You enter the sudoku from the newspaper into the
cells.

When done you press DONE.

It tells you if this is a hard medium or easy problem.

Each cell has a small grid of 9 with small gray digits -
1
to 9. They are surrounded by red borders - red
meaning this cell has not yet been calculated.
The cells that you filled with numbers have:
a bold (4px) green border around them, meaning
that
this cell is originally defined. The number in the grid
that was selected is bold yellow in a green
background.

Above each column and to the left of each row
there
is an extra "title" cell with a grid of 9 with small gray
digits 1 to 9.

Each "square" that is 3x3 cells, has a rectangular
"title
cell" set into the top mid cell, with a flattened out
grid of digits 1-9.

The numbers you entered are marked with color (say
bold, red on yellow background.)

When hovering on a number, several options are
shown as icons. In turn, each icon is explained in
text
as you hover on it.

The options are: Mark all. Mark square. Mark row,
Mark
vertical column.

On ALL squares you also have the option: Calculate
square. Calculate row. Calculate column.

Marking - marks the area (row column or square)
setting the digit selected as unavailable (grayed
out).
A small triangle adjacent to the digit points to the
direction of the existing digit. A small x in the
corner
means existing digit is in the square.

Calculate - marks the area (row, column or square)
setting all available digits as bold, and unavailable
digits as grayed out, with the same UI as "Marking".
A surrounding bold border is set around pairs of
available numbers (that is: if there are only two
options for a single digit in an area) with an arrow or
x
pointing to the second option.

When an area is calculated its "title" (column, row or
square-title-rectangle) is updated and marked with a
(2px) green border.

Cells that numbers have been entered in

When you enter a number into a cell, the titles of
the
areas (row, column and square) are "invalidated" so
the border turns red, and so does the border of the
areas themselves.

There is UNDO, and view history.

History shows order entry and values:

. . . ; . . . ; . . .
. 0(4) . ; 0(3) . . ; 1(2) 0 0
. . . ; . . . ; . . .
. . . ; . . . ; . . 2(3)
. . . ; . . 0:2 0 ; . . .
. . . ; . . . ; . 3(5) .
. 4(9) . ; . . . ; . . .

etc.