/* Copyright (C) 2024 Aiden Gall

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>. */

struct ray {
	float3 orig;
	float3 dir;
};

static float3
at(const struct ray ray, const float t)
{
	return ray.orig + ray.dir * t;
}

static float
length_squared(const float3 v)
{
	return v.x * v.x + v.y * v.y + v.z * v.z;
}

__kernel void
gen_rays(__global float3 *const rays, const float3 camera_centre,
         const float3 pixel_delta_u, const float3 pixel_delta_v,
         const float3 corner00)
{
	size_t idx, w, i, j;

	w = get_global_size(0);

	i = get_global_id(0);
	j = get_global_id(1);

	idx = (w * j + i) * 2;

	rays[idx] = camera_centre;
	rays[idx + 1] = corner00 + ((i * pixel_delta_u) + (j * pixel_delta_v)) -
	                camera_centre;
}

static float
hit_sphere(const float3 center, const float radius, const struct ray ray)
{
	float3 oc;
	float a, half_b, c, disc;

	oc = ray.orig - center;
	a = length_squared(ray.dir);
	half_b = dot(oc, ray.dir);
	c = length_squared(oc) - radius * radius;
	disc = half_b * half_b - a * c;

	if (disc < 0.0f)
		return -1.0f;

	return (-half_b - sqrt(disc)) / a;
}

void
write_pixel(__global uchar *const pixel, float3 colour)
{
	/* gamma correction */
	colour = 256.0f * clamp(pow(colour, 1.0f/2.2f), 0.0f, 0.999f);

	pixel[0] = colour.x;
	pixel[1] = colour.y;
	pixel[2] = colour.z;
	pixel[3] = 255;
}

__kernel void
ray_colour(__global uchar *const canvas, __global const float3 *const rays)
{
	struct ray ray;

	size_t ray_idx, w, i, j;
	float3 colour;
	float t;

	w = get_global_size(0);

	i = get_global_id(0);
	j = get_global_id(1);

	ray_idx = (w * j + i) * 2;

	ray.orig = rays[ray_idx];
	ray.dir = rays[ray_idx + 1];

	t = hit_sphere((float3)(0.0f, 0.0f, -1.0f), 0.5f, ray);

	if (t > 0.0) {
		colour = 0.5f *
		         (normalize(at(ray, t) - (float3)(0.0f, 0.0f, -1.0f)) +
		          1.0f);
	} else {
		float3 unit_direction;
		float a;

		unit_direction = normalize(ray.dir);
		a = 0.5f * (unit_direction.y + 1.0f);

		colour = (1.0f - a) * 1.0f + a * (float3)(0.5f, 0.7f, 1.0f);
	}

	write_pixel(canvas + 4 * (w * j + i), colour);
}