using System.Collections.Immutable;
using StellaOps.Concelier.SbomIntegration.Models;

namespace StellaOps.Scanner.Reachability.Dependencies;

/// <summary>
/// Static graph traversal reachability for dependency graphs.
/// </summary>
public sealed class StaticReachabilityAnalyzer
{
    public ReachabilityReport Analyze(
        DependencyGraph graph,
        ImmutableArray<string> entryPoints,
        ReachabilityPolicy? policy = null)
    {
        ArgumentNullException.ThrowIfNull(graph);

        var resolvedPolicy = policy ?? new ReachabilityPolicy();
        var results = new Dictionary<string, ReachabilityStatus>(StringComparer.Ordinal);
        var findings = new List<ReachabilityFinding>();

        if (entryPoints.IsDefaultOrEmpty)
        {
            foreach (var node in graph.Nodes)
            {
                results[node] = ReachabilityStatus.Unknown;
                findings.Add(new ReachabilityFinding
                {
                    ComponentRef = node,
                    Status = ReachabilityStatus.Unknown,
                    Reason = "no-entrypoints"
                });
            }

            return ReachabilityReportBuilder.Build(graph, results, findings);
        }

        var reachable = new HashSet<string>(StringComparer.Ordinal);
        var potential = new HashSet<string>(StringComparer.Ordinal);
        var predecessor = new Dictionary<string, string?>(StringComparer.Ordinal);
        var pathKind = new Dictionary<string, PathKind>(StringComparer.Ordinal);
        var queue = new Queue<(string Node, PathKind Kind)>();

        foreach (var entry in entryPoints)
        {
            if (string.IsNullOrWhiteSpace(entry))
            {
                continue;
            }

            var normalized = entry.Trim();
            if (reachable.Add(normalized))
            {
                predecessor[normalized] = null;
                pathKind[normalized] = PathKind.Required;
                queue.Enqueue((normalized, PathKind.Required));
            }
        }

        while (queue.Count > 0)
        {
            var (node, kind) = queue.Dequeue();
            if (!graph.Edges.TryGetValue(node, out var edges))
            {
                continue;
            }

            foreach (var edge in edges)
            {
                if (!IsScopeIncluded(edge.Scope, resolvedPolicy.ScopeHandling))
                {
                    continue;
                }

                var nextKind = Combine(kind, edge.Scope, resolvedPolicy.ScopeHandling);
                var target = edge.To;
                if (nextKind == PathKind.Required)
                {
                    if (reachable.Add(target))
                    {
                        predecessor[target] = node;
                        pathKind[target] = PathKind.Required;
                        queue.Enqueue((target, PathKind.Required));
                    }
                    else if (pathKind.TryGetValue(target, out var existing) &&
                             existing == PathKind.Optional)
                    {
                        predecessor[target] = node;
                        pathKind[target] = PathKind.Required;
                        queue.Enqueue((target, PathKind.Required));
                    }
                }
                else
                {
                    if (reachable.Contains(target))
                    {
                        continue;
                    }

                    if (potential.Add(target))
                    {
                        predecessor[target] = node;
                        pathKind[target] = PathKind.Optional;
                        queue.Enqueue((target, PathKind.Optional));
                    }
                }
            }
        }

        foreach (var node in graph.Nodes)
        {
            if (reachable.Contains(node))
            {
                results[node] = ReachabilityStatus.Reachable;
                findings.Add(new ReachabilityFinding
                {
                    ComponentRef = node,
                    Status = ReachabilityStatus.Reachable,
                    Path = BuildPath(node, predecessor)
                });
                continue;
            }

            if (potential.Contains(node))
            {
                results[node] = ReachabilityStatus.PotentiallyReachable;
                findings.Add(new ReachabilityFinding
                {
                    ComponentRef = node,
                    Status = ReachabilityStatus.PotentiallyReachable,
                    Path = BuildPath(node, predecessor),
                    Reason = "optional-dependency"
                });
                continue;
            }

            results[node] = ReachabilityStatus.Unreachable;
            findings.Add(new ReachabilityFinding
            {
                ComponentRef = node,
                Status = ReachabilityStatus.Unreachable,
                Reason = "no-path"
            });
        }

        return ReachabilityReportBuilder.Build(graph, results, findings);
    }

    private static ImmutableArray<string> BuildPath(
        string target,
        IReadOnlyDictionary<string, string?> predecessor)
    {
        var path = new List<string>();
        var current = target;
        var seen = new HashSet<string>(StringComparer.Ordinal);
        while (true)
        {
            if (!seen.Add(current))
            {
                break;
            }

            path.Add(current);

            if (!predecessor.TryGetValue(current, out var previous) ||
                string.IsNullOrWhiteSpace(previous))
            {
                break;
            }

            current = previous;
        }

        path.Reverse();
        return path.ToImmutableArray();
    }

    private static bool IsScopeIncluded(DependencyScope scope, ReachabilityScopePolicy policy)
    {
        return scope switch
        {
            DependencyScope.Runtime => policy.IncludeRuntime,
            DependencyScope.Development => policy.IncludeDevelopment,
            DependencyScope.Test => policy.IncludeTest,
            DependencyScope.Optional => policy.IncludeOptional != OptionalDependencyHandling.Exclude,
            _ => true
        };
    }

    private static PathKind Combine(
        PathKind current,
        DependencyScope scope,
        ReachabilityScopePolicy policy)
    {
        if (scope != DependencyScope.Optional)
        {
            return current;
        }

        return policy.IncludeOptional switch
        {
            OptionalDependencyHandling.Reachable => current,
            OptionalDependencyHandling.AsPotentiallyReachable => PathKind.Optional,
            _ => current
        };
    }

    private enum PathKind
    {
        Required,
        Optional
    }
}