namespace StellaOps.ElkSharp;

internal static partial class ElkEdgeRouterIterative
{
    /// <summary>
    /// When two edges converge on the same target face with inadequate
    /// Y-separation, spreads their approach lanes apart to create adequate
    /// clearance. Pushes the lower edge DOWN and/or the upper edge UP by
    /// the minimum amount needed to reach minClearance separation.
    /// </summary>
    private static ElkRoutedEdge[]? ReassignConvergentTargetFace(
        ElkRoutedEdge[] edges,
        ElkPositionedNode[] nodes,
        ElkLayoutDirection direction)
    {
        if (direction != ElkLayoutDirection.LeftToRight)
        {
            return null;
        }

        var nodesById = nodes.ToDictionary(n => n.Id, StringComparer.Ordinal);
        var serviceNodes = nodes.Where(n => n.Kind is not "Start" and not "End").ToArray();
        var minClearance = serviceNodes.Length > 0
            ? Math.Min(serviceNodes.Average(n => n.Width), serviceNodes.Average(n => n.Height)) / 2d
            : 50d;
        var joinSeverity = new Dictionary<string, int>(StringComparer.Ordinal);
        ElkEdgeRoutingScoring.CountTargetApproachJoinViolations(edges, nodes, joinSeverity, 1);
        if (joinSeverity.Count < 2)
        {
            return null;
        }

        // Find pairs of edges targeting the same node with join violations.
        var joinEdgeIds = joinSeverity.Keys.ToHashSet(StringComparer.Ordinal);
        var edgesByTarget = edges
            .Where(e => joinEdgeIds.Contains(e.Id))
            .GroupBy(e => e.TargetNodeId ?? string.Empty, StringComparer.Ordinal)
            .Where(g => g.Count() >= 2);

        ElkRoutedEdge[]? result = null;

        foreach (var group in edgesByTarget)
        {
            if (!nodesById.TryGetValue(group.Key, out var targetNode))
            {
                continue;
            }

            var groupEdges = group.OrderBy(e =>
            {
                var path = ExtractPath(e);
                return path.Count >= 2 ? path[^2].Y : 0d;
            }).ToArray();

            if (groupEdges.Length < 2)
            {
                continue;
            }

            // Get approach Y-positions (the horizontal segment Y before the target).
            var paths = groupEdges.Select(ExtractPath).ToArray();
            var approachYs = new double[groupEdges.Length];
            var approachSegIndices = new int[groupEdges.Length];
            for (var i = 0; i < groupEdges.Length; i++)
            {
                // Find the last horizontal segment (the approach lane).
                approachSegIndices[i] = -1;
                for (var j = paths[i].Count - 2; j >= 0; j--)
                {
                    if (Math.Abs(paths[i][j].Y - paths[i][j + 1].Y) <= 2d)
                    {
                        approachYs[i] = paths[i][j].Y;
                        approachSegIndices[i] = j;
                        break;
                    }
                }
            }

            if (approachSegIndices.Any(idx => idx < 0))
            {
                continue;
            }

            // Compute the current gap and required spread.
            var currentGap = Math.Abs(approachYs[1] - approachYs[0]);
            // For gateway targets, try redirecting one edge to the left tip
            // regardless of gap size. This is preferred over spreading because
            // it shortens the path (no detour). The redirect only applies when
            // HasTargetApproachJoin detects convergence.
            if (ElkShapeBoundaries.IsGatewayShape(targetNode)
                && ElkEdgeRoutingScoring.HasTargetApproachJoin(
                    paths[0], paths[1], minClearance, 3))
            {
                result = TryRedirectGatewayFaceOverflowEntry(
                    result, edges, groupEdges, paths, targetNode, approachYs);
                continue;
            }

            if (currentGap >= minClearance)
            {
                continue;
            }

            var spreadNeeded = minClearance - currentGap + 8d;
            var halfSpread = spreadNeeded / 2d;

            // Push upper edge up, lower edge down.
            var upperIdx = approachYs[0] < approachYs[1] ? 0 : 1;
            var lowerIdx = 1 - upperIdx;

            for (var which = 0; which < 2; which++)
            {
                var edgeIdx = which == 0 ? upperIdx : lowerIdx;
                var delta = which == 0 ? -halfSpread : halfSpread;
                var segIdx = approachSegIndices[edgeIdx];
                var newY = approachYs[edgeIdx] + delta;

                var globalIdx = Array.FindIndex(edges, e =>
                    string.Equals(e.Id, groupEdges[edgeIdx].Id, StringComparison.Ordinal));
                if (globalIdx < 0)
                {
                    continue;
                }

                var path = paths[edgeIdx];
                var newPath = BuildTargetJoinSpreadPath(path, segIdx, newY);
                if (newPath.Count == 0)
                {
                    continue;
                }

                result = ReplaceEdgePath(result, edges, globalIdx, edges[globalIdx], newPath);
            }

