Features of DLXSolver 2.0

Here is a dynamic display of the solving process as you can see it with DLXSolver 2.0

Just click in the start button on the bottom left to play the video !!!

DLXSolver 2.0 supports 9 x 9 and 16 x 16 Sudoku boards.

Just as a fast reminder... Sudoku Boards are made up of cells (81 cells in a 9*9 Board and 256 cells in a 16*16 Board). Boards are divided in Rows (horizontal lines), Columns (vertical lines) and Boxes containing 9 (3*3) or 16 (4*4) cells. The rows, columns and boxes are called Groups. Every group must contain all the numbers from 1 to 9 (in 9*9 Board) or from 1 to 9 and A to G (in 16*16 Board). At the start, some cells are filled with numbers called Clues or Givens. The numbers added when solving the game are called candidates, solved cells or Quest.

The givens of the Sudoku boards are the numbers from 1 to 9 (in 9*9 Boards) or from 1 to 9 and A to G (in 16*16 Boards). The empty cells or candidates are filled with . or 0.

The grids can be entered manually or pasted from the clipboard. The installer of DLXSolver 2.0 adds several text files containing grids with different levels of difficulty. These can be used with the DLX Solver.

An other possibilty is to generate the grids with the freeware TGMDev Isanaki Sudoku and copying the grids to the clipboard.

Any other characters than the valid givens (as the last character A in the exemple below) will interrupt the solving process and genereates an invalid grid with No Solution.

The button 'Solve' is enabled only if the entered grids contain exactly 81 or 216 characters.

Once a valid grid is entered, the button 'Solve' is enabled.

Two modes of solving are available:

1. Solve Sudoku: this is the fastest mode. The grid is solved using the Dancing Links method and only the complete solution is displayed.
2. Show Progress: this processing is a little bit slower as the progress of the solving process is shown on screen by updating the baord and the list box (see more below)

The Solving progress mode can be tailored by changing the Processing Delay (from 0 ms to 1 second). This gives the opportunity to visualize the progress of the solving process eaisly.

With a delay of 1 second, the processing can be quite lengthy. Hint: Set the delay to 0 ms while the process is running will speed the termination of the solving process.

In Solving Progress Mode, DLXSolver 2.0 updates:

1. the Sudoku board by updating gradually the different candidates (in blue color) to the board.
2. the action list by adding the actions defined by the solver: either a candidate addition or a candidate replacement. The last actions are always on the top of the list.

For very simple grids, the solver performs only simple candidate additions. There is no backtracking that requires to re-process the grid and proceed with the replacement of a candidate already placed on board. But hard-to-solve grids require backtracking and several actions of candidates replacement.

If you look a the solving process in slow mode (1 sec delay), you can see it clearly with the following grid:

.4........2...61.5..61...7.1..8.....3.2.4.8.6.....9..7.6...49..9.52...6........8.

The relationship between grid difficulty level and solver processing has been tested with grids generated by the freeware TGMDev Isanaki Sudoku. Grids with increasing difficulty levels and different symetries were tested with the DLXSudokuSolver.

The results, obtained on 23 march 2025 using DLXSudokuSolver 2.0 and Isanaki Sudoku 2.6b , show that as the difficulty level increases, the solver makes more candidate replacements. This relationship is not always respected but this may be due to the subjective level of difficulty, based on the use of increasingly complex human resolution strategies to find the candidates.

