langton.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define WIDTH 64
#define HEIGHT 64
#define AREA (WIDTH * HEIGHT)

#define WHITE 255
#define BLACK 0

#define INTERVAL 100

uint8_t grid[WIDTH * HEIGHT];

struct {
	size_t x;
	size_t y;
	enum { UP, RIGHT, DOWN, LEFT, } or;
} ant;

void
dump(void) {
	uint8_t *p;
	size_t n;
	ssize_t w;

	p = grid;
	n = AREA;

	while (n > 0) {
		w = write(1, p, n);
		if (w == -1)
			exit(1);

		p += w;
		n -= w;
	}
}

int
ok(void) {
	if (ant.x == 0 && ant.or != RIGHT)
		return 0;

	if (ant.x == WIDTH-1 && ant.or != LEFT)
		return 0;

	if (ant.y == 0 && ant.or != DOWN)
		return 0;

	if (ant.y == HEIGHT-1 && ant.or != UP)
		return 0;

	return 1;
}

int
main(void) {
	size_t c, i;

	memset(grid, WHITE, AREA);

	ant.x = WIDTH / 2;
	ant.y = HEIGHT / 2;

	c = 0;

	while (ok()) {
		if (++c == INTERVAL) {
			dump();
			c = 0;
		}

		i = ant.y * WIDTH + ant.x;

		if (grid[i] == WHITE) {
			grid[i] = BLACK;
			ant.or = (ant.or + 1) % 4;
		}
		else {
			grid[i] = WHITE;
			ant.or = (ant.or + 3) % 4;
		}

		switch (ant.or) {
		case UP:
			ant.y--;
			break;
		case RIGHT:
			ant.x++;
			break;
		case DOWN:
			ant.y++;
			break;
		case LEFT:
			ant.x--;
			break;
		}
	}
}