The Blowfish Cipher

uint32_t P[18];
uint32_t S[4][256];

uint32_t f (uint32_t x) {
   uint32_t h = S[0][x >> 24] + S[1][x >> 16 & 0xff];
   return ( h ^ S[2][x >> 8 & 0xff] ) + S[3][x & 0xff];
}

void encrypt (uint32_t & L, uint32_t & R) {
   for (int i=0 ; i<16 ; i += 2) {
      L ^= P[i];
      R ^= f(L);
      R ^= P[i+1];
      L ^= f(R);
   }
   L ^= P[16];
   R ^= P[17];
   swap (L, R);
}

void decrypt (uint32_t & L, uint32_t & R) {
   for (int i=16 ; i > 0 ; i -= 2) {
      L ^= P[i+1];
      R ^= f(L);
      R ^= P[i];
      L ^= f(R);
   }
   L ^= P[1];
   R ^= P[0];
   swap (L, R);
}

  // ...
  // initializing the P-array and S-boxes with values derived from pi; omitted in the example
  // ...
{
   for (int i=0 ; i<18 ; ++i)
      P[i] ^= key[i % keylen];
   uint32_t L = 0, R = 0;
   for (int i=0 ; i<18 ; i+=2) {
      encrypt (L, R);
      P[i] = L; P[i+1] = R;
   }
   for (int i=0 ; i<4 ; ++i)
      for (int j=0 ; j<256; j+=2) {
         encrypt (L, R);
         S[i][j] = L; S[i][j+1] = R;
      }
}