#include "ff.h"
#include "vec3.h"

#include <limits.h>
#include <math.h>
#include <stdio.h>
#include <unistd.h>

#define W 800
#define H 600

#define VP_H 2.0
#define VP_W (VP_H * W / H)

#define FOCLEN 1.0

typedef struct {
	vec3_t orig, dir;
} ray_t;

typedef struct {
	vec3_t centre;
	double rad;
} sphere_t;

static const vec3_t lightblue = { 0.4, 0.6, 1.0 };
static const vec3_t white = { 1.0, 1.0, 1.0 };
static const vec3_t red = { 1.0, 0.1, 0.2 };

static const vec3_t camera = { 0, 0, 0 };
static const vec3_t vp_disp = { 0, 0, FOCLEN };
static const vec3_t vp_u = { VP_W, 0, 0 };
static const vec3_t vp_v = { 0, -VP_H, 0 };
static const vec3_t pix_x_step = { VP_W / W, 0, 0 };
static const vec3_t pix_y_step = { 0, -VP_H / H, 0 };

static const sphere_t obj = {
	.centre = { 1.0, 0.0, -3.0 },
	.rad = 1.0,
};

static pix_t pix[W * H];

static double intersect(sphere_t sphere, ray_t ray)
{
	const vec3_t disp = vec3_sub(sphere.centre, ray.orig);

	const double a = vec3_dot(ray.dir, ray.dir);
	const double b = -2.0 * vec3_dot(ray.dir, disp);
	const double c = vec3_dot(disp, disp) - sphere.rad * sphere.rad;

	const double discriminant = b * b - 4 * a * c;
	if (discriminant < 0)
		return -1.0;
	else
		return (-b - sqrt(discriminant)) / (2.0 * a);
}

static vec3_t raycol(ray_t ray)
{
	const double t = intersect(obj, ray);
	if (t > 0.0) {
		const vec3_t p = vec3_add(ray.orig, vec3_scale(ray.dir, t));
		const vec3_t normal = vec3_unit(vec3_sub(p, obj.centre));
		return vec3_scale(vec3_add(normal, white), 0.5);
	}

	const double a = (ray.dir.y + 1.0) / 2.0;
	return vec3_add(vec3_scale(lightblue, a), vec3_scale(white, 1 - a));
}

static void setpix(vec3_t col, pix_t *out)
{
	out->r = UINT16_MAX * col.x;
	out->g = UINT16_MAX * col.y;
	out->b = UINT16_MAX * col.z;
	out->a = UINT16_MAX;
}

int main()
{
	img_t img = { .w = W, .h = H, .pix = pix };

	const vec3_t vp_topleft = vec3_sub(
	    vec3_sub(vec3_sub(camera, vp_disp), vec3_scale(vp_u, 0.5)),
	    vec3_scale(vp_v, 0.5));
	const vec3_t pix_orig = vec3_add(
	    vec3_add(vp_topleft, vec3_scale(pix_x_step, 0.5)),
	    vec3_scale(pix_y_step, 0.5));

	for (unsigned y = 0; y < H; ++y) {
		fprintf(stderr, "\r[%3d/%3d]", y, H);
		fflush(stderr);

		const vec3_t row = vec3_add(pix_orig, vec3_scale(pix_y_step, y));
		for (unsigned x = 0; x < W; ++x) {
			const ray_t ray = {
				.orig = camera,
				.dir = vec3_unit(vec3_add(row, vec3_scale(pix_x_step, x))),
			};
			const vec3_t col = raycol(ray);
			setpix(col, pix + (W * y + x));
		}
	}

	fprintf(stderr, "\r[%3d/%3d]\n", H, H);

	ff_write(STDOUT_FILENO, img);

	return 0;
}