Difficulty Level	Symetry	Grid	Givens	Cand.	Add.	Repl.
9 * 9 grids
Very Easy	Symetrical	61...9.8....2.35...3.....195.8.9.4.7.......9....6...52...85..63..6..18.......79..	28	53	53	0
Very Easy	Non-Sym.	.....52.6..61..43..593...8.....713.....453.....729.....1...472..72..85..5.49.....	31	50	50	0
Easy	Symetrical	3.2...8....96...7....1.5....16.5....9.....28.7....3....2..67...8..32......7...9..	24	57	57	0
Easy	Non-Sym.	....2.6.3.....18..4.5...72.1....6.828...7...435.8....6.17...9.8..83.....2.3.9....	29	52	52	0
Medium	Symetrical	....1....96...73.2....4..7..9.8.1.3..3.2.....7.5......2......4......9..88....3769	25	56	56	0
Medium	Non-Sym.	1....76...8.......7.51.9.....1...4.66.45.31.73.8...5.....3.69.4.......7...74....2	28	53	53	0
Hard	Symetrical	.81.....4...7.......6..2.8..7..9.4....31.....5....31...57.1.82....3..9....2...3..	24	57	57	1
Hard	Non-Sym.	.3..4..21......67...7...453.439..2.....462.....6..374.754...3...91......86..3..1.	31	50	50	0
Very Hard	Symetrical	..7.3..9...8.....4..97.5.38.9....6.....526....13.74...3...5.4....6.......51.8....	26	55	55	14
Very Hard	Non-Sym.	1..4....2....5..34..8.1..7..7.9.....3.58.79.1.....2.6..1..8.7..45..2....8....5..6	28	53	53	6
Fiendish	Symetrical	..8..73....26...5...6..19...1.98...6......8.5.9....4.........7.3.4..5....6..32...	24	57	57	49
Fiendish	Non-Sym.	.4........2...61.5..61...7.1..8.....3.2.4.8.6.....9..7.6...49..9.52...6........8.	25	56	56	79
16 * 16 grids
Very Easy	Symetrical	.....2.E....57.F9..7..4..25A6.D.....95..G7.B..A...2A.3.791.E...8...B....3.A1.F.C7....G....D......4DEB96......85.2.F...3A59..1.E.B.9G..8...3.7.F..7.5.E.B2F.94.3.D..2....CEG...6B..4.7.536.1...G.E57..F...3C8.D46.B61ED79.A2G.3..GCA.361.BDE.F92.3..D..C.769.A.1.	123	133	133	0
Very Easy	Non-Sym.	21E..5B......G6...F.E.7ABD....24..G9.28.51...DA.B..A6.9..2.F3178C.12.B5.63.7G.....8D..1.F...E..5.....DG6E5.8AB.25B.4...E..DG...78...BF..G...2.D99.572.C148B.....A..F...D.9..C4.....G9.47.FC.65.114C65.D..E.27..3.92...3C.48.DA..G8....A2D6.5.F...A7......G9..25B	128	128	128	0
Easy	Symetrical	3B8.26.9.G..EDF.4.A...8C.EFD6......6.EGA.8.9....2....4FD36CAB.1..C....A..2....7F7..D...2.A8G5.....4.F..56..192.....F...E..3C8BD...C1A23.....D9.E......BFA.23.6.5EA.5D9.G81B..C27FG2...57D94.A......B..2.E....AG8G5..8A9.C.6.1..D..DA1..4..GBC.62.F...D.6..A834..	127	127	129	0
Easy	Non-Sym.	.....G...93A47F.19CF.823......6...D.79...82..3.G3BG..E..4D.C.829.13G..C..57.FB.87..E138.DGB2A9...2A.B....14...3.9.B......3.....22.....7......D.F.G...1A....D.48...48FD9E.6173..BB.79.25..A..6C1.CF2.A.EG..9..6B3D.6..B1...E4.F...4......BFD.G275.79B8FD...A.....	124	132	132	0
Medium	Symetrical	.3.A18.6.C....GE57.C3.F..8...A6D6.GD5..73A..148.8..E.CD..G6..9...E.B..157......G3.8.C62B5.E....A.D.4..G.231.6.5..F...D9........BA.....5D......F...7964.1....A...D6B.....A...57.1.8..27AGC......4.2.3D56AE1...C9F.....B4.F.GC.6.2....G.8.49.3E..5..E..27F.B5A.14.	119	137	137	0
Medium	Non-Sym.	.A.....6F........G7D..B..9A.6.4.6.93.4..BE.2D.58.BE418D....G..C.4.G18.C3.5...72.32.57F.1.6GA9.E.D..7.2...F1......6.9DGEB.2............3.2DB4A.9......9F...6.C..B.3.B2C1.A.F7E.65.D6...8.C1.324.7.4..5....7D1GEA.E5.2B.7F..8.39.C.7.A.12..B..458........85.....7.	122	134	134	0
Hard	Symetrical	.62..9...E.CBF.A...92...FA....4.48.F1.5A2..D3C.....C.3.D14B9.......G..3.9D8.A4.C6.9..24..FA38.B...A..1.B....F5..8.35.ED.71..629.9256....G.1.D..7.1.7G....93F..C.BEF.9....C27G...G...F.17.5.A.....G62BA.......9D5D34.E.6..795.B....7....9B8D..G6.E..B...1.G..4.A.	121	135	135	0
Hard	Non-Sym.	7A19B.G....8F.DC2.35.D9.6......G8C4E1.F37.9...2.6..F8..E..1A.3....C.3.B...A5..GD3.E..7.5.........8FB...D27.....35..G28E.31...7A..57...AC.FD21..EA.....71C...3D8.........G.7..C.ACG..DF...A.E.B....9.A2..4..3G..5.2...3.7BD.6A918F......G.25.4E.6G4.6E....9.FD237	122	134	134	17
Very Hard	Symetrical	.D..94..FGE....C.C2...3169DB4..F..5F.....7.........GA.BF8...7.2..8F7..6GD1..34AB9.E....7CAB.D.G..6..F.E54...19....GC......F..87.7..5.6G....F.....4.921.A5D6.C.F.....D.4.BE8....6D...5.FB721.A.8..7186FD...A9.23.F543..18E.7.....G9.D3..4...8E..1....G...3...F...	117	139	139	26
Very Hard	Non-Sym.	1..2....7C9.EG...GC.9..ABE.276.14FB...E.1...9...9.....GCA.F.4...5.DBA..6.24..E78.24.E1.8.5.....G8...G57.6.DE..24.3G.C..F8..B..D...1D....46..G..EB.245F..CA.G.7.9.8.....4.7...2B..A....CG..B.....245..C...FA...G7.E..3..12458C.6AFB....87..C129E5G.....5.9B....1.	120	136	136	74
Fiendish	Symetrical	1.A2.8.E.C....D338B51.9D7.6.....4.C.A.52..D..8..7..964....A2..B....37C..4.GB...2.2.4..D.37....G.A..B4.69.1.D378F5....G.....E4.6DB5...DG.E.F.C3......EA1.B.....9G.498.......3.5.EE67..53...2...18...1D7..6.E4..3....DF......79.4C.B4EG...CD..86..8C.7.E.......D.1	116	140	140	236
Fiendish	Non-Sym.	2.G...6C....5.....E.41D.372..G.A6FA....E..DG4.7......BG.5..8.....4...G.B2DEFA.8...3A..1.C.5...B2.E.B..58.1.3.DG.CG...6..9A8B7......4F8C3..9...2G.59.D.B.A6..8.3.B2...7.6.8..D5...8.3529GF.B...A.....8..5.2F......9.5B3..8....2FDF.8..D79.315.C.....D....74...8.1	116	140	140	211