log2

main.cpp

#include <cstdio>
#include <cstdlib>

// int main()
// {
//   const size_t size = sizeof(char)*8;
//   for(size_t l=0; l<8; ++l) {
//     const size_t i_begin = 1<<l;
//     const size_t i_end = 1<<(l+1);
//     for(size_t i=i_begin; i<i_end; ++i) {
//       fprintf(stdout, "case %zu: ", i);
//     }
//     fprintf(stdout, "\n");
//     fprintf(stdout, "return %zu;\n", l);

//   }
// }


int log2_uchar(const unsigned char c) {
  switch(c) {
  case 1: 
    return 0;
  case 2: case 3: 
    return 1;
  case 4: case 5: case 6: case 7: 
    return 2;
  case 8: case 9: case 10: case 11: case 12: case 13: case 14: case 15: 
    return 3;
  case 16: case 17: case 18: case 19: case 20: case 21: case 22: case 23: case 24: case 25: case 26: case 27: case 28: case 29: case 30: case 31: 
    return 4;
  case 32: case 33: case 34: case 35: case 36: case 37: case 38: case 39: case 40: case 41: case 42: case 43: case 44: case 45: case 46: case 47: case 48: case 49: case 50: case 51: case 52: case 53: case 54: case 55: case 56: case 57: case 58: case 59: case 60: case 61: case 62: case 63: 
    return 5;
  case 64: case 65: case 66: case 67: case 68: case 69: case 70: case 71: case 72: case 73: case 74: case 75: case 76: case 77: case 78: case 79: case 80: case 81: case 82: case 83: case 84: case 85: case 86: case 87: case 88: case 89: case 90: case 91: case 92: case 93: case 94: case 95: case 96: case 97: case 98: case 99: case 100: case 101: case 102: case 103: case 104: case 105: case 106: case 107: case 108: case 109: case 110: case 111: case 112: case 113: case 114: case 115: case 116: case 117: case 118: case 119: case 120: case 121: case 122: case 123: case 124: case 125: case 126: case 127: 
    return 6;
  case 128: case 129: case 130: case 131: case 132: case 133: case 134: case 135: case 136: case 137: case 138: case 139: case 140: case 141: case 142: case 143: case 144: case 145: case 146: case 147: case 148: case 149: case 150: case 151: case 152: case 153: case 154: case 155: case 156: case 157: case 158: case 159: case 160: case 161: case 162: case 163: case 164: case 165: case 166: case 167: case 168: case 169: case 170: case 171: case 172: case 173: case 174: case 175: case 176: case 177: case 178: case 179: case 180: case 181: case 182: case 183: case 184: case 185: case 186: case 187: case 188: case 189: case 190: case 191: case 192: case 193: case 194: case 195: case 196: case 197: case 198: case 199: case 200: case 201: case 202: case 203: case 204: case 205: case 206: case 207: case 208: case 209: case 210: case 211: case 212: case 213: case 214: case 215: case 216: case 217: case 218: case 219: case 220: case 221: case 222: case 223: case 224: case 225: case 226: case 227: case 228: case 229: case 230: case 231: case 232: case 233: case 234: case 235: case 236: case 237: case 238: case 239: case 240: case 241: case 242: case 243: case 244: case 245: case 246: case 247: case 248: case 249: case 250: case 251: case 252: case 253: case 254: case 255: 
    return 7;

  default:
    return 8;
  }
}

template<typename INT, size_t size>
struct log2_impl {
  enum ENUMHACK {
    bit_length = size*8,
    bit_length_half = size*4,
    size_half = size>>1,
  };
  static int apply(const INT input) {
    const INT input_upper = input>>(bit_length_half);
    return (input_upper)?
      bit_length_half + log2_impl<INT, size_half>::apply(input_upper) :
      log2_impl<INT, size_half>::apply(input);
  };
};

template<>
struct log2_impl<unsigned char, 1> {
  static int apply(const unsigned char input) {
    return log2_uchar(input);
  };
};


template<typename INT>
struct log2_impl<INT, 1> {
  static int apply(const INT input) {
    return log2_impl<unsigned char, 1>::apply(input);
  };
};




template<typename INT>
int log2(const INT input) {
  return  log2_impl<INT, sizeof(INT)>::apply(input);
}




int main(const int argc, char* const* argv) {

  const int i = 1234567;
  printf("log2(%d)=%d\n", i, log2(i)); 

  return 0;
}