IsDefault ? 0 : document.Rules.Length;

        VisitMetadata(document.Metadata.Values, metrics);
        VisitMetadata(document.Settings.Values, metrics);
        VisitProfiles(document.Profiles, metrics);

        if (!document.Rules.IsDefaultOrEmpty)
        {
            foreach (var rule in document.Rules)
            {
                metrics.ConditionCount++;
                VisitExpression(rule.When, metrics, depth: 0);

                VisitActions(rule.ThenActions, metrics);
                VisitActions(rule.ElseActions, metrics);
            }
        }

        var score = CalculateScore(metrics);
        var roundedScore = Math.Round(score, 3, MidpointRounding.AwayFromZero);

        return new PolicyComplexityReport(
            roundedScore,
            metrics.RuleCount,
            metrics.ActionCount,
            metrics.ExpressionCount,
            metrics.InvocationCount,
            metrics.MemberAccessCount,
            metrics.IdentifierCount,
            metrics.LiteralCount,
            metrics.MaxDepth,
            metrics.ProfileCount,
            metrics.ProfileBindings,
            metrics.ConditionCount,
            metrics.ListItems);
    }

    private static void VisitProfiles(ImmutableArray<PolicyIrProfile> profiles, ComplexityMetrics metrics)
    {
        if (profiles.IsDefaultOrEmpty)
        {
            return;
        }

        foreach (var profile in profiles)
        {
            metrics.ProfileCount++;

            if (!profile.Maps.IsDefaultOrEmpty)
            {
                foreach (var map in profile.Maps)
                {
                    if (map.Entries.IsDefaultOrEmpty)
                    {
                        continue;
                    }

                    foreach (var entry in map.Entries)
                    {
                        metrics.ProfileBindings++;
                        metrics.LiteralCount++; // weight values contribute to literal count
                    }
                }
            }

            if (!profile.Environments.IsDefaultOrEmpty)
            {
                foreach (var environment in profile.Environments)
                {
                    if (environment.Entries.IsDefaultOrEmpty)
                    {
                        continue;
                    }

                    foreach (var entry in environment.Entries)
                    {
                        metrics.ProfileBindings++;
                        metrics.ConditionCount++;
                        VisitExpression(entry.Condition, metrics, depth: 0);
                    }
                }
            }

            if (!profile.Scalars.IsDefaultOrEmpty)
            {
                foreach (var scalar in profile.Scalars)
                {
                    metrics.ProfileBindings++;
                    VisitLiteral(scalar.Value, metrics);
                }
            }
        }
    }

    private static void VisitMetadata(IEnumerable<PolicyIrLiteral> literals, ComplexityMetrics metrics)
    {
        foreach (var literal in literals)
        {
            VisitLiteral(literal, metrics);
        }
    }

    private static void VisitLiteral(PolicyIrLiteral literal, ComplexityMetrics metrics)
    {
        switch (literal)
        {
            case PolicyIrListLiteral list when !list.Items.IsDefaultOrEmpty:
                foreach (var item in list.Items)
                {
                    VisitLiteral(item, metrics);
                }
                break;
        }

        metrics.LiteralCount++;
    }

    private static void VisitActions(ImmutableArray<PolicyIrAction> actions, ComplexityMetrics metrics)
    {
        if (actions.IsDefaultOrEmpty)
        {
            return;
        }

