主題
補充之前寫的，
樣板元编程(Template Metaprogramming)範例2 - CRC32雜湊碼
之前只能算出unsigned int
#define CRC2INT(x) (MM<sizeof(x) - 1>::CRC32(x))
這次新增
#define CRC2CARR(x) HexdecimalCArray<CRC2INT(x)>
能把unsigned int轉成16進位數值，並儲存在11個bytes的char array內
譬如CRC2CARR("1")為"0x83DCEFB7"，字串長度10，
但陣列大小為11，因為結尾要多儲存'\0'代表字串結束
新增相關實際使用範例，以及StrLen和StrEqual，用來驗證char array方面的程式碼
//計算字串長度
constexpr size_t StrLen(const char *str)
{
return *str ? 1 + StrLen(str + 1) : 0;
}
//比較兩個字串是否相同
constexpr bool StrEqual(char const *a, char const *b)
{
return *a == *b && (*a == '\0' || StrEqual(a + 1, b + 1));
}
static constexpr char HexBaseCArray[16] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'};
template<char... Args>
struct HexMetaString
{
int ContentSize = (sizeof... (Args));
const char Data[sizeof... (Args)] = {Args...};
};
//利用遞迴，反覆對unsigned int除以16，取得商數和餘數，並且把餘數轉換成16進位碼(HexBaseCArray)
template<int Width, unsigned int N, char... Args>
struct HexdecimalCArrayBuilder
{
typedef typename HexdecimalCArrayBuilder<Width - 1, N / 16, HexBaseCArray[N % 16], Args...>::Type Type;
};
template<unsigned int N, char... Args>
struct HexdecimalCArrayBuilder<0, N, Args...>
{
typedef HexMetaString<'0', 'x', HexBaseCArray[N], Args...> Type;
};
template<unsigned int N>
class HexdecimalCArray
{
private:
typedef typename HexdecimalCArrayBuilder<8 - 1, N, '\0'>::Type t;
static constexpr t metaStr {};
public:
static constexpr int GetContentSize()
{
return metaStr.ContentSize;
}
static constexpr const char *GetData()
{
return metaStr.Data;
}
static constexpr size_t GetStrLen()
{
return StrLen(metaStr.Data);
}
};
template<unsigned int N>
constexpr typename HexdecimalCArray<N>::t HexdecimalCArray<N>::metaStr;
//CRC2CARR，計算輸入字串的CRC32，輸出成16進位數值，並儲存在11個bytes的char array內
#define CRC2CARR(x) HexdecimalCArray<CRC2INT(x)>
//計算和驗證CRC2CARR
//不用編譯和執行程式，滑鼠移到變數上，就能看到測試結果是否正常
//CRCCArrSize和CRCStrLen如果改成別的數字，相關的int值會得到-1，代表錯誤
//CRCCArrSizeXXX等於11代表正常
//CRCStrLenXXX等於10代表正常
//ChkCRCCArrXXX是用來比較和驗證，CRC2CARR算出來的16進位碼，
//和網站上算出來的是否相同，true代表相同，測試正常
static constexpr int CRCCArrSize = 11;
static constexpr int CRCStrLen = 10;
static constexpr const char *CRCCArrEmpty = CRC2CARR("")::GetData();
static constexpr int CRCCArrSizeEmpty = CRC2CARR("")::GetContentSize() == CRCCArrSize ? CRCCArrSize : -1;
static constexpr int CRCStrLenEmpty = (CRC2CARR("")::GetStrLen() == CRCStrLen && StrLen(CRCCArrEmpty) == CRCStrLen) ? CRCStrLen : -1;
static constexpr bool ChkCRCCArrEmpty = StrEqual(CRCCArrEmpty, "0x00000000") && !StrEqual(CRCCArrEmpty, "0x0000000") && !StrEqual(CRCCArrEmpty, "0x00000000_");
static constexpr const char *CRCCArrSpace = CRC2CARR(" ")::GetData();
static constexpr int CRCCArrSizeSpace = CRC2CARR(" ")::GetContentSize() == CRCCArrSize ? CRCCArrSize : -1;
static constexpr int CRCStrLenSpace = (CRC2CARR(" ")::GetStrLen() == CRCStrLen && StrLen(CRCCArrSpace) == CRCStrLen) ? CRCStrLen : -1;
static constexpr bool ChkCRCCArrSpace = StrEqual(CRCCArrSpace, "0xE96CCF45") && !StrEqual(CRCCArrSpace, "0xE96CCF4") && !StrEqual(CRCCArrSpace, "0xE96CCF45_");
