SudoCue - HaniDoku Introduction

HaniDoku sample This picture shows a HaniDoku puzzle.

The rule of this game: Complete the grid by placing a digit in each cell in such a way that all lines of adjacent cells contain a consecutive series of digits. Each puzzle has a a single solution, which can be found using logic only.

It has a hexagonal shape, with hexagonal cells. Lines connect the cells in 3 directions: horizontal, 60 degrees up and 60 degrees down. According to the rule, each line must contain a consecutive series of digits between 1 and 9. Because the lines have different sizes, you do not know which series of digits must be placed in each line. This is a totally new aspect of HaniDoku, setting it apart from many other Sudoku variations.

Also, there are no boxes of any size. This makes it impossible to use some popular Sudoku solving techniques. Please check our HaniDoku solving strategies page for a nice set of alternative techniques for this variation.

This site offers a daily HaniDoku with an archive of previous puzzles and solutions.

Check our freeware HaniCue program. You can play HaniDoku on your Windows PC with this unique program.

What is HaniDoku?

A Hanidoku puzzle looks like a honeycomb. It has hexagonal cells. Like in regular Sudoku, there are 9 rows, but there are no columns. Instead, there are lines that run in 60 degree angles from the horizontal rows. We call the ascending lines inclines and the descending lines declines. There are 9 of each type of line. The rows are numbered from top to bottom and the other lines are numbered from left to right. To be able to communicate about these lines, use this shorthand code: /3/ is the 3rd incline, \5\ is the 5th decline and =4= is row 4.

HaniDoku line samples This picture shows samples of each line type. You can see =5= in blue, /4/ in green and \6\ in yellow.

To identify cells in the grid, I advice you to use RnCn notation. R4C7 is the 7th cell in row 4, counting from left to right.

The lines have different lengths. Each line type comes in sizes 5-6-7-8-9-8-7-6-5. There is only one line for each type that has size 9, all shorter sizes appear twice.

The lines of length 9 must contain all digits 1 through 9. This rule is similar to the rule of Sudoku. However, the shorter lines must contain consecutive numbers. For example, =4= contains 8 cells. It can either contain digits 1 through 8 or digits 2 through 9. One of the new aspects of HaniDoku is that you do not know which of these two series is used. In your solving strategies, you must make a distinction between mandatory digits (2-8) and optional digits (1,9). A line of size 5 has many alternative series: 12345, 23456, 34567, 45678 or 56789. The only mandatory digit in such a line is 5.

The name HaniDoku stems from the Japanese word “hanikamu”, which is used for honeycomb. Take the shape of this puzzle, combine it with “sudoku”, and here is the entymology for this new word.

HaniDoku Specifications

The most interesting part of this format are the lines in 3 directions, with different sizes. Here is a table that shows the possible digit series for each line size:

812345678 or 234567892,3,4,5,6,7,81,9
71234567 or 2345678 or 34567893,4,5,6,71,2,8,9
6123456 or 234567 or 345678 or 4567894,5,61,2,3,7,8,9
512345 or 23456 or 34567 or 45678 or 5678951,2,3,4,6,7,8,9
HaniDoku zones This picture shows colored zones. The 3 lines that cross the blue zone require all 9 digits. Each subsequent zone has 2 additional optional digits. In the red zone, only digit 5 is mandatory. As you can see, only lines of size 5 do not leave their designated zone. Lines of length 9 cross all 5 zones. This can be very useful.

A hidden single for digit 9 in a line of size 9 can be placed anywhere in that line, but when it is located in another zone, it will make digit 9 and all optional digits between 5 & 9 mandatory in all lines that contain the cell with the hidden single.

An optional digit can become mandatory in an individual puzzle, when more information becomes available when you solve it. There are different reasons that make an optional digit mandatory:

  • The digit is given as part of the puzzle. It is already placed in a cell.
  • The digit is a naked or hidden single in one of the cells on the line. It must be placed in that cell.
  • The digit falls in the gap between two placed or mandatory digits.
  • There are no candidates left for the alternative. When no candidates for digit 1 are left on a line of size 5, digit 6 automatically becomes mandatory.
  • The digit is part of a disjoint subset. This is a familiar sudoku technique that can also be used in HaniDoku.

Optional digits can also be eliminated from a line. On a line of size 5, 6 or 7, you can find optional digits that require another optional digit to be present. For example, digit 9 requires digit 8 to be available, otherwise it cannot be used in a series. When digit 8 has no candidates left in a line, digit 9 can also be eliminated from the entire line.

You can save or print the worksheet page, to create worksheets that enable you to solve HaniDoku manually.

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