#include "Pixel_Frame.h"

#include <algorithm>
#include <cstring>
#include <limits>

namespace renderive::web {
namespace {

void write_u32_le(char* target, std::uint32_t value) {
    target[0] = static_cast<char>(value & 0xffU);
    target[1] = static_cast<char>((value >> 8U) & 0xffU);
    target[2] = static_cast<char>((value >> 16U) & 0xffU);
    target[3] = static_cast<char>((value >> 24U) & 0xffU);
}

std::uint8_t flatten(std::uint8_t premultiplied, std::uint8_t alpha,
                     std::uint8_t background) {
    return static_cast<std::uint8_t>(
        std::min(255U, static_cast<unsigned>(premultiplied) +
                           (static_cast<unsigned>(background) * (255U - alpha) + 127U) / 255U));
}

} // namespace

std::string encode_pixel_frame(Image_View image, Color background) {
    if (image.empty() || image.format != Pixel_Format::Premultiplied_32 ||
        image.stride < image.width * static_cast<int>(sizeof(Pixel))) {
        return {};
    }
    const auto width = static_cast<std::size_t>(image.width);
    const auto height = static_cast<std::size_t>(image.height);
    if (width > (std::numeric_limits<std::size_t>::max() - pixel_frame_header_size) /
                    (height * sizeof(Pixel))) {
        return {};
    }

    const std::size_t pixel_bytes = width * height * sizeof(Pixel);
    std::string frame(pixel_frame_header_size + pixel_bytes, '\0');
    frame[0] = 'R';
    frame[1] = 'V';
    frame[2] = 'P';
    frame[3] = '1';
    write_u32_le(frame.data() + 4, static_cast<std::uint32_t>(image.width));
    write_u32_le(frame.data() + 8, static_cast<std::uint32_t>(image.height));
    write_u32_le(frame.data() + 12,
                 static_cast<std::uint32_t>(image.width * static_cast<int>(sizeof(Pixel))));

    auto* output = reinterpret_cast<std::uint8_t*>(frame.data() + pixel_frame_header_size);
    for (int y = 0; y < image.height; ++y) {
        const auto* row = image.data + static_cast<std::ptrdiff_t>(y) * image.stride;
        for (int x = 0; x < image.width; ++x) {
            Pixel pixel{};
            std::memcpy(&pixel, row + static_cast<std::ptrdiff_t>(x) * sizeof(Pixel), sizeof(pixel));
            const auto alpha = static_cast<std::uint8_t>((pixel >> 24U) & 0xffU);
            const auto red = static_cast<std::uint8_t>((pixel >> 16U) & 0xffU);
            const auto green = static_cast<std::uint8_t>((pixel >> 8U) & 0xffU);
            const auto blue = static_cast<std::uint8_t>(pixel & 0xffU);
            *output++ = flatten(red, alpha, background.r);
            *output++ = flatten(green, alpha, background.g);
            *output++ = flatten(blue, alpha, background.b);
            *output++ = 255;
        }
    }
    return frame;
}

} // namespace renderive::web