FindPerimeterLoop.cs

using System.Collections.Generic;
using System.Linq;
using UnityEngine;

public class HexPerimeter
{

  public static List<HexCell> FindPerimeterLoop(List<HexCell> cells){
    //start by finding the top right most cell to start a loop from
    var startCell = cells.OrderByDescending(t => t.coordinates.Z).ThenByDescending(t => t.coordinates.X).FirstOrDefault();

    //trace right and down as much as we can until the bottom is found, then start going left and up
    //It's possible to go back the way we came if there is a one cell peninsula
    var perim = new List<HexCell>();
    var travelDirection = HexDirection.SE;
    var currentCell = startCell;
    do
    {
      var directionPriorities = directionPriority(travelDirection);
      foreach (var direction in directionPriorities)
      {
        var nextCell = currentCell.GetNeighbor(direction);
        //Add if in the original set of cells
        if (cells.Any(c => nextCell == c))
        {
          perim.Add(currentCell);
          travelDirection = direction;
          currentCell = nextCell;
          break;
        }
      }
    }
    while (currentCell != startCell);

    return perim;
  }

  //Which way should we go given which way we came from?
  //The way the directions are set up this works out to going around clockwise given the start at top
  static IEnumerable<HexDirection> directionPriority(HexDirection dir){
    yield return dir.Previous();
    yield return dir;
    yield return dir.Next();
    yield return dir.Next2();
    yield return dir.Opposite(); //Last resort go back the way we came
  }
}