Question

Below is my current char* to hex string function. I wrote it as an exercise in bit manipulation. It takes ~7ms on a AMD Athlon MP 2800+ to hexify a 10 million byte array. Is there any trick or other way that I am missing?

How can I make this faster?

Compiled with -O3 in g++

static const char _hex2asciiU_value[256][2] =      { {'0','0'}, {'0','1'}, /* snip..., */ {'F','E'},{'F','F'} };  std::string char_to_hex( const unsigned char* _pArray, unsigned int _len ) {     std::string str;     str.resize(_len*2);     char* pszHex = &str[0];     const unsigned char* pEnd = _pArray + _len;      clock_t stick, etick;     stick = clock();     for( const unsigned char* pChar = _pArray; pChar != pEnd; pChar++, pszHex += 2 ) {         pszHex[0] = _hex2asciiU_value[*pChar][0];         pszHex[1] = _hex2asciiU_value[*pChar][1];     }     etick = clock();      std::cout << 'ticks to hexify ' << etick - stick << std::endl;      return str; } 

Updates

Added timing code

Brian R. Bondy: replace the std::string with a heap alloc’d buffer and change ofs*16 to ofs << 4 – however the heap allocated buffer seems to slow it down? – result ~11ms

Antti Sykäri:replace inner loop with

 int upper = *pChar >> 4;  int lower = *pChar & 0x0f;  pszHex[0] = pHex[upper];  pszHex[1] = pHex[lower]; 

result ~8ms

Robert: replace _hex2asciiU_value with a full 256-entry table, sacrificing memory space but result ~7ms!

HoyHoy: Noted it was producing incorrect results

Answer 1

At the cost of more memory you can create a full 256-entry table of the hex codes:

static const char _hex2asciiU_value[256][2] =     { {'0','0'}, {'0','1'}, /* ..., */ {'F','E'},{'F','F'} }; 

Then direct index into the table, no bit fiddling required.

const char *pHexVal = pHex[*pChar]; pszHex[0] = pHexVal[0]; pszHex[1] = pHexVal[1];