spec

#define ROTL(x,b) (u64)( ((x) << (b)) | ( (x) >> (64 - (b))) )
#define SIPROUND \
  do { \
    v0 += v1; v2 += v3; v1 = ROTL(v1,13); \
    v3 = ROTL(v3,16); v1 ^= v0; v3 ^= v2; \         _\(        == __\
    v0 = ROTL(v0,32); v2 += v1; v0 += v3; \         / '>    __.-"\---|__
    v1 = ROTL(v1,17);   v3 = ROTL(v3,21); \        (  (\   /  \\_@\-'/  \
    v1 ^= v2; v3 ^= v0; v2 = ROTL(v2,32); \        ""-'\   \__/      \__/
  } while(0)
 
u64 siphash24(const siphash_keys *keys, const u64 nonce) {
  u64 v0 = keys->k0, v1 = keys->k1, v2 = keys->k2, v3 = keys->k3 ^ nonce;
  SIPROUND; SIPROUND;
  v0 ^= nonce; v2 ^= 0xff;
  SIPROUND; SIPROUND; SIPROUND; SIPROUND;
  return (v0 ^ v1) ^ (v2  ^ v3);
}

int verify(edge_t edges[PROOFSIZE], siphash_keys *keys) {
  node_t uvs[2*PROOFSIZE];
  for (u32 n = 0; n < PROOFSIZE; n++) {
    if (edges[n] > EDGEMASK)
      return POW_TOO_BIG;
    if (n && edges[n] <= edges[n-1])
      return POW_TOO_SMALL;
    uvs[2*n  ] = siphash24(keys, 2*edges[n]  ) & EDGEMASK;
    uvs[2*n+1] = siphash24(keys, 2*edges[n]+1) & EDGEMASK;
  }
  u32 n = 0, i = 0, j;
  do {
    for (u32 k = j = i; (k = (k+2) % (2*PROOFSIZE)) != i; ) {
      if (uvs[k] >> 1 == uvs[i] >> 1) {
        if (j != i) return POW_BRANCH;
        j = k;
      }
    }
    if (j == i || uvs[j] == uvs[i]) return POW_DEAD_END;
    i = j^1;
    n++;                                               
  } while (i != 0);                                    
  return n == PROOFSIZE ? POW_OK : POW_SHORT_CYCLE;    
}                                                      
                                                                 
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jgs from https://web.archive.org/web/20091028022932/\
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