#include <array>
#include <chrono>
#include <cstddef>
#include <cstdint>
#include <iostream>

constexpr std::array<std::uint32_t, 256> make_crc32_table() {
    std::array<std::uint32_t, 256> table{};
    constexpr std::uint32_t kPolynomial = 0xEDB88320u;

    for (std::size_t i = 0; i < 256; ++i) {
        std::uint32_t crc = static_cast<std::uint32_t>(i);
        for (int j = 0; j < 8; ++j) {
            if (crc & 1) {
                crc = (crc >> 1) ^ kPolynomial;
            } else {
                crc >>= 1;
            }
        }
        table[i] = crc;
    }
    return table;
}

// 编译期生成完整的 CRC-32 查找表
constexpr auto kCrc32Table = make_crc32_table();

// 运行时使用：只需要做查表操作
constexpr std::uint32_t crc32_compute(const std::uint8_t* data, std::size_t len) {
    std::uint32_t crc = 0xFFFFFFFFu;
    for (std::size_t i = 0; i < len; ++i) {
        crc = (crc >> 8) ^ kCrc32Table[(crc ^ data[i]) & 0xFF];
    }
    return crc ^ 0xFFFFFFFFu;
}

// 运行时版本的 CRC 表生成
std::array<std::uint32_t, 256> make_crc32_table_runtime() {
    std::array<std::uint32_t, 256> table{};
    constexpr std::uint32_t kPolynomial = 0xEDB88320u;
    for (std::size_t i = 0; i < 256; ++i) {
        std::uint32_t crc = static_cast<std::uint32_t>(i);
        for (int j = 0; j < 8; ++j) {
            if (crc & 1) {
                crc = (crc >> 1) ^ kPolynomial;
            } else {
                crc >>= 1;
            }
        }
        table[i] = crc;
    }
    return table;
}

int main() {
    // 运行时生成
    auto start = std::chrono::high_resolution_clock::now();
    auto runtime_table = make_crc32_table_runtime();
    auto end = std::chrono::high_resolution_clock::now();
    std::cout << "Runtime generation: "
              << std::chrono::duration<double, std::micro>(end - start).count() << " us\n";

    // constexpr 版本：直接使用 kCrc32Table，耗时为 0
    std::cout << "CRC table first entry: " << kCrc32Table[0] << "\n";
    std::cout << "Runtime table first entry: " << runtime_table[0] << "\n";

    return 0;
}