git.stella-ops.org/src/__Libraries/StellaOps.ElkSharp/ElkEdgePostProcessor.GatewayBoundary.TargetBacktracking.cs

namespace StellaOps.ElkSharp;

internal static partial class ElkEdgePostProcessor
{
    private static List<ElkPoint> TrimTargetApproachBacktracking(
        IReadOnlyList<ElkPoint> sourcePath,
        ElkPositionedNode targetNode,
        string side,
        ElkPoint explicitEndpoint)
    {
        if (sourcePath.Count < 4)
        {
            return sourcePath
                .Select(point => new ElkPoint { X = point.X, Y = point.Y })
                .ToList();
        }

        const double tolerance = 0.5d;
        var startIndex = Math.Max(0, sourcePath.Count - 5);
        var firstOffendingIndex = -1;
        for (var i = startIndex; i < sourcePath.Count - 1; i++)
        {
            if (IsOnWrongSideOfTarget(sourcePath[i], targetNode, side, tolerance))
            {
                firstOffendingIndex = i;
                break;
            }
        }

        if (firstOffendingIndex < 0)
        {
            return sourcePath
                .Select(point => new ElkPoint { X = point.X, Y = point.Y })
                .ToList();
        }

        var trimmed = sourcePath
            .Take(Math.Max(1, firstOffendingIndex))
            .Select(point => new ElkPoint { X = point.X, Y = point.Y })
            .ToList();
        if (trimmed.Count == 0 || !ElkEdgeRoutingGeometry.PointsEqual(trimmed[^1], explicitEndpoint))
        {
            trimmed.Add(explicitEndpoint);
        }

        return NormalizeEntryPath(trimmed, targetNode, side, explicitEndpoint);
    }

    private static bool TryNormalizeNonGatewayBacktrackingEntry(
        IReadOnlyList<ElkPoint> sourcePath,
        ElkPositionedNode targetNode,
        out List<ElkPoint> repairedPath)
    {
        repairedPath = sourcePath
            .Select(point => new ElkPoint { X = point.X, Y = point.Y })
            .ToList();
        if (sourcePath.Count < 2)
        {
            return false;
        }

        if (!TryResolveNonGatewayBacktrackingEndpoint(sourcePath, targetNode, out var side, out var endpoint))
        {
            return false;
        }

        var candidate = NormalizeEntryPath(sourcePath, targetNode, side, endpoint);
        if (HasTargetApproachBacktracking(candidate, targetNode))
        {
            return false;
        }

        repairedPath = candidate;
        return true;
    }

    private static bool TryResolveNonGatewayBacktrackingEndpoint(
        IReadOnlyList<ElkPoint> sourcePath,
        ElkPositionedNode targetNode,
        out string side,
        out ElkPoint endpoint)
    {
        side = string.Empty;
        endpoint = default!;
        if (sourcePath.Count < 2)
        {
            return false;
        }

        var anchor = sourcePath[^2];
        var centerX = targetNode.X + (targetNode.Width / 2d);
        var centerY = targetNode.Y + (targetNode.Height / 2d);
        var deltaX = anchor.X - centerX;
        var deltaY = anchor.Y - centerY;
        var dominantHorizontal = Math.Abs(deltaX) >= Math.Abs(deltaY) * 1.15d;
        side = dominantHorizontal
            ? (deltaX <= 0d ? "left" : "right")
            : (deltaY <= 0d ? "top" : "bottom");

        if (side is "left" or "right")
        {
            endpoint = new ElkPoint
            {
                X = side == "left" ? targetNode.X : targetNode.X + targetNode.Width,
                Y = Math.Clamp(anchor.Y, targetNode.Y + 4d, targetNode.Y + targetNode.Height - 4d),
            };
        }
        else
        {
            endpoint = new ElkPoint
            {
                X = Math.Clamp(anchor.X, targetNode.X + 4d, targetNode.X + targetNode.Width - 4d),
                Y = side == "top" ? targetNode.Y : targetNode.Y + targetNode.Height,
            };
        }

        return true;
    }

    private static bool HasTargetApproachBacktracking(
        IReadOnlyList<ElkPoint> path,
        ElkPositionedNode targetNode)
    {
        if (path.Count < 3 || ElkShapeBoundaries.IsGatewayShape(targetNode))
        {
            return false;
        }

        var side = ElkEdgeRoutingGeometry.ResolveBoundarySide(path[^1], targetNode);
        if (side is not "left" and not "right" and not "top" and not "bottom")
        {
            return false;
        }

        const double tolerance = 0.5d;
        if (HasShortOrthogonalTargetHook(path, targetNode, side, tolerance))
        {
            return true;
        }