        foreach (var action in actions)
        {
            metrics.ActionCount++;
            switch (action)
            {
                case PolicyIrAssignmentAction assignment:
                    VisitExpression(assignment.Value, metrics, depth: 0);
                    break;
                case PolicyIrAnnotateAction annotate:
                    VisitExpression(annotate.Value, metrics, depth: 0);
                    break;
                case PolicyIrIgnoreAction ignore when ignore.Until is not null:
                    VisitExpression(ignore.Until, metrics, depth: 0);
                    break;
                case PolicyIrEscalateAction escalate:
                    VisitExpression(escalate.To, metrics, depth: 0);
                    VisitExpression(escalate.When, metrics, depth: 0);
                    break;
                case PolicyIrRequireVexAction require when !require.Conditions.IsEmpty:
                    foreach (var condition in require.Conditions.Values)
                    {
                        VisitExpression(condition, metrics, depth: 0);
                    }
                    break;
                case PolicyIrWarnAction warn when warn.Message is not null:
                    VisitExpression(warn.Message, metrics, depth: 0);
                    break;
                case PolicyIrDeferAction defer when defer.Until is not null:
                    VisitExpression(defer.Until, metrics, depth: 0);
                    break;
            }
        }
    }

    private static void VisitExpression(PolicyExpression? expression, ComplexityMetrics metrics, int depth)
    {
        if (expression is null)
        {
            return;
        }

        metrics.ExpressionCount++;
        var currentDepth = depth + 1;
        if (currentDepth > metrics.MaxDepth)
        {
            metrics.MaxDepth = currentDepth;
        }

        switch (expression)
        {
            case PolicyLiteralExpression:
                metrics.LiteralCount++;
                break;
            case PolicyListExpression listExpression:
                if (!listExpression.Items.IsDefaultOrEmpty)
                {
                    foreach (var item in listExpression.Items)
                    {
                        metrics.ListItems++;
                        VisitExpression(item, metrics, currentDepth);
                    }
                }
                break;
            case PolicyIdentifierExpression:
                metrics.IdentifierCount++;
                break;
            case PolicyMemberAccessExpression member:
                metrics.MemberAccessCount++;
                VisitExpression(member.Target, metrics, currentDepth);
                break;
            case PolicyInvocationExpression invocation:
                metrics.InvocationCount++;
                VisitExpression(invocation.Target, metrics, currentDepth);
                if (!invocation.Arguments.IsDefaultOrEmpty)
                {
                    foreach (var argument in invocation.Arguments)
                    {
                        VisitExpression(argument, metrics, currentDepth);
                    }
                }
                break;
            case PolicyIndexerExpression indexer:
                VisitExpression(indexer.Target, metrics, currentDepth);
                VisitExpression(indexer.Index, metrics, currentDepth);
                break;
            case PolicyUnaryExpression unary:
                VisitExpression(unary.Operand, metrics, currentDepth);
                break;
            case PolicyBinaryExpression binary:
                VisitExpression(binary.Left, metrics, currentDepth);
                VisitExpression(binary.Right, metrics, currentDepth);
                break;
            default:
                break;
        }
    }

    private static double CalculateScore(ComplexityMetrics metrics)
    {
        return metrics.RuleCount * 5d
               + metrics.ActionCount * 1.5d
               + metrics.ExpressionCount * 0.75d
               + metrics.InvocationCount * 1.5d
               + metrics.MemberAccessCount * 1.0d
               + metrics.IdentifierCount * 0.5d
               + metrics.LiteralCount * 0.25d
               + metrics.ProfileBindings * 0.5d
               + metrics.ConditionCount * 1.25d
               + metrics.MaxDepth * 2d
               + metrics.ListItems * 0.25d;
    }

    private sealed class ComplexityMetrics
    {
        public int RuleCount;
        public int ActionCount;
        public int ExpressionCount;
        public int InvocationCount;
        public int MemberAccessCount;
        public int IdentifierCount;
        public int LiteralCount;
        public int ProfileCount;
        public int ProfileBindings;
        public int ConditionCount;
        public int MaxDepth;
        public int ListItems;
    }
}

internal sealed record PolicyComplexityReport(
    double Score,
    int RuleCount,
    int ActionCount,
    int ExpressionCount,
    int InvocationCount,
    int MemberAccessCount,
    int IdentifierCount,
    int LiteralCount,
    int MaxExpressionDepth,
    int ProfileCount,
    int ProfileBindingCount,
    int ConditionCount,
    int ListItemCount);