DNA complement (A<>T, G<>C, N<>N) with only bit operations.

dna_complement.c

#include <stdio.h>

char dna_complement(char dna)
{
  dna ^= 0x15;
  dna ^= (dna & 0x02) << 3;
  dna ^= (dna & 0x02) >> 1;
  dna ^= (dna & 0x08) >> 1;
  return dna;
}

int64_t dna_complement_int64(int64_t dnas)
{
  dnas ^= 0x1515151515151515;
  dnas ^= (dnas & 0x0202020202020202) << 3;
  dnas ^= (dnas & 0x0202020202020202) >> 1;
  dnas ^= (dnas & 0x0808080808080808) >> 1;
  return dnas;
}

int main()
{
  printf("%c\n", dna_complement('A')); // => T
  printf("%c\n", dna_complement('T')); // => A
  printf("%c\n", dna_complement('G')); // => C
  printf("%c\n", dna_complement('C')); // => G
  printf("%c\n", dna_complement('N')); // => N

  char *dnas = "ATGCNNNN";
  int64_t dnas_comp = dna_complement_int64(*((int64_t *) dnas));
  char *s = (char *) &dnas_comp;
  printf("%c%c%c%c%c%c%c%c\n", // => TACGNNNN
			s[0], s[1], s[2], s[3], s[4], s[5], s[6], s[7]);
}