First Light

#include "prism/renderer.hpp"
#include <algorithm>
#include <cmath>
#include <fstream>
#include <limits>
#include <locale>
#include <new>
#include <numbers>
#include <optional>

namespace {
using prism::Vec3;
Vec3 add(Vec3 a, Vec3 b) { return {a.x + b.x, a.y + b.y, a.z + b.z}; }
Vec3 subtract(Vec3 a, Vec3 b) { return {a.x - b.x, a.y - b.y, a.z - b.z}; }
Vec3 scale(Vec3 a, double s) { return {a.x * s, a.y * s, a.z * s}; }
double dot(Vec3 a, Vec3 b) { return a.x * b.x + a.y * b.y + a.z * b.z; }
Vec3 cross(Vec3 a, Vec3 b) { return {a.y*b.z - a.z*b.y, a.z*b.x - a.x*b.z, a.x*b.y - a.y*b.x}; }
// All callers establish a nonzero finite length: validate_scene checks camera
// and light, and sphere normals have a strictly positive radius.
Vec3 unit(Vec3 a) { return scale(a, 1.0 / std::sqrt(dot(a, a))); }
struct Ray { Vec3 origin, direction; };
struct Hit { double distance; Vec3 point, normal; prism::Color color; };
constexpr double epsilon = 1e-5;

std::optional<double> sphere_distance(const Ray& ray, const prism::Sphere& sphere, double nearest) {
    const auto offset = subtract(ray.origin, sphere.center);
    // Unit ray direction makes the quadratic coefficient a = 1.
    const double half_b = dot(offset, ray.direction);
    const double c = dot(offset, offset) - sphere.radius * sphere.radius;
    const double discriminant = half_b * half_b - c;
    if (discriminant < 0) return std::nullopt;
    const double root = std::sqrt(discriminant);
    double distance = -half_b - root;
    if (distance <= epsilon) distance = -half_b + root; // Ray may start inside.
    if (distance <= epsilon || distance >= nearest) return std::nullopt;
    return distance;
}

std::optional<Hit> intersect(const Ray& ray, const prism::Scene& scene) {
    std::optional<Hit> hit;
    double nearest = std::numeric_limits<double>::infinity();
    for (const auto& sphere : scene.spheres) {
        if (const auto distance = sphere_distance(ray, sphere, nearest)) {
            nearest = *distance;
            const auto point = add(ray.origin, scale(ray.direction, nearest));
            hit = Hit{nearest, point, unit(subtract(point, sphere.center)), sphere.color};
        }
    }
    if (std::abs(ray.direction.y) > epsilon) {
        const double distance = (scene.ground_y - ray.origin.y) / ray.direction.y;
        if (distance > epsilon && distance < nearest) {
            const auto point = add(ray.origin, scale(ray.direction, distance));
            // Bounded hit distance is unnecessary for shading. Avoid conversion
            // of a large floor(double) to an integer; fmod stays in floating point.
            const bool alternate = std::fmod(std::floor(point.x) + std::floor(point.z), 2.0) != 0.0;
            auto color = scene.ground_color;
            const double factor = alternate ? 0.87 : 1.0;
            color = {color.r * factor, color.g * factor, color.b * factor};
            hit = Hit{distance, point, {0, ray.direction.y < 0 ? 1.0 : -1.0, 0}, color};
        }
    }
    return hit;
}

prism::Color shade(const Ray& ray, const prism::Scene& scene, Vec3 light) {
    const auto hit = intersect(ray, scene);
    if (!hit) return scene.background;
    const Ray shadow{add(hit->point, scale(hit->normal, epsilon * 4)), light};
    bool occluded = false;
    // Plane shading is one-sided; sphere geometry casts the useful shadows in
    // this small direct-light renderer. No recursive bounces or hidden workers.
    for (const auto& sphere : scene.spheres) {
        if (sphere_distance(shadow, sphere, std::numeric_limits<double>::infinity())) {
            occluded = true;
            break;
        }
    }
    const double diffuse = occluded ? 0 : std::max(0.0, dot(hit->normal, light));
    const double brightness = 0.18 + 0.82 * diffuse;
    return {hit->color.r * brightness, hit->color.g * brightness, hit->color.b * brightness};
}

