char[] to hex string exercise

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 -…

Accepted Answer

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];
```

char[] to hex string exercise

Solution

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