This project is another demo program for DlxLib. I recently added support for secondary columns as described in Donald E. Knuth's original Dancing Links paper around pages 17-18 where he discusses puzzles involving tetra sticks.

A tetra stick is a shape composed of four short line segments of equal length. There are 16 distinct tetra sticks known as F H I J L N O P R T U V W X Y Z because of the similarity of the shapes to these letters. It turns out that if one of H J L N Y is omitted, the remaining 15 tetra sticks can be arranged to form a 5 x 5 square. Here is one possible solution (omitting L):

Secondary columns are needed to prevent the tetra sticks from crossing:

Each tetra stick can be placed on the board with:

• a specific location e.g. (0, 0)
• a specific orientation i.e. North, South, East or West
• a specific reflection mode i.e. Normal or MirrorY

When building the internal data structure, which in turn will be used to build the DLX matrix, we need to consider each valid location/orientation/reflection mode of each tetra stick. This is achieved with the following code (see RowBuilder.cs):

public static IImmutableList<PlacedTetraStick> BuildRows(IImmutableList<TetraStick> tetraSticks)
{
    var locations = (
        from x in Enumerable.Range(0, 5)
        from y in Enumerable.Range(0, 5)
        select new Coords(x, y)
        ).ToImmutableList();
    var orientations = Enum.GetValues(typeof (Orientation)).Cast<Orientation>().ToImmutableList();
    var reflectionModes = Enum.GetValues(typeof (ReflectionMode)).Cast<ReflectionMode>().ToImmutableList();

    var placedTetraSticks =
        from tetraStick in tetraSticks
        from location in locations
        from orientation in orientations
        from reflectionMode in reflectionModes
        let placedTetraStick = new PlacedTetraStick(tetraStick, location, orientation, reflectionMode)
        where IsFullyWithinGrid(placedTetraStick)
        select placedTetraStick;

    placedTetraSticks = placedTetraSticks.Distinct(new PlacedTetraStickComparer());
    placedTetraSticks = PinTetraStickToBeOrientedNorth(tetraSticks.First(), placedTetraSticks);

    return placedTetraSticks.ToImmutableList();
}

Notes:

• We use Distinct with a specific comparer to remove duplicate rows e.g. O covers the same points in all orientations.

• We arbitrarily pick one tetra stick (the first one) and pin it so that we only consider it's North orientation. This is to avoid finding each solution four times where the only difference is the orientation of the board.

• get basic grid working
  • draw empty grid
• model a tetra stick
  • each tetra stick will have:
    • a tag
    • a list of lines each of which is a list of segments
    • a list of interior junction points
• model all 16 tetra sticks
• draw a tetra stick
  • draw a line segment (0, 0) => (1, 0)
  • draw with given location
  • draw several line segments
  • draw several lines (i.e. a whole tetra stick)
  • draw with correct colour
  • draw with given orientation
  • draw with given reflection mode
• need to enumerate all possible locations for each tetra stick
  • with all possible orientations and reflection modes
• need to be able to generate a matrix row for a given tetra stick/location/orientation
  • initially, ignore interior junction points
• build the DLX matrix with all rows (but no secondary columns)
  • we only want to do this for 15 of the 16 tetra sticks
    • initially, hardcode which 15 tetra sticks to use
• solve the DLX matrix
  • initially on the main thread
  • display first solution
  • then solve on a background thread
  • then add the ability to cancel the solving
• extend DlxLib to support secondary columns
• extend DLX matrix by adding secondary columns for the interior junction points when these are not on the edges of the grid
• add the ability to select which of the tetra sticks (H, J, L, N, Y) to omit
  • simple dropdown showing the name of the tetra stick e.g. "L"
  • enhance this by adding a tiny picture of the tetra stick next to the name (drawn North with the correct colour)
• add the ability to step forward through each solution
  • add Next Solution and Cancel buttons
  • enable/disable the buttons appropriately
  • the background thread should invoke onSolutionFound for each solution
  • view model needs a CurrentSolutionIndex property (initially null)
  • onSolutionFound impl will just add each solution to a list in the view model
    • if this is the first solution then automatically display it
  • OnNextSolution displays the next available solution
    • only enabled when there is another solution to display
  • add a label to indicate we are currently displaying solution 'n' of 'total' where 'total' auto-updates as the background thread finds more solutions
• improve the drawing of the tetra sticks on the board (currently it is very basic and looks a bit naff)
• add the ability to display each step of the algorithm
• add a status bar
• add an app icon
• display messages in the status bar
• display stats in the status bar (size of matrix and elapsed time to solve it)
• Use the MVVM pattern

• Dancing Links
• http://puzzler.sourceforge.net/docs/polysticks-intro.html#tetrasticks