参考プログラム

/*
 * hilbert-eggx.c --- Hilbert 曲線描画
 *
 * コンパイル: egg hilbert-eggx.c -o hilbert-eggx
 */

#include <stdio.h>
#include <stdlib.h>             /* atoi() */
#include <math.h>               /* pow() */
#include <eggxlib.h>

/* EGGX のウィンドウ番号 */
int win;
/* ペンの現在位置 */
double cur_x, cur_y;

void move2(double x, double y)
{
    eggx_line(win, x, y, PENUP);
    cur_x = x;
    cur_y = y;
}

void draw2(double x, double y)
{
    eggx_line(win, x, y, PENDOWN);
    cur_x = x;
    cur_y = y;
}

void rdraw2(double dx, double dy)
{
    draw2(cur_x + dx, cur_y + dy);
}

void hilbert(int n, double dx, double dy, int s)
{
    if (--n >= 0) {
        hilbert(n, -s * dy, s * dx, -s);
        rdraw2(-s * dy, s * dx);
        hilbert(n, dx, dy, s);
        rdraw2(dx, dy);
        hilbert(n, dx, dy, s);
        rdraw2(s * dy, -s * dx);
        hilbert(n, s * dy, -s * dx, -s);
    }
}

int main(int argc, char *argv[])
{
    int n;
    double ds;

    n = (argc > 1) ? atoi(argv[1]) : 3;

    win = eggx_gopen(657, 657);
    eggx_gsetbgcolor(win, "white");
    eggx_gclr(win);
    eggx_newcolor(win, "black");
    eggx_window(win, -1.0 / 8, -15.0 / 64, 9.0 / 8, 65.0 / 64);
    eggx_drawstr(win, 0.15, -0.2, 24, 0, "Hilbert function");
    eggx_drawrect(win, 0.0, 0.0, 1.0, 1.0);
    ds = pow(2.0, - (double)n);
    move2(ds / 2, ds / 2);
    hilbert(n, ds, 0.0, 1);
    getchar();
    eggx_gclose(win);
    return 0;
}