            ElkLayoutDiagnostics.LogProgress(
                $"Target-join spread: {groupEdges[0].Id}+{groupEdges[1].Id} gap={currentGap:F0}→{currentGap + spreadNeeded:F0}px");
        }

        return result;
    }

    private static List<ElkPoint> BuildTargetJoinSpreadPath(
        IReadOnlyList<ElkPoint> path,
        int approachSegmentIndex,
        double newY)
    {
        if (approachSegmentIndex < 0 || approachSegmentIndex >= path.Count - 1)
        {
            return [];
        }

        var segmentStart = path[approachSegmentIndex];
        var segmentEnd = path[approachSegmentIndex + 1];
        var segmentLength = Math.Abs(segmentEnd.X - segmentStart.X);
        if (segmentLength <= 4d)
        {
            return [];
        }

        var inset = Math.Clamp(segmentLength / 4d, 12d, 24d);
        var transitionX = segmentEnd.X >= segmentStart.X
            ? Math.Max(segmentStart.X + 4d, segmentEnd.X - inset)
            : Math.Min(segmentStart.X - 4d, segmentEnd.X + inset);
        if (Math.Abs(transitionX - segmentStart.X) <= 2d
            || Math.Abs(segmentEnd.X - transitionX) <= 2d)
        {
            transitionX = (segmentStart.X + segmentEnd.X) / 2d;
        }

        var newPath = new List<ElkPoint>(path.Count + 2);
        for (var i = 0; i < approachSegmentIndex; i++)
        {
            AddUnique(newPath, path[i]);
        }

        AddUnique(newPath, segmentStart);
        AddUnique(newPath, new ElkPoint { X = transitionX, Y = segmentStart.Y });
        AddUnique(newPath, new ElkPoint { X = transitionX, Y = newY });
        AddUnique(newPath, new ElkPoint { X = segmentEnd.X, Y = newY });

        for (var i = approachSegmentIndex + 2; i < path.Count; i++)
        {
            AddUnique(newPath, path[i]);
        }

        return newPath;
    }

    private static void AddUnique(List<ElkPoint> points, ElkPoint point)
    {
        if (points.Count > 0 && ElkEdgeRoutingGeometry.PointsEqual(points[^1], point))
        {
            return;
        }

        points.Add(new ElkPoint { X = point.X, Y = point.Y });
    }

    /// <summary>
    /// When two edges converge on a gateway face with insufficient room for
    /// proper slot spacing, redirects the edge whose horizontal approach is
    /// closest to the node center Y to the left tip vertex (for LTR layout).
    /// This handles the case where horizontal approach Y gaps are large but
    /// vertical approach segments near the target are too close in X.
    /// </summary>
    private static ElkRoutedEdge[]? TryRedirectGatewayFaceOverflowEntry(
        ElkRoutedEdge[]? result,
        ElkRoutedEdge[] edges,
        ElkRoutedEdge[] groupEdges,
        IReadOnlyList<ElkPoint>[] paths,
        ElkPositionedNode targetNode,
        double[] approachYs)
    {
        if (groupEdges.Length < 2)
        {
            return result;
        }

        var centerY = targetNode.Y + targetNode.Height / 2d;

        // Pick the edge whose horizontal approach Y is closest to the node
        // center -- that edge naturally wants to enter from the upstream face
        // (left for LTR) rather than the bottom/top face.
        var redirectIdx = -1;
        var bestDistToCenter = double.MaxValue;
        for (var i = 0; i < groupEdges.Length; i++)
        {
            var dist = Math.Abs(approachYs[i] - centerY);
            if (dist < bestDistToCenter)
            {
                bestDistToCenter = dist;
                redirectIdx = i;
            }
        }

        if (redirectIdx < 0)
        {
            return result;
        }

        var redirectPath = paths[redirectIdx];
        var leftTipX = targetNode.X;
        var leftTipY = centerY;

        // Find the last path point that is clearly outside the target node's
        // left boundary. Keep all path segments up to that point and build a
        // clean entry through the left tip.
        var lastOutsideIdx = -1;
        for (var j = redirectPath.Count - 1; j >= 0; j--)
        {
            if (redirectPath[j].X < leftTipX - 4d)
            {
                lastOutsideIdx = j;
                break;
            }
        }

        if (lastOutsideIdx < 0)
        {
            return result;
        }

        // Build the redirected path: keep everything up to the last outside
        // point, then route horizontally to a stub 24px left of the tip,
        // bend vertically to the tip Y, and enter at the tip. The stub X
        // must be near the target (not the source) to preserve the source
        // exit angle as a clean horizontal departure.
        var outsidePoint = redirectPath[lastOutsideIdx];
        var stubX = leftTipX - 24d;
        var newPath = new List<ElkPoint>(lastOutsideIdx + 5);
        for (var j = 0; j <= lastOutsideIdx; j++)
        {
            AddUnique(newPath, redirectPath[j]);
        }

        // Horizontal approach to the stub X (preserves source exit angle).
        if (Math.Abs(outsidePoint.X - stubX) > 2d)
        {
            AddUnique(newPath, new ElkPoint { X = stubX, Y = outsidePoint.Y });
        }

        // Vertical transition to the tip Y.
        var currentY = newPath.Count > 0 ? newPath[^1].Y : outsidePoint.Y;
        if (Math.Abs(currentY - leftTipY) > 2d)
        {
            AddUnique(newPath, new ElkPoint { X = stubX, Y = leftTipY });
        }

        // Enter at the left tip vertex.
        AddUnique(newPath, new ElkPoint { X = leftTipX, Y = leftTipY });

        var globalIdx = Array.FindIndex(edges, e =>
            string.Equals(e.Id, groupEdges[redirectIdx].Id, StringComparison.Ordinal));
        if (globalIdx < 0)
        {
            return result;
        }

        result = ReplaceEdgePath(result, edges, globalIdx, edges[globalIdx], newPath);
        ElkLayoutDiagnostics.LogProgress(
            $"Gateway face redirect: {groupEdges[redirectIdx].Id} to left tip of {targetNode.Id}");
        return result;
    }
}