std::uint8_t encode(double linear) {
    // A simple gamma-2 display transform, explicitly not a full color pipeline.
    const auto display = std::sqrt(std::clamp(linear, 0.0, 1.0));
    return static_cast<std::uint8_t>(std::lround(display * 255.0));
}
} // namespace

namespace prism {
std::expected<Image, Error> render(const Scene& scene) {
    if (auto checked = validate_scene(scene); !checked) return std::unexpected(checked.error());
    const auto width = static_cast<std::size_t>(scene.width);
    const auto height = static_cast<std::size_t>(scene.height);
    const auto count = width * height; // validate_scene established the product bound.
    if (count > std::numeric_limits<std::size_t>::max() / 3)
        return std::unexpected(Error{ErrorCode::invalid_scene, "RGB byte count would overflow."});
    try {
        Image image{scene.width, scene.height, std::vector<std::uint8_t>(count * 3)};
        const auto forward = unit(subtract(scene.camera.look_at, scene.camera.position));
        const auto right = unit(cross(forward, {0, 1, 0}));
        const auto up = cross(right, forward);
        const double half_height = std::tan(scene.camera.vertical_fov_degrees * std::numbers::pi / 360.0);
        const double half_width = half_height * static_cast<double>(width) / static_cast<double>(height);
        const auto light = unit(scene.light_direction);
        for (std::size_t y = 0; y < height; ++y) {
            for (std::size_t x = 0; x < width; ++x) {
                const double u = (2.0 * (static_cast<double>(x) + 0.5) / static_cast<double>(width) - 1.0) * half_width;
                const double v = (1.0 - 2.0 * (static_cast<double>(y) + 0.5) / static_cast<double>(height)) * half_height;
                const Ray ray{scene.camera.position, unit(add(forward, add(scale(right, u), scale(up, v))))};
                const auto color = shade(ray, scene, light);
                const auto offset = (y * width + x) * 3;
                image.rgb[offset] = encode(color.r);
                image.rgb[offset + 1] = encode(color.g);
                image.rgb[offset + 2] = encode(color.b);
            }
        }
        return image;
    } catch (const std::bad_alloc&) {
        return std::unexpected(Error{ErrorCode::allocation, "Image allocation failed."});
    }
}

std::expected<void, Error> write_ppm(const std::filesystem::path& path, const Image& image) {
    // Public boundaries validate their own contract, including caller-built Image.
    if (image.width == 0 || image.height == 0 || image.width > max_dimension || image.height > max_dimension)
        return std::unexpected(Error{ErrorCode::invalid_scene, "Invalid image dimensions."});
    const auto width = static_cast<std::size_t>(image.width);
    const auto height = static_cast<std::size_t>(image.height);
    if (height > max_pixels / width || image.rgb.size() != width * height * 3)
        return std::unexpected(Error{ErrorCode::invalid_scene, "Image RGB storage does not match its dimensions."});
    std::ofstream output{path, std::ios::binary | std::ios::trunc};
    if (!output) return std::unexpected(Error{ErrorCode::io, "Could not open the output file."});
    // PPM's numeric header is ASCII, regardless of the caller's global locale.
    output.imbue(std::locale::classic());
    output << "P6\n" << image.width << ' ' << image.height << "\n255\n";
    output.write(reinterpret_cast<const char*>(image.rgb.data()), static_cast<std::streamsize>(image.rgb.size()));
    output.close(); // Check close errors as well as buffered writes.
    if (!output) return std::unexpected(Error{ErrorCode::io, "PPM write failed; output may be incomplete."});
    return {};
}
} // namespace prism