path: root/src/main.zig

const std = @import("std");
const debug = std.debug;
const math = std.math;
const ArrayList = std.ArrayList;

const Vec3 = struct {
    x: f64,
    y: f64,
    z: f64,

    pub fn norm(v: Vec3) f64 {
        return @sqrt(v.normSquared());
    }

    pub fn normSquared(v: Vec3) f64 {
        return v.x * v.x + v.y * v.y + v.z * v.z;
    }

    pub fn dot(u: Vec3, v: Vec3) f64 {
        return u.x * v.x + u.y * v.y + u.z * v.z;
    }

    pub fn cross(u: Vec3, v: Vec3) Vec3 {
        return Vec3{
            .x = u.y * v.z - u.z * v.y,
            .y = u.z * v.x - u.x * v.z,
            .z = u.x * v.y - u.y * v.x,
        };
    }

    pub fn normalized(v: Vec3) Vec3 {
        const n = v.norm();
        if (n == 0.0) {
            return v;
        } else {
            return v.div(n);
        }
    }

    pub fn add(u: Vec3, v: Vec3) Vec3 {
        return Vec3{
            .x = u.x + v.x,
            .y = u.y + v.y,
            .z = u.z + v.z,
        };
    }

    pub fn sub(u: Vec3, v: Vec3) Vec3 {
        return Vec3{
            .x = u.x - v.x,
            .y = u.y - v.y,
            .z = u.z - v.z,
        };
    }

    pub fn mul(v: Vec3, t: f64) Vec3 {
        return Vec3{
            .x = v.x * t,
            .y = v.y * t,
            .z = v.z * t,
        };
    }

    pub fn div(v: Vec3, t: f64) Vec3 {
        return Vec3{
            .x = v.x / t,
            .y = v.y / t,
            .z = v.z / t,
        };
    }

    pub fn random01(rand: std.rand.Random) Vec3 {
        return Vec3{
            .x = randomReal01(rand),
            .y = randomReal01(rand),
            .z = randomReal01(rand),
        };
    }

    pub fn random(rand: std.rand.Random, min: f64, max: f64) Vec3 {
        return Vec3{
            .x = randomReal(rand, min, max),
            .y = randomReal(rand, min, max),
            .z = randomReal(rand, min, max),
        };
    }

    // for debugging
    pub fn pp(v: Vec3) void {
        debug.print("{} {} {}\n", .{ v.x, v.y, v.z });
    }
};

fn randomPointInUnitSphere(rand: std.rand.Random) Vec3 {
    while (true) {
        const p = Vec3.random(rand, -1.0, 1.0);
        if (p.norm() >= 1) continue;
        return p;
    }
}

fn randomUnitVector(rand: std.rand.Random) Vec3 {
    return randomPointInUnitSphere(rand).normalized();
}

const Point3 = Vec3;
const Color = Vec3;

const Ray = struct {
    origin: Vec3,
    dir: Vec3,

    pub fn at(r: Ray, t: f64) Point3 {
        return r.origin.add(r.dir.mul(t));
    }
};

const HitRecord = struct {
    // The point where the ray and the hittable hits.
    p: Point3,
    // The normal of the hittable at p.
    normal: Vec3,
    // p = ray.at(t)
    t: f64,
    // True if the ray hits the hittable from the front face, i.e., outside of it.
    front_face: bool,
};

const HittableTag = enum {
    sphere,
    list,
};

const Hittable = union(HittableTag) {
    sphere: Sphere,
    list: HittableList,

    fn hit(h: Hittable, r: Ray, t_min: f64, t_max: f64, record: *HitRecord) bool {
        return switch (h) {
            HittableTag.sphere => |sphere| sphere.hit(r, t_min, t_max, record),
            HittableTag.list => |list| list.hit(r, t_min, t_max, record),
        };
    }
};

const Sphere = struct {
    center: Point3,
    radius: f64,

    fn hit(sphere: Sphere, r: Ray, t_min: f64, t_max: f64, record: *HitRecord) bool {
        const oc = r.origin.sub(sphere.center);
        const a = r.dir.normSquared();
        const half_b = Vec3.dot(oc, r.dir);
        const c = oc.normSquared() - sphere.radius * sphere.radius;

        const discriminant = half_b * half_b - a * c;
        if (discriminant < 0.0) {
            // r does not intersect the sphere.
            return false;
        }
        const sqrtd = @sqrt(discriminant);

        // Find the nearest root that lies in the acceptable range.
        var root = (-half_b - sqrtd) / a;
        if (root < t_min or t_max < root) {
            root = (-half_b + sqrtd) / a;
            if (root < t_min or t_max < root) {
                // out of range
                return false;
            }
        }

        record.t = root;
        record.p = r.at(root);
        const outward_normal = (record.p.sub(sphere.center)).div(sphere.radius);
        record.front_face = Vec3.dot(outward_normal, r.dir) < 0.0;
        if (record.front_face) {
            record.normal = outward_normal;
        } else {
            record.normal = outward_normal.mul(-1.0);
        }

        return true;
    }
};

const HittableList = struct {
    objects: ArrayList(*const Hittable),

    fn hit(list: HittableList, r: Ray, t_min: f64, t_max: f64, record: *HitRecord) bool {
        var hit_anything = false;
        var closest_so_far = t_max;

        for (list.objects.items) |object| {
            var rec: HitRecord = undefined;
            if (object.hit(r, t_min, closest_so_far, &rec)) {
                hit_anything = true;
                closest_so_far = rec.t;
                record.* = rec;
            }
        }
        return hit_anything;
    }
};

const inf = math.inf(f64);
const pi = math.pi(f64);

fn deg2rad(degree: f64) f64 {
    return degree * pi / 180.0;
}

// [0, 1)
fn randomReal01(rand: std.rand.Random) f64 {
    return rand.float(f64);
}

