Function pfcountCommand

src/hyperloglog.cpp:1230–1317

PFCOUNT var -> approximated cardinality of set. */

/* PFCOUNT var -> approximated cardinality of set. */
void pfcountCommand(client *c) {
    robj *o;
    struct hllhdr *hdr;
    uint64_t card;

    /* Case 1: multi-key keys, cardinality of the union.
     *
     * When multiple keys are specified, PFCOUNT actually computes
     * the cardinality of the merge of the N HLLs specified. */
    if (c->argc > 2) {
        uint8_t max[HLL_HDR_SIZE+HLL_REGISTERS], *registers;
        int j;

        /* Compute an HLL with M[i] = MAX(M[i]_j). */
        memset(max,0,sizeof(max));
        hdr = (struct hllhdr*) max;
        hdr->encoding = HLL_RAW; /* Special internal-only encoding. */
        registers = max + HLL_HDR_SIZE;
        for (j = 1; j < c->argc; j++) {
            /* Check type and size. */
            robj_roptr o = lookupKeyRead(c->db,c->argv[j]);
            if (o == nullptr) continue; /* Assume empty HLL for non existing var.*/
            if (isHLLObjectOrReply(c,o) != C_OK) return;

            /* Merge with this HLL with our 'max' HLL by setting max[i]
             * to MAX(max[i],hll[i]). */
            if (hllMerge(registers,HLL_REGISTERS,o) == C_ERR) {
                addReplyError(c,invalid_hll_err);
                return;
            }
        }

        /* Compute cardinality of the resulting set. */
        addReplyLongLong(c,hllCount(hdr,NULL));
        return;
    }

    /* Case 2: cardinality of the single HLL.
     *
     * The user specified a single key. Either return the cached value
     * or compute one and update the cache. */
    o = lookupKeyWrite(c->db,c->argv[1]);
    if (o == NULL) {
        /* No key? Cardinality is zero since no element was added, otherwise
         * we would have a key as HLLADD creates it as a side effect. */
        addReply(c,shared.czero);
    } else {
        if (isHLLObjectOrReply(c,o) != C_OK) return;
        o = dbUnshareStringValue(c->db,c->argv[1],o);

        /* Check if the cached cardinality is valid. */
        hdr = (hllhdr*)ptrFromObj(o);
        if (HLL_VALID_CACHE(hdr)) {
            /* Just return the cached value. */
            card = (uint64_t)hdr->card[0];
            card |= (uint64_t)hdr->card[1] << 8;
            card |= (uint64_t)hdr->card[2] << 16;
            card |= (uint64_t)hdr->card[3] << 24;