space consisting of 21 by 17 squares of equal size
2. An organism consisting of 4 types of cells: green, red, yellow, and
3. A cell is represented by one square.
4. Each type of cell reproduces cells of the same type to build a tissue.
5. A tissue has at least two cells.
6. Tissues grow in a two-dimensional array of squares.
7. The initial distribution of 4 types of tissues is marked by colored
squares, with the end cells marked by +.
1. No cells
die once reproduced.
2. Both ends of a tissue grow whenever possible, by reproducing one cell
per unit time in a vacant contiguous square. If there is no vacant contiguous
square at either end, that end stops growing. If there is more than one
vacant contiguous square at either end, the direction of the growth is
governed by the preferential order given by Rules 3, 4, and 5.
3. If, along the straight line defined by the end cell and the penultimate
cell (next to the end cell) there are less than or equal to three cells
of the same type (but may be of different tissues) consecutively, the
preferred direction is along the same straight line. If that direction
is blocked, follow Rule 5.
4. If, along the straight line defined by the end cell and the penultimate
cell, there are more than or equal to four cells of the same type (but
may be of different tissues) consecutively, the preferred direction of
the growth is a left turn. If a left turn is impossible, make a right
5. If, when a straight growth is preferred, the straight growth is impossible
because the square ahead is already occupied, do the following: If the
square to which the straight growth would take place is filled with a
cell of the same type as the growing tissue, make a left turn. If left
turn is impossible, make a right turn. If the square ahead is filled with
a cell whose type is different from that of the growing tissue, make a
right turn. If right turn is impossible, make a left turn.
6. If a boundary is encountered, it is to be treated as a cell of different
type. And the above rules apply as appropriate.
7. The growth of the four types of tissues is time-wise out of phase with
each other: green first, red second, yellow third, ad blue last within
a cycle of one unit time.