        var startIndex = Math.Max(0, path.Count - (side is "left" or "right" ? 4 : 3));
        var axisValues = new List<double>(path.Count - startIndex);
        for (var i = startIndex; i < path.Count; i++)
        {
            var value = side is "left" or "right"
                ? path[i].X
                : path[i].Y;
            if (axisValues.Count == 0 || Math.Abs(axisValues[^1] - value) > tolerance)
            {
                axisValues.Add(value);
            }
        }

        if (axisValues.Count < 3)
        {
            return false;
        }

        var targetAxis = side switch
        {
            "left" => targetNode.X,
            "right" => targetNode.X + targetNode.Width,
            "top" => targetNode.Y,
            "bottom" => targetNode.Y + targetNode.Height,
            _ => double.NaN,
        };

        var overshootsTargetSide = side switch
        {
            "left" or "top" => axisValues.Any(value => value > targetAxis + tolerance),
            "right" or "bottom" => axisValues.Any(value => value < targetAxis - tolerance),
            _ => false,
        };
        if (overshootsTargetSide)
        {
            return true;
        }

        var expectsIncreasing = side is "left" or "top";
        var sawProgress = false;
        for (var i = 1; i < axisValues.Count; i++)
        {
            var delta = axisValues[i] - axisValues[i - 1];
            if (Math.Abs(delta) <= tolerance)
            {
                continue;
            }

            if (expectsIncreasing)
            {
                if (delta > tolerance)
                {
                    sawProgress = true;
                }
                else if (sawProgress)
                {
                    return true;
                }
            }
            else
            {
                if (delta < -tolerance)
                {
                    sawProgress = true;
                }
                else if (sawProgress)
                {
                    return true;
                }
            }
        }

        return false;
    }

    private static bool HasShortOrthogonalTargetHook(
        IReadOnlyList<ElkPoint> path,
        ElkPositionedNode targetNode,
        string side,
        double tolerance)
    {
        if (path.Count < 3)
        {
            return false;
        }

        var boundaryPoint = path[^1];
        var runStartIndex = path.Count - 2;
        if (side is "left" or "right")
        {
            while (runStartIndex > 0 && Math.Abs(path[runStartIndex - 1].Y - boundaryPoint.Y) <= tolerance)
            {
                runStartIndex--;
            }
        }
        else
        {
            while (runStartIndex > 0 && Math.Abs(path[runStartIndex - 1].X - boundaryPoint.X) <= tolerance)
            {
                runStartIndex--;
            }
        }

        if (runStartIndex == 0)
        {
            return false;
        }

        var overallDeltaX = path[^1].X - path[0].X;
        var overallDeltaY = path[^1].Y - path[0].Y;
        var overallAbsDx = Math.Abs(overallDeltaX);
        var overallAbsDy = Math.Abs(overallDeltaY);
        var sameRowThreshold = Math.Max(24d, targetNode.Height / 3d);
        var sameColumnThreshold = Math.Max(24d, targetNode.Width / 3d);
        var looksHorizontal = overallAbsDx >= overallAbsDy * 1.15d
            && overallAbsDy <= sameRowThreshold
            && Math.Sign(overallDeltaX) != 0;
        var looksVertical = overallAbsDy >= overallAbsDx * 1.15d
            && overallAbsDx <= sameColumnThreshold
            && Math.Sign(overallDeltaY) != 0;
        var contradictsDominantApproach = side switch
        {
            "left" or "right" => looksVertical,
            "top" or "bottom" => looksHorizontal,
            _ => false,
        };
        if (!contradictsDominantApproach)
        {
            return false;
        }

        var runStart = path[runStartIndex];
        var boundaryDepth = side is "left" or "right"
            ? Math.Abs(boundaryPoint.X - runStart.X)
            : Math.Abs(boundaryPoint.Y - runStart.Y);
        var requiredDepth = side is "left" or "right"
            ? targetNode.Width
            : targetNode.Height;
        if (boundaryDepth + tolerance >= requiredDepth)
        {
            return false;
        }

        var predecessor = path[runStartIndex - 1];
        var predecessorDx = Math.Abs(runStart.X - predecessor.X);
        var predecessorDy = Math.Abs(runStart.Y - predecessor.Y);
        return side switch
        {
            "left" or "right" => predecessorDy > predecessorDx * 3d,
            "top" or "bottom" => predecessorDx > predecessorDy * 3d,
            _ => false,
        };
    }

    private static bool IsOnWrongSideOfTarget(
        ElkPoint point,
        ElkPositionedNode targetNode,
        string side,
        double tolerance)
    {
        return side switch
        {
            "left" => point.X > targetNode.X + tolerance,
            "right" => point.X < (targetNode.X + targetNode.Width) - tolerance,
            "top" => point.Y > targetNode.Y + tolerance,
            "bottom" => point.Y < (targetNode.Y + targetNode.Height) - tolerance,
            _ => false,
        };
    }

}