Function raxAddChild

app/redis-6.2.6/src/rax.c:256–387

Add a new child to the node 'n' representing the character 'c' and return * its new pointer, as well as the child pointer by reference. Additionally * '***parentlink' is populated with the raxNode pointer-to-pointer of where * the new child was stored, which is useful for the caller to replace the * child pointer if it gets reallocated. * * On success the new parent node pointer is returned

 * of the realloc, so the caller should discard 'n' and use the new value).
 * On out of memory NULL is returned, and the old node is still valid. */
raxNode *raxAddChild(raxNode *n, unsigned char c, raