うるう年判定：ビット演算＆LUT利用

http://qiita.com/le_panda_noir/items/70eaab385c2b33c8fb63 より

うるう年判定器

uint64_t leapyear_lut[] =
  {0x1111111111111111, 0x1111110111111111,
   0x1111111111111111, 0x1111111111111011,
   0x1111011111111111, 0x1111111111111111,
   0x1111111111111111, 0x1111111111111111};

int is_leapyear(int year) {
  year %= 400;
  return !!(leapyear_lut[year >> 6] & (1ULL << (year & 0x3f)));
}

ルックアップテーブル(LUT)生成

# include <inttypes.h>
# include <stdio.h>

int main()
{
  uint64_t lut[8];
  int exclude[] = {100, 200, 300};
  int i;

  for (i = 0; i < 8; i++)
    lut[i] = 0x1111111111111111ULL;
  for (i = 0; i < 3; i++) {
    int year = exclude[i];
    lut[year >> 6] &= ~(1ULL << (year & 0x3f));
  }

  printf("uint64_t leapyear_lut[] = {");
  for (i = 0; i < 8; i++)
    printf("%s0x%016" PRIx64, (i ? "," : ""), lut[i]);
  printf("};\n");
}

うるう年判定器（圧縮版）

uint64_t leapyear_lut[] = {0xfffbfffffdffffff,0xfffffffffffff7ff};

int is_leapyear(int year) {
  if (year & 3) return 0;
  int x = (year >> 2) % 100;
  return !!(leapyear_lut[x >> 6] & (1ULL << (x & 0x3f)));
}

ルックアップテーブル(LUT)生成

# include <inttypes.h>
# include <stdio.h>

int main() {
  uint64_t lut[2] = { ~0UL, ~0UL };
  int exclude[] = {100, 200, 300};

  for (int i = 0; i < 3; i++) {
    int x = exclude[i] >> 2;
    lut[x >> 6] &= ~(1ULL << (x & 0x3f));
  }
  printf("uint64_t leapyear_lut[] = "
   "{0x%016"PRIx64",0x%016"PRIx64"};", lut[0], lut[1]);
}