Algorithms and Data Structures

N.Wirth. Algorithms and Data Structures. Oberon version 104
main procedure determines the initial point of the curve, i.e., the initial coordinates of the pen denoted by
x0 and y0, and the unit increment u. The square in which the curves are drawn is placed into the middle of
the page with given width and height. These parameters as well as the drawing procedure line are taken
from a module Draw. Note that this module retains the current position of the pen.
DEFINITION Draw; (* ADenS33_Draw *)
CONST width = 1024; height = 800;
PROCEDURE Clear; (*clear drawing plane*)
PROCEDURE SetPen(x, y: INTEGER);
PROCEDURE line(dir, len: INTEGER);
(*draw line of length len in direction dir*45 degrees; move pen accordingly*)
END Draw.
Procedure Hilbert draws the n Hilbert curves H 1 ... H n . It uses the four auxiliary procedures A, B, C
and D recursively:
VAR u: INTEGER; (* ADenS33_Hilbert *)
PROCEDURE A (i: INTEGER);
BEGIN
IF i > 0 THEN
D(i-1); Draw.line(4, u); A(i-1); Draw.line(6, u); A(i-1); Draw.line(0, u); B(i-1)
END
END A;
PROCEDURE B (i: INTEGER);
BEGIN
IF i > 0 THEN
C(i-1); Draw.line(2, u); B(i-1); Draw.line(0, u); B(i-1); Draw.line(6, u); A(i-1)
END
END B;
PROCEDURE C (i: INTEGER);
BEGIN
IF i > 0 THEN
B(i-1); Draw.line(0, u); C(i-1); Draw.line(2, u); C(i-1); Draw.line(4, u); D(i-1)
END
END C;
PROCEDURE D (i: INTEGER);
BEGIN
IF i > 0 THEN
A(i-1); Draw.line(6, u); D(i-1); Draw.line(4, u); D(i-1); Draw.line(2, u); C(i-1)
END
END D;
PROCEDURE Hilbert (n: INTEGER);
CONST SquareSize = 512;
VAR i, x0, y0: INTEGER;
BEGIN
Draw.Clear;
x0 := Draw.width DIV 2; y0 := Draw.height DIV 2;
u := SquareSize; i := 0;
REPEAT
INC(i); u := u DIV 2;