// A solution to asn 2 image generation with malloc and 1D pointer arithmetic
// compile as "gcc -O circle_demo.c -lm" since this uses trig functions
// run as "a.out 1024 768 20 > circles.ppm" or any other image size
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
void circle(unsigned char *, int, int, int, int, int);

main(int argc, char *argv[]) {
	unsigned char *image, *tempptr;
	int x, y, width, height, csize;
	float a;
	if(argc < 4) {
		fprintf(stderr, "Usage: %s width height radius\n", argv[0]);
		exit(1);
	}
	width = atoi(argv[1]);
	height = atoi(argv[2]);
	csize = atoi(argv[3]);
	// allocate a pre-zeroed space, all black, for the RGB color image
	image = calloc(1, height*width*3*sizeof(unsigned char));
	// clock scan to generate circle centers around a larger circle pattern
	for(a = 2*M_PI; a >= 0.0; a -= M_PI/9) {
		x = width/2 + sin(a) * width/3;
		y = height/2 + cos(a) * height/3;
		circle(image, x, y, csize, width, height);
	}
	printf("P6\n%d %d\n255\n", width, height);	// PPM format header
	fflush(stdout);	// force the header to be written to file descriptor 1
	// this write appends the actual image data to stdout which is FD 1
	write(1, image, width*height*3*sizeof(unsigned char));
	return(0);
}

// arguments are the address of the center, the radius and overall image width
void circle(unsigned char *image, int cx, int cy, int csize, int wid, int ht) {
	int x, y;
	unsigned char *p;
	for(y = -csize; y <= csize; y++) {
		for(x = -csize; x <= csize; x++) {
			if(x*x + y*y > csize*csize)
				continue; // reject points outside of circle
			if(x+cx < 0 || x+cx >= wid || y+cy < 0 || y+cy >= ht)
				continue; // reject if outside the image bounds
			// compute the 1D address of this pixel
			p = image + 3*((y+cy)*wid + (x+cx));
			// put yellow into the 3 consecutive RGB bytes
			*p++ = 255;
			*p++ = 255;
			*p++ = 0;
		}
	}
}