convert binary string to hex expression each other.

hex.cpp

#include <stdlib.h>
#include <string>

namespace {
const char kBin2Hex[] = { "0123456789ABCDEF" };

char Hex2Bin(char ch) {
  switch(ch) {
    case '0': return 0;
    case '1': return 1;
    case '2': return 2;
    case '3': return 3;
    case '4': return 4;
    case '5': return 5;
    case '6': return 6;
    case '7': return 7;
    case '8': return 8;
    case '9': return 9;
    case 'A':
    case 'a': return 0x0a;
    case 'B':
    case 'b': return 0x0b;
    case 'C':
    case 'c': return 0x0c;
    case 'D':
    case 'd': return 0x0d;
    case 'E':
    case 'e': return 0x0e;
    case 'F':
    case 'f': return 0x0f;
  }
}
}

std::string bin2hex(const std::string& binary) {
  std::string hex;
  char chs[3];
  const char* ps = binary.data();
  for (size_t i = 0; i < binary.size(); ++i) {
    unsigned char ch(ps[i]);
    chs[0] = kBin2Hex[ch >> 4];
    chs[1] = kBin2Hex[ch & 0x0F];
    chs[2] = '\0';
    hex.append(chs);
  }
  return hex;
}

std::string hex2bin(const std::string& hex) {
  // TODO: hex string length should be even number                                       
  // TODO: endian issue                                                                  
  std::string binary;
  const char* pt = hex.data();
  size_t nlen = hex.size() / 2;
  for (size_t i = 0; i < nlen; ++i) {
    unsigned char ch = ((Hex2Bin(pt[i*2]) << 4) & 0xF0) + Hex2Bin(pt[i*2+1]);
    binary.append(1, ch);
  }
  return binary;
}