unsigned char *ziplistMerge(unsigned char **first, unsigned char **second) {
    /* If any params are null, we can't merge, so NULL. */
    if (first == NULL || *first == NULL || second == NULL || *second == NULL)
        return NULL;

    /* Can't merge same list into itself. */
    if (*first == *second)
        return NULL;

    size_t first_bytes = intrev32ifbe(ZIPLIST_BYTES(*first));
    size_t first_len = intrev16ifbe(ZIPLIST_LENGTH(*first));

    size_t second_bytes = intrev32ifbe(ZIPLIST_BYTES(*second));
    size_t second_len = intrev16ifbe(ZIPLIST_LENGTH(*second));

    int append;
    unsigned char *source, *target;
    size_t target_bytes, source_bytes;
    /* Pick the largest ziplist so we can resize easily in-place.
     * We must also track if we are now appending or prepending to
     * the target ziplist. */
    if (first_len >= second_len) {
        /* retain first, append second to first. */
        target = *first;
        target_bytes = first_bytes;
        source = *second;
        source_bytes = second_bytes;
        append = 1;
    } else {
        /* else, retain second, prepend first to second. */
        target = *second;
        target_bytes = second_bytes;
        source = *first;
        source_bytes = first_bytes;
        append = 0;
    }

    /* Calculate final bytes (subtract one pair of metadata) */
    size_t zlbytes = first_bytes + second_bytes -
                     ZIPLIST_HEADER_SIZE - ZIPLIST_END_SIZE;
    size_t zllength = first_len + second_len;

    /* Combined zl length should be limited within UINT16_MAX */
    zllength = zllength < UINT16_MAX ? zllength : UINT16_MAX;

    /* larger values can't be stored into ZIPLIST_BYTES */
    assert(zlbytes < UINT32_MAX);

    /* Save offset positions before we start ripping memory apart. */
    size_t first_offset = intrev32ifbe(ZIPLIST_TAIL_OFFSET(*first));
    size_t second_offset = intrev32ifbe(ZIPLIST_TAIL_OFFSET(*second));

    /* Extend target to new zlbytes then append or prepend source. */
    target = zrealloc(target, zlbytes);
    if (append) {
        /* append == appending to target */
        /* Copy source after target (copying over original [END]):
         *   [TARGET - END, SOURCE - HEADER] */
        memcpy(target + target_bytes - ZIPLIST_END_SIZE,
               source + ZIPLIST_HEADER_SIZE,
               source_bytes - ZIPLIST_HEADER_SIZE);
    } else {
        /* !append == prepending to target */
        /* Move target *contents* exactly size of (source - [END]),
         * then copy source into vacated space (source - [END]):
         *   [SOURCE - END, TARGET - HEADER] */
        memmove(target + source_bytes - ZIPLIST_END_SIZE,
                target + ZIPLIST_HEADER_SIZE,
                target_bytes - ZIPLIST_HEADER_SIZE);
        memcpy(target, source, source_bytes - ZIPLIST_END_SIZE);
    }

    /* Update header metadata. */
    ZIPLIST_BYTES(target) = intrev32ifbe(zlbytes);
    ZIPLIST_LENGTH(target) = intrev16ifbe(zllength);
    /* New tail offset is:
     *   + N bytes of first ziplist
     *   - 1 byte for [END] of first ziplist
     *   + M bytes for the offset of the original tail of the second ziplist
     *   - J bytes for HEADER because second_offset keeps no header. */
    ZIPLIST_TAIL_OFFSET(target) = intrev32ifbe(
                                   (first_bytes - ZIPLIST_END_SIZE) +
                                   (second_offset - ZIPLIST_HEADER_SIZE));

    /* __ziplistCascadeUpdate just fixes the prev length values until it finds a
     * correct prev length value (then it assumes the rest of the list is okay).
     * We tell CascadeUpdate to start at the first ziplist's tail element to fix
     * the merge seam. */
    target = __ziplistCascadeUpdate(target, target+first_offset);

    /* Now free and NULL out what we didn't realloc */
    if (append) {
        zfree(*second);
        *second = NULL;
        *first = target;
    } else {
        zfree(*first);
        *first = NULL;
        *second = target;
    }
    return target;
}