How can one short rule decide the shape of every block in a town?

The neighbor rulecode ← 0IF (north is filled){ code ← code + 1 }IF (east is filled){ code ← code + 2 }IF (south is filled){ code ← code + 4 }IF (west is filled){ code ← code + 8 }piece ← PIECE[code]blocks placed: 0
blocks
0
code of (4, 3)
-
cells that re-picked
-
area
-
perimeter
-

Press the steps in order, or press Auto. Every block runs the same few lines of pseudocode. The only input is which of its four neighbors are filled.

Pseudocode in the assignment-arrow style of the AP Computer Science Principles exam reference sheet (College Board). Coordinates, area and perimeter on the grid as in Common Core Grade 6 Geometry, 6.G.A.3, thecorestandards.org. Every number is counted from the grid drawn. Example values: the town's shape and colors. Our own drawing.