static constexpr const char *CRCCArr1 = CRC2CARR("1")::GetData();
static constexpr int CRCCArrSize1 = CRC2CARR("1")::GetContentSize() == CRCCArrSize ? CRCCArrSize : -1;
static constexpr int CRCStrLen1 = (CRC2CARR("1")::GetStrLen() == CRCStrLen && StrLen(CRCCArr1) == CRCStrLen) ? CRCStrLen : -1;
static constexpr bool ChkCRCCArr1 = StrEqual(CRCCArr1, "0x83DCEFB7") && !StrEqual(CRCCArr1, "0x83DCEFB") && !StrEqual(CRCCArr1, "0x83DCEFB7_");
static constexpr const char *CRCCArr123456789 = CRC2CARR("123456789")::GetData();
static constexpr int CRCCArrSize123456789 = CRC2CARR("123456789")::GetContentSize() == CRCCArrSize ? CRCCArrSize : -1;
static constexpr int CRCStrLen123456789 = (CRC2CARR("123456789")::GetStrLen() == CRCStrLen && StrLen(CRCCArr123456789) == CRCStrLen) ? CRCStrLen : -1;
static constexpr bool ChkCRCCArr123456789 = StrEqual(CRCCArr123456789, "0xCBF43926") && !StrEqual(CRCCArr123456789, "0xCBF4392") && !StrEqual(CRCCArr123456789, "0xCBF43926_");
static constexpr const char *CRCCArrCrcVal01 = CRC2CARR("CrcVal01")::GetData();
static constexpr int CRCCArrSizeCrcVal01 = CRC2CARR("CrcVal01")::GetContentSize() == CRCCArrSize ? CRCCArrSize : -1;
static constexpr int CRCStrLenCrcVal01 = (CRC2CARR("CrcVal01")::GetStrLen() == CRCStrLen && StrLen(CRCCArrCrcVal01) == CRCStrLen) ? CRCStrLen : -1;
static constexpr bool ChkCRCCArrCrcVal01 = StrEqual(CRCCArrCrcVal01, "0xCC3B0811") && !StrEqual(CRCCArrCrcVal01, "0xCC3B081") && !StrEqual(CRCCArrCrcVal01, "0xCC3B0811_");
static constexpr const char *CRCCArrFilePath = CRC2CARR(__FILE__)::GetData();
static constexpr int CRCCArrSizeFilePath = CRC2CARR(__FILE__)::GetContentSize() == CRCCArrSize ? CRCCArrSize : -1;
static constexpr int CRCStrLenFilePath = (CRC2CARR(__FILE__)::GetStrLen() == CRCStrLen && StrLen(CRCCArrFilePath) == CRCStrLen) ? CRCStrLen : -1;
static constexpr bool ChkCRCCArrFilePath = StrEqual(CRCCArrFilePath, "0xB413D6BF") && !StrEqual(CRCCArrFilePath, "0xB413D6B") && !StrEqual(CRCCArrFilePath, "0xB413D6BF_");
對檔案路徑取CRC32 (CRC2CARR(__FILE__))，應用面可以用在之前寫的文章，譬如
C++11取得和轉換thread::id
C++物件的singleton constructor補充
程式內固定路徑的檔案或單一的物件(singleton class)，能在編譯前就先給定獨一無二的CRC32，當作唯一碼
//擴充之前寫的ThreadExtensions
static constexpr const char *CRCCArrFilePathThreadExt = CRC2CARR(__FILE__)::GetData();
static constexpr int CRCCArrSizeFilePathThreadExt = CRC2CARR(__FILE__)::GetContentSize() == CRCCArrSize ? CRCCArrSize : -1;
static constexpr int CRCStrLenFilePathThreadExt = (CRC2CARR(__FILE__)::GetStrLen() == CRCStrLen && StrLen(CRCCArrFilePathThreadExt) == CRCStrLen) ? CRCStrLen : -1;
static constexpr bool ChkCRCCArrFilePathThreadExt = StrEqual(CRCCArrFilePathThreadExt, "0xD5166410") && !StrEqual(CRCCArrFilePathThreadExt, "0xD516641") && !StrEqual(CRCCArrFilePathThreadExt, "0xD5166410_");
參考
CRC32是很簡易的雜湊運算，每77164筆CRC32計算，就有50%機率重複(Birthday Problem)，
因此在檔案很多的大型專案上，應該不太適合大量使用CRC2INT和CRC2CARR，避免衝突