// [min, max)
fn randomReal(rand: std.rand.Random, min: f64, max: f64) f64 {
    return min + randomReal01(rand) * (max - min);
}

const Camera = struct {
    origin: Point3,
    horizontal: Vec3,
    vertical: Vec3,
    lower_left_corner: Point3,

    fn init(viewport_width: f64, viewport_height: f64, focal_length: f64) Camera {
        const origin = Point3{ .x = 0.0, .y = 0.0, .z = 0.0 };
        const horizontal = Vec3{ .x = viewport_width, .y = 0.0, .z = 0.0 };
        const vertical = Vec3{ .x = 0.0, .y = viewport_height, .z = 0.0 };
        const lower_left_corner = origin.sub(horizontal.div(2.0)).sub(vertical.div(2.0)).sub(Vec3{ .x = 0.0, .y = 0.0, .z = focal_length });

        return Camera{
            .origin = origin,
            .horizontal = horizontal,
            .vertical = vertical,
            .lower_left_corner = lower_left_corner,
        };
    }

    fn getRay(camera: Camera, u: f64, v: f64) Ray {
        const dir = camera.lower_left_corner.add(camera.horizontal.mul(u)).add(camera.vertical.mul(v)).sub(camera.origin);
        return Ray{
            .origin = camera.origin,
            .dir = dir,
        };
    }
};

fn rayColor(r: Ray, world: Hittable, rand: std.rand.Random, depth: u32) Color {
    var rec: HitRecord = undefined;
    if (depth == 0) {
        // If we've exceeded the ray bounce limit, no more ligth is gathered.
        return Color{ .x = 0.0, .y = 0.0, .z = 0.0 };
    }
    if (world.hit(r, 0.001, inf, &rec)) {
        const target = rec.p.add(rec.normal).add(randomUnitVector(rand));
        return rayColor(Ray{ .origin = rec.p, .dir = target.sub(rec.p) }, world, rand, depth - 1).mul(0.5);
    }
    const unit_dir = r.dir.normalized();
    const s = 0.5 * (unit_dir.y + 1.0);
    return (Color{ .x = 1.0, .y = 1.0, .z = 1.0 }).mul(1.0 - s).add((Color{ .x = 0.5, .y = 0.7, .z = 1.0 }).mul(s));
}

fn writeColor(out: anytype, c: Color, samples_per_pixel: u32) !void {
    const scale = 1.0 / @intToFloat(f64, samples_per_pixel);
    try out.print("{} {} {}\n", .{
        @floatToInt(u8, 256.0 * math.clamp(@sqrt(c.x * scale), 0.0, 0.999)),
        @floatToInt(u8, 256.0 * math.clamp(@sqrt(c.y * scale), 0.0, 0.999)),
        @floatToInt(u8, 256.0 * math.clamp(@sqrt(c.z * scale), 0.0, 0.999)),
    });
}

pub fn main() !void {
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    const allocator = gpa.allocator();
    defer debug.assert(!gpa.deinit());

    var rng = std.rand.DefaultPrng.init(42);
    var rand = rng.random();

    // Image
    const aspect_ratio = 16.0 / 9.0;
    const image_width = 400;
    const image_height = @floatToInt(comptime_int, @divTrunc(image_width, aspect_ratio));
    const samples_per_pixel = 100;
    const max_depth = 50;

    // World
    const sphere1 = Hittable{ .sphere = Sphere{ .center = Point3{ .x = 0.0, .y = 0.0, .z = -1.0 }, .radius = 0.5 } };
    const sphere2 = Hittable{ .sphere = Sphere{ .center = Point3{ .x = 0.0, .y = -100.5, .z = -1.0 }, .radius = 100.0 } };
    var hittable_objects = ArrayList(*const Hittable).init(allocator);
    try hittable_objects.append(&sphere1);
    try hittable_objects.append(&sphere2);
    const world = Hittable{ .list = HittableList{ .objects = hittable_objects } };
    defer hittable_objects.deinit();

    // Camera
    const viewport_height = 2.0;
    const viewport_width = aspect_ratio * viewport_height;
    const focal_length = 1.0;
    const camera = Camera.init(viewport_width, viewport_height, focal_length);

    // Render
    const stdout_file = std.io.getStdOut().writer();
    var bw = std.io.bufferedWriter(stdout_file);
    const stdout = bw.writer();

    try stdout.print("P3\n{} {}\n255\n", .{ image_width, image_height });

    var j: i32 = image_height - 1;
    while (j >= 0) : (j -= 1) {
        std.debug.print("\rScanlines remaining: {}     ", .{j});
        var i: i32 = 0;
        while (i < image_width) : (i += 1) {
            var s: u32 = 0;
            var pixelColor = Color{ .x = 0.0, .y = 0.0, .z = 0.0 };
            while (s < samples_per_pixel) : (s += 1) {
                const u = (@intToFloat(f64, i) + randomReal01(rand)) / (image_width - 1);
                const v = (@intToFloat(f64, j) + randomReal01(rand)) / (image_height - 1);
                const r = camera.getRay(u, v);
                pixelColor = pixelColor.add(rayColor(r, world, rand, max_depth));
            }
            try writeColor(stdout, pixelColor, samples_per_pixel);
        }
    }

    try bw.flush();
}