zlib-ng bi-reverse table

bi_reverse.c

#include <stdio.h>
#include <cstdlib>
#include <map>


unsigned bi_reverse(unsigned code, int len) {
    /* code: the value to invert */
    /* len: its bit length */
    unsigned long res = 0;
    do {
        res |= code & 1;
        code >>= 1, res <<= 1;
    } while (--len > 0);
    return (unsigned)(res >> 1);
}

static const unsigned char bi_reverse_table[256] = {
   0x00,0x80,0x40,0xc0,0x20,0xa0,0x60,0xe0,
    0x10,0x90,0x50,0xd0,0x30,0xb0,0x70,0xf0,
    0x08,0x88,0x48,0xc8,0x28,0xa8,0x68,0xe8,
    0x18,0x98,0x58,0xd8,0x38,0xb8,0x78,0xf8,
    0x04,0x84,0x44,0xc4,0x24,0xa4,0x64,0xe4,
    0x14,0x94,0x54,0xd4,0x34,0xb4,0x74,0xf4,
    0x0c,0x8c,0x4c,0xcc,0x2c,0xac,0x6c,0xec,
    0x1c,0x9c,0x5c,0xdc,0x3c,0xbc,0x7c,0xfc,
    0x02,0x82,0x42,0xc2,0x22,0xa2,0x62,0xe2,
    0x12,0x92,0x52,0xd2,0x32,0xb2,0x72,0xf2,
    0x0a,0x8a,0x4a,0xca,0x2a,0xaa,0x6a,0xea,
    0x1a,0x9a,0x5a,0xda,0x3a,0xba,0x7a,0xfa,
    0x06,0x86,0x46,0xc6,0x26,0xa6,0x66,0xe6,
    0x16,0x96,0x56,0xd6,0x36,0xb6,0x76,0xf6,
    0x0e,0x8e,0x4e,0xce,0x2e,0xae,0x6e,0xee,
    0x1e,0x9e,0x5e,0xde,0x3e,0xbe,0x7e,0xfe,
    0x01,0x81,0x41,0xc1,0x21,0xa1,0x61,0xe1,
    0x11,0x91,0x51,0xd1,0x31,0xb1,0x71,0xf1,
    0x09,0x89,0x49,0xc9,0x29,0xa9,0x69,0xe9,
    0x19,0x99,0x59,0xd9,0x39,0xb9,0x79,0xf9,
    0x05,0x85,0x45,0xc5,0x25,0xa5,0x65,0xe5,
    0x15,0x95,0x55,0xd5,0x35,0xb5,0x75,0xf5,
    0x0d,0x8d,0x4d,0xcd,0x2d,0xad,0x6d,0xed,
    0x1d,0x9d,0x5d,0xdd,0x3d,0xbd,0x7d,0xfd,
    0x03,0x83,0x43,0xc3,0x23,0xa3,0x63,0xe3,
    0x13,0x93,0x53,0xd3,0x33,0xb3,0x73,0xf3,
    0x0b,0x8b,0x4b,0xcb,0x2b,0xab,0x6b,0xeb,
    0x1b,0x9b,0x5b,0xdb,0x3b,0xbb,0x7b,0xfb,
    0x07,0x87,0x47,0xc7,0x27,0xa7,0x67,0xe7,
    0x17,0x97,0x57,0xd7,0x37,0xb7,0x77,0xf7,
    0x0f,0x8f,0x4f,0xcf,0x2f,0xaf,0x6f,0xef,
    0x1f,0x9f,0x5f,0xdf,0x3f,0xbf,0x7f,0xff
};
static unsigned bi_reverse2(unsigned code, int len) {
    return ((unsigned)bi_reverse_table[code & 0xff] << 24 |
            (unsigned)bi_reverse_table[(code >> 8) & 0xff] << 16 |
            (unsigned)bi_reverse_table[(code >> 16) & 0xff] << 8 |
             bi_reverse_table[(code >> 24) & 0xff]) >> (32 - len);
}

int main()
{
    unsigned int code = 0x66881045;
    printf("original %08x reverse (%02x%02x%02x%02x)\n", code, 
        code & 0xff, code >> 8 & 0xff, code >> 16 & 0xff, code >> 24 & 0xff);
    printf("%08x\n", (unsigned)bi_reverse_table[code & 0xff] << 24);
    printf("%08x\n", (unsigned)bi_reverse_table[(code >> 8) & 0xff] << 16);
    printf("%08x\n", (unsigned)bi_reverse_table[code & 0xff] << 24
    | (unsigned)bi_reverse_table[(code >> 8) & 0xff] << 16);
    printf("%08x\n", (unsigned)bi_reverse_table[(code >> 16) & 0xff] << 8);
    printf("%08x\n", (unsigned)bi_reverse_table[(code >> 24) & 0xff]);
    
    printf("bi_reverse %08x\n", bi_reverse(code,12));
    printf("bi_reverse2 %08x\n", bi_reverse2(code,12));

    printf("static const char bi_reverse_table = {\n    ");
    for (int i = 0; i < 256; i++) {
        printf("0x%02x", bi_reverse(i, 8));
        if (i < 255) {
            printf(",");
        }
        if ((i+1) % 16 == 0) {
            printf("\n    ");
        }
    }
    printf("};");

    return 0;
}

std::map<int, int> ConstructRandomMap(int size) {
  std::map<int, int> m;
  for (int i = 0; i < size; ++i) {
    m.insert(std::make_pair(std::rand() % size, std::rand() % size));
  }
  return m;
}

class MapFixture : public ::benchmark::Fixture {
 public:
  void SetUp(const ::benchmark::State& st) {
    m = ConstructRandomMap(10 * 1024);
  }

  void TearDown(const ::benchmark::State&) { m.clear(); }

  std::map<int, int> m;
};

BENCHMARK_DEFINE_F(MapFixture, bireverse1)(benchmark::State& state) {
  for (auto _ : state) {
    for (std::pair<int, int> e : m) {
      unsigned reversed = bi_reverse((unsigned)e.first, e.second);
      benchmark::DoNotOptimize(reversed);
    }
  }
}

BENCHMARK_REGISTER_F(MapFixture, bireverse1);


BENCHMARK_DEFINE_F(MapFixture, bireverse2)(benchmark::State& state) {
  for (auto _ : state) {
    for (std::pair<int, int> e : m) {
      unsigned reversed = bi_reverse2((unsigned)e.first, e.second);
      benchmark::DoNotOptimize(reversed);
    }
  }
}

BENCHMARK_REGISTER_F(MapFixture, bireverse2);