Updated Spline Raycast with Unity Dreamteck Splines, adds support for raycasting across a closed Spline.

SplineRaycast.cs

using System.Collections.Generic;
using UnityEngine;

namespace Dreamteck.Splines
{
  static class SplineUtilities
  {
    /// <summary>
    /// Casts a ray along the transformed spline against all scene colliders.
    /// </summary>
    public static bool Raycast(SplineComputer spline, float xOffset, float yOffset, ref SplineSample sample, out RaycastHit hit, out double hitPercent, LayerMask layerMask, double from = 0.0, double to = 1.0, double resolution = 1.0, Spline.Direction direction = Spline.Direction.Forward, QueryTriggerInteraction hitTriggers = QueryTriggerInteraction.UseGlobal)
    {
      Spline.FormatFromTo(ref from, ref to, false);
      resolution = DMath.Clamp01(resolution);
      from = DMath.Clamp01(from);
      to = DMath.Clamp01(to);
      double percent = from;

      spline.Evaluate(percent, ref sample);
      Vector3 fromPos = sample.position + sample.up * yOffset + sample.right * xOffset;

      hitPercent = 0f;
      while (true)
      {
        double prevPercent = percent;
        double nextStep = spline.moveStep / resolution;

        percent = direction == Spline.Direction.Forward ?
          MoveForward(percent, to, nextStep) :
          MoveBackward(percent, to, nextStep);
        // percent = DMath.Move(percent, to, nextStep);

        spline.Evaluate(percent, ref sample);
        Vector3 toPos = sample.position + sample.up * yOffset + sample.right * xOffset;

        Debug.DrawLine(fromPos, toPos, Color.red, Time.deltaTime);

        if (Physics.Linecast(fromPos, toPos, out hit, layerMask, hitTriggers))
        {
          double segmentPercent = (hit.point - fromPos).sqrMagnitude / (toPos - fromPos).sqrMagnitude;
          hitPercent = DMath.Lerp(prevPercent, percent, segmentPercent);
          return true;
        }
        fromPos = toPos;
        if (percent >= to) break;
      }
      return false;
    }

    public static double MoveForward(double from, double to, double amount)
    {
      double current = from;
      if (current + amount > to)
        current = to;
      else if (current > 1.0)
        current = 1.0 - current;
      else
        current += amount;

      return current;
    }
    public static double MoveBackward(double from, double to, double amount)
    {
      double current = from;
      if (current - amount < to)
        current = to;
      else if (current < 0.0)
        current = 1.0 + current;
      else
        current -= amount;

      return current;
    }
  }
}