A Su Doku solver

The goal of a Su Doku problem is to complete the published grid so that no number appears more than once in any row, column or 3x3 subgrid. The rules are described more fully on the official Su Doku website, to which a link is provided above. To set up the solver, simply type the numbers printed in The Times into the corresponding squares in the grid below. (The Tab and Shift-Tab keys allow the grid to be navigated without use of the mouse). Squares that are blank in The Times should be left blank in the grid. Once all the values have been copied across from the newspaper, use the buttons to the right of the grid.

Instructions appear underneath.

Solve

There are three solve buttons:

Solve completes the grid (provided that a solution exists).
Unsolve resets any squares on the grid filled by a previous press of the Solve button.
Reset resets the entire grid.

Evaluate

Evaluate checks to see whether the current incomplete grid has a unique solution. The Time field records the time taken to solve the puzzle. (The command-line solver runs considerably faster than the applet because it doesn't cache the intermediate solutions or state grids).

Shuffle and Labelling Format

The Shuffle button performs a random series of reflections, rotations and digit rearrangements in order to create a new puzzle that has a similar structure to before.

The drop-down list to the right of the Evaluate button allows the user to switch between the 'Numeric from 1' format used by the majority of sources, the 'Alphanumeric from 0' format used by the 4x4 puzzles in The Independent and the 'Text' format used by the Wisden Cricketer magazine, where the numbers 1-9 seen in the traditional puzzle are replaced by the nine letters of a cricket-related word or phrase such as 'TENDULKAR' or 'SIXANDOUT'.

The Clipboard

Copy transfers the current grid into a separate clipboard window (the system clipboard cannot be used for security reasons) in the format selected in the drop-down box on the far right of the row. The supported formats are Plain Text, Library Book (the XML format used by the Pappocom Sudoku software) and the various state grid types. Once Copy has been pressed, simply select the displayed grid in the clipboard window, press Ctrl-Insert to copy it, then press Shift-Insert from within some other document, such as a new e-mail message. The grid will look its best rendered in a monospaced font such as Courier. Paste performs the opposite operation, i.e. it transfers a grid from the clipboard window (which must have been previously opened by a press of the Copy button) to the solver. (Only the Plain Text format is supported by the Paste operation). Suppose you have received an e-mail that contains a Su Doku puzzle in the approved clipboard format. Select the grid from within the document, then press Ctrl-Insert in order to copy the grid to the system clipboard. Press Shift-Insert within the clipboard window to copy the text there, then press the Paste button to copy the grid from the clipboard window into the solver.

Resize

Although classical Su Doku puzzles use a 3x3 grid of boxes, the Su Doku Solver supports the alternative box grids that are sometimes seen. Use the Resize button to change the grid size. Be aware that the grid might not display properly if a very large grid has been chosen.

Compose

Compose composes a fresh puzzle. The new puzzle is certain to be symmetrical, have a unique solution and feature the requested number of filled cells. Note that complicated puzzles (in the case of the classic 3x3 Su Doku puzzle, 'complicated' usually means with less than 24 filled cells) might take a considerable time to generate. Press Break in order to interrupt the procedure.

Patterns

The Patterns section dictates which patterns should be detected during the Solve phase. The meanings of the various patterns are discussed on the Strategy Types page. The numbers to the right refer (except in the case of Guess) to the total number of candidate move eliminations performed due to the discovery of each type of pattern. In the case of Guess, the figure is the total number of guesses performed.

Reasoning

The text box at the bottom displays the reasoning that has been used to solve the puzzle. Click part-way through the reasoning in order to view partial solutions.

Important Note

Since Release 1.21, the Su Doku Solver has been compiled with the JDK 5.0 version of the javac compiler, which has meant that users with version 1.4 or earlier of the Java Runtime Environment (JRE) installed on their PCs have observed irregular behaviour. Error messages such as:

java.lang.NoSuchMethodError: java.lang.StringBuffer.append(Ljava/lang/String;)Ljava/lang/AbstractStringBuilder;

have appeared in the Sun Java Console (which is accessed from the Tools menu on Internet Explorer). To see which version of the JRE is installed on any PC, type:

java -version

at the Command Prompt. A message such as:

'java' is not recognized as an internal or external command, operable program or batch file.

means that the JRE is not installed on the PC at all, whereas a message such as:

java version "1.5.0_04"

means that the latest (as I type) version is installed. (Version 1.5 is a synonym for Version 5.0). Users with an earlier version (or no JRE at all) should download JRE 5.0 from the Sun Java website.

Alternatively, users should click on the Solver 1.20 link in the left-hand panel, which will display the last version of the Solver to be compatible with JRE 1.4.

Recommended Browser

The Su Doku Solver applet is best viewed through the Firefox browser. Click on the button in order to download.