I'm excited to introduce a new scheme to capture and transport the board and progress in minimal fashion for Sudoku and many other puzzles including Killers, KenKen and Str8ts. I've been collaborating with Quintin May a.k.a. Sasha behind the scenes to come up with a better approach. This page and the follow-up page on Killer/KenKen definitions is a very compressed overview of the approach - but enough to explain how. Quintin has created a superb and comprehensive documentation library - and if you intend to code - I recommend you browse the following:

• Source code for library project and top-level entry point:
  https://github.com/BlueAnt1/puzzle-coding
• Documentation:
  https://blueant1.github.io/puzzle-coding/documentation/puzzlecoding/
• Encoding formats reference:
  https://blueant1.github.io/puzzle-coding/documentation/puzzlecoding/encodingformats

There are also some Test Strings built into links to each solver.

The chief deficits of version 1 were:

• A single candidate note was being interpreted as a solved cell
• Unable to expand the scheme for puzzles with jigsaw boxes
• Unable to expand the scheme for operators in clues like KenKen and KenDoku
• No version number in case of changes to the definition

The solvers will continue to support the old packed definitions for the time being since there will be many hard coded links to the solvers out in the wild or stored in databases.

The unpacked 81 character sudoku definition remains the same as before in all places and uses. You can continue to import a Sudoku with 81 numbers (and other symbols like '.' or '_' for blanks). I will be building a more comprehensive importer in the near future to cope with line breaks and other more 'noisy' puzzle definitions.

So lets dive into the new version for Sudoku including Jigsaw Sudoku, Sudoku X, Windoku and Coloured Sudoku. The next page will cater for Killer, KenKen and KenDoku. Lets consider the sorts of information we want to transport:

• All puzzles need the ability to store progress - which consists of solved cells and candidates.
• We should also retain which large numbers are clues.
• It will be very helpful to include a version header to give vital context for interpreting the data

That is all for Sudoku and variants with extra constraints like Sudoku X. However other puzzles contains more information we will need to encode:

• Jigsaw Sudoku has irregular boxes. Our new approach moves away from a shape number as this is peculiar to SudokuWiki.org and not universal.
• For Killers and variants we have the concept of cages and clue operands (+ for Killer, -, x and / for others)
• For Str8ts we need to encode the black cells

The following table lays it all out. We can also predict the minimum number of bytes per cell the new packed strings will achieve. (Str8ts is still under development).

Leave Out
Encode
	Prefix	Clues	Solved	Notes	Boxes	Cages	Clue Operands	Black Cells	Bytes / Cell
Sudoku	S9B	Yes	Yes	Yes	Regular	-	-	-	2
Sudoku X	X9B	Yes	Yes	Yes	Regular	-	-	-	2
Windoku	W9B	Yes	Yes	Yes	Regular	-	-	-	2
Colour Sudoku	C9B	Yes	Yes	Yes	Regular	-	-	-	2
Jigsaw Sudoku	J9B	Yes	Yes	Yes	Irregular	-	-	-	3
Killer Sudoku	L9B	No	Yes	Yes	Regular	Yes	-	-	4
Killer Jigsaw	M9B	No	Yes	Yes	Irregular	Yes	-	-	5
KenKen	K6C	No	Yes	Yes	-	Yes	4 (+-x/)	-	4
KenDoku	D6C	No	Yes	Yes	Regular	Yes	4 (+-x/)	-	4
Str8ts	T9B	Yes	Yes	Yes	-	-	-	Yes	?
Str8ts X	U9B	Yes	Yes	Yes	-	-	-	Yes	?
Str8ts B	B9B	Yes	Yes	Yes	Regular	-	-	Yes	?
Str8ts BX	V9X	Yes	Yes	Yes	Regular	-	-	Yes	?
		OFF SET			SHIFTED	SHIFTED	SHIFTED	SHIFTED

Each packed string definition will be prefixed by three characters.

• A puzzle type letter
• + board size (exclusively 6 or 9 at the moment)
• + version letter. Since this is version 2 we're going with 'B'.

Sudoku and any variants with a fixed pattern or extra constraint will have the same packed definition. We can tell what variant by the first letter of the packed string (yellow column).

As in the previous packed version we store candidates as bits. (1=1, 2=2, 3=4, 4=8, 5=16, 6=32, 7=64, 8=128 and 9=256). The key difference between this packing method and the previous is that we're going to create smaller numbers for the clues/solved/candidates. In fact they are going to be no larger than 511+18. Previously we shifted the bits to make room for the clue flag meaning the largest number was 1022. However, the new method recognises that a clue and a solved cell and candidates are mutually exclusive.

• We can store a clue number (1 to 9) as 1 to 9
• We can store a solved call (1 to 9) as 10 to 18 - offset.
• And candidates can be any number between 1 and 511 + 18

It so happens that the largest number 529 can be stored in 10 bits. All this fits in two bytes. For Jigsaws we need three bits to store the box number. So we shift the box number 10 bits and add it to the clues/candidates number. Finally we convert to base 36 and pad with zeros. Jigsaws fit produce cell strings 3 bytes long.

All the data for each cell will fit into a single integer. But we're not using all 64 bits. Only the last 16 (two bytes)
unused
|  |box |cell no |
000 010 1010010001
|byte 1 | byte 2 |

A bit of javascript code shows how this is done

for (y = 0; y < 9; y++)
	for (x = 0; x < 9; x++) {
		n = get the cell value or set of candidates as bits
		if( candidates )
			n += 18; //  offset by 18
		else if ( !clue )
			n += 9; //  offset by 9
		if( jigsaw ) 
			n += (box_num << 10); //  shift 10 bits and add
		h = n.toString(36); // convert to base 36
		if (h.length < 2) h = '0' + h; //  pad the number if not 2 digits
		if (jigsaw && h.length < 3) h = '0' + h; //  pad if not 3 digits
		s += h; // append to string being made
	}

To unpack the string (after checking the version is S9B, X9B, W9B, C9B or J9B) we do the following

// This splits a string into an array of elements each 2 characters long 
var narr = theString.match(/.{1,2}/g);   // should be {1,3} for Jigsaws
for (y = 0; y < 9; y++)
	for (x = 0; x < 9; x++) {
		p = parseInt(narr[y * 9 + x], 36); // convert base 36 to decimal
		n = p & 1023; // first 10 bits
		clue(y, x) = (n < 10) ? 1 : 0; // extract the clue flag
		if (n <= 18) {
			if (n > 9 ) n -= 9; { // removed 'solved cell' offset
			// save the number as a clue or solution
		} else {
			// save the number as a set of candidates
			n -= 18;
		}
		p >>= 10; // remove first 10 bits
		 // p is the box number for this cell
		 // Will be 0 for vanilla Sudoku
	}

Click on the link to read about how the scheme extends to Killer, KenKen and KenDoku