/// <summary>
/// Solves the Tower of Hanoi problem recursively.
/// </summary>
/// <param name="discNumber">
/// The current disc number counted from 1. The number is also
/// identical to the size of the (sub-)tower to be solved.
/// </param>
/// <param name="source">
/// The source/initial tower in form of a <see cref="Stack{T}"/>.
/// </param>
/// <param name="temporary">
/// The tower for temporary operations in form of a
/// <see cref="Stack{T}"/>.
/// </param>
/// <param name="destination">
/// The destination/goal tower in form of a <see cref="Stack{T}"/>.
/// </param>
/// <exception cref="ArgumentException">
/// Thrown when <paramref name="discNumber"/> is smaller than 1 or
/// any of the stack references (<paramref name="source"/>,
/// <paramref name="temporary"/>, <paramref name="destination"/>)
/// is null.
/// </exception>
/// <remarks>
/// The essential part to understand the algorithm is to realize that
/// it is the target/goal tower that varies according to the
/// sub-problem.
/// At least some previous knowledge of recursive methods is
/// absolutely required or you might not take the leap at all.
/// If you have played the puzzle on paper for towers of 1 to 3 discs
/// you may intuitively - thinking ahead - have moved discs
/// in similar constellations differently and may fail to see the
/// generic pattern and why behind it.
/// For a 1-disc tower it is trivial - you directly move from
/// the start peg to the goal peg.
/// For a 2-disc tower you (hopefully) are not going to move
/// top disc 1 directly to goal but to the temporary peg instead
/// as you are anticipating that you could then move disc 2
/// directly to its final destination, the goal peg.
/// Finally you move disc 1 from current peg (temporary) to goal.
/// For a 3-disc tower you should realize that it makes sense to
/// first get the 2-disc "sub-tower" out of the way by erecting it
/// on the temporary peg. Then you can simply move disc 3 to the
/// final destination, the goal peg. Now the problem is reduced
/// again to a 2-disc tower problem where the sub-tower is
/// currently on the temporary peg and has to move to the goal peg.
/// Hopefully you may start to see that it is only the "goal tower"
/// that varies by the current needs. You move the current n - 1
/// tower of discs out of the way to be able to move the last or
/// nth disc to the final destination. Then you move the n - 1
/// tower to the final destination.
/// The more discs there are the more similar groups of steps
/// there are that repeat over and over with varying source
/// and destination pegs.
/// </remarks>
private void HanoiSolve(int discNumber, Stack <int> source, Stack <int> temporary, Stack <int> destination)
{
// Argument checking - discNumber must be >= 1
if (discNumber < 1)
{
throw new ArgumentException(
$"Disc number must equal 1 or greater.",
nameof(discNumber));
}
// Null checks
if (source == null ||
temporary == null ||
destination == null)
{
throw new ArgumentException(
"Neither stack must be null.");
}
// Base case - 1 disc; recursion ends
if (discNumber == 1)
{
// Move directly from source to goal
var disc = source.Pop();
destination.Push(disc);
// Cause a notification event to eg. draw and delay UI.
DiscMoved?.Invoke(this, new DiscMovedEventArgs <int>(discNumber, source, destination));
}
else
{
// Recursive procedure - first move all discs less than
// current discNumber out of the way thus using temporary
// as goal and goal as temporary storage.
HanoiSolve(discNumber - 1, source, destination, temporary);
// We freed the current disc on the current from-stack.
// Now move the current disc from target to goal.
var disc = source.Pop();
destination.Push(disc);
// Cause a notification event to eg. draw and delay UI.
DiscMoved?.Invoke(this, new DiscMovedEventArgs <int>(discNumber, source, destination));
// Finally move the "sub-tower" of (n - 1) parked discs
// (ie. < discNumber) to the goal peg.
// We know we are currently on temporary and have to go to
// goal, using from as temporary as necessary.
HanoiSolve(discNumber - 1, temporary, source, destination);
}
}
/// <summary>
/// Runs the game simulation.
/// </summary>
/// <param name="numberOfDiscs">
/// The number of discs to use in the simulation.
/// Defaults to 5.
/// </param>
/// <exception cref="ArgumentException">
/// Thrown when <paramref name="numberOfDiscs"/> is smaller than 1 or
/// larger than the number of discs that could fit the current console
/// window.
/// </exception>
public void Run(int numberOfDiscs = 5)
{
var discCandidates =
OddNumbers(3)
.Take(_maximumDiscs)
.ToList();
var maximumDiscSelection =
discCandidates
.Where(n => n <= _maximumDiscWidth)
.ToList();
WriteLine("~~~ Hanoi Stats ~~~");
WriteLine($"Console height: {_consoleHeight}");
WriteLine($"Console width: {_consoleWidth}");
WriteLine($"Maximum number of discs: {_maximumDiscs}");
WriteLine($"Width candidates: {String.Join(", ", discCandidates)}");
WriteLine($"Maximum disc width: {_maximumDiscWidth}");
WriteLine($"Maximum disc selection: {String.Join(", ", maximumDiscSelection)}");
WriteLine($"User-requested discs: {numberOfDiscs}");
if (numberOfDiscs < 1)
{
throw new ArgumentException(
"The simulation must be run with a number of discs of 1 or greater.",
nameof(numberOfDiscs));
}
if (numberOfDiscs > _maximumDiscs)
{
throw new ArgumentException(
$"Too many discs: A console of {_consoleWidth}x{_consoleHeight} can support a maximum of {_maximumDiscs} discs.",
nameof(numberOfDiscs));
}
Reset();
_userNumberOfDiscs = numberOfDiscs;
var gameDiscs =
maximumDiscSelection
.Take(numberOfDiscs);
WriteLine($"User discs: {String.Join(", ", gameDiscs)}");
WriteLine();
// Now put the selection of game discs on the source stack
foreach (var gameDisc in gameDiscs.Reverse())
{
_sourceStack.Push(gameDisc);
}
DiscMoved += Visualize;
// Initial dummy disc move (no discs moved) to get the
// starting point drawn to screen
DiscMoved?.Invoke(this, DiscMovedEventArgs <int> .Empty);
// Recursively solve to move from-stack to goal-stack
HanoiSolve(_userNumberOfDiscs, _sourceStack, _temporaryStack, _destinationStack);
DiscMoved -= Visualize;
}
