/// <summary>
/// Finds the shortest possible <see cref="ITrip"/> between two <see cref="ITown"/>s.
/// </summary>
/// <param name="railwaySystem">An instance of <see cref="IRailwaySystem"/>.</param>
/// <param name="from">The <see cref="ITown"/> from where the trip begins.</param>
/// <param name="to">The <see cref="ITown"/> to where the trip ends.</param>
/// <exception cref="InvalidRouteException">Thrown whenever no <see cref="ITrip"/> between the two <see cref="ITown"/>s exists.</exception>
/// <returns>The instance of the shortest <see cref="ITrip"/> found.</returns>
public ITrip FindShortest(IRailwaySystem railwaySystem, ITown from, ITown to)
{
var shortestPath = new Dictionary <ITown, Route>();
var distances = new Dictionary <ITown, int>();
var actualTowns = railwaySystem.GetTowns() as List <ITown> ?? railwaySystem.GetTowns().ToList();
foreach (var town in actualTowns)
{
distances[town] = town.Equals(from) ? 0 : int.MaxValue;
}
if (from.CloneIfEquals(to, out var clonedFrom, $"{from.Name}_"))
{
actualTowns.Add(clonedFrom);
distances[to] = int.MaxValue;
distances[clonedFrom] = 0;
}
while (actualTowns.Count != 0)
{
// This is bad. Should use a Binary Heap instead, but .NET doesn't have one by default like Java's Priority Queue.
var actualShortest = actualTowns.OrderBy(t => distances[t]).First();
actualTowns.Remove(actualShortest);
if (distances[actualShortest] == int.MaxValue)
{
break;
}
if (actualShortest.Equals(to))
{
var path = new Trip(from);
while (shortestPath.ContainsKey(actualShortest))
{
path.AddRoute(shortestPath[actualShortest]);
actualShortest = shortestPath[actualShortest].From;
}
return(path);
}
foreach (var route in actualShortest.Routes)
{
var actualDistance = distances[actualShortest] + route.Distance;
if (actualDistance >= distances[route.To])
{
continue;
}
distances[route.To] = actualDistance;
shortestPath[route.To] = route;
}
}
throw new InvalidRouteException($"There's no such route from '{from}' to '{to}'.");
}
