// 29_alignment.cpp
// 对齐与填充验证：sizeof / alignof / offsetof 对比，成员重排与 #pragma pack 的优化效果

#include <cstddef>
#include <cstdint>
#include <iostream>

// --- 结构体定义 ---

struct BadLayout {
    char a;
    int b;
    char c;
};

struct GoodLayout {
    int b;
    char a;
    char c;
};

struct alignas(16) AlignedBuffer {
    int data[3]; // 12 字节，补齐到 16
};

#pragma pack(push, 1)
struct PackedHeader {
    uint8_t version;
    uint16_t length;
    uint32_t crc;
};
#pragma pack(pop)

struct MixedTypes {
    char flag;
    double value;
    int count;
    short id;
};

struct ReorderedMixed {
    double value;
    int count;
    short id;
    char flag;
};

// --- 工具函数 ---

/// 打印结构体信息和成员偏移量
template <typename T> void print_struct_info(const char* name) {
    std::cout << name << ":\n";
    std::cout << "  sizeof = " << sizeof(T) << ", alignof = " << alignof(T) << "\n";
}

int main() {
    std::cout << "=== sizeof 和 alignof 对比 ===\n\n";

    print_struct_info<BadLayout>("BadLayout");
    std::cout << "  偏移量: a=" << offsetof(BadLayout, a) << ", b=" << offsetof(BadLayout, b)
              << ", c=" << offsetof(BadLayout, c) << "\n\n";

    print_struct_info<GoodLayout>("GoodLayout");
    std::cout << "  偏移量: b=" << offsetof(GoodLayout, b) << ", a=" << offsetof(GoodLayout, a)
              << ", c=" << offsetof(GoodLayout, c) << "\n\n";

    print_struct_info<AlignedBuffer>("AlignedBuffer");
    std::cout << "  偏移量: data=" << offsetof(AlignedBuffer, data) << "\n\n";

    print_struct_info<PackedHeader>("PackedHeader");
    std::cout << "  偏移量: version=" << offsetof(PackedHeader, version)
              << ", length=" << offsetof(PackedHeader, length)
              << ", crc=" << offsetof(PackedHeader, crc) << "\n\n";

    print_struct_info<MixedTypes>("MixedTypes");
    std::cout << "  偏移量: flag=" << offsetof(MixedTypes, flag)
              << ", value=" << offsetof(MixedTypes, value)
              << ", count=" << offsetof(MixedTypes, count) << ", id=" << offsetof(MixedTypes, id)
              << "\n\n";

    print_struct_info<ReorderedMixed>("ReorderedMixed");
    std::cout << "  偏移量: value=" << offsetof(ReorderedMixed, value)
              << ", count=" << offsetof(ReorderedMixed, count)
              << ", id=" << offsetof(ReorderedMixed, id)
              << ", flag=" << offsetof(ReorderedMixed, flag) << "\n\n";

    std::cout << "=== 优化效果 ===\n";
    std::cout << "BadLayout  -> GoodLayout: " << sizeof(BadLayout) << " -> " << sizeof(GoodLayout)
              << " (节省 " << sizeof(BadLayout) - sizeof(GoodLayout) << " 字节)\n";
    std::cout << "MixedTypes -> ReorderedMixed: " << sizeof(MixedTypes) << " -> "
              << sizeof(ReorderedMixed) << " (节省 " << sizeof(MixedTypes) - sizeof(ReorderedMixed)
              << " 字节)\n";

    return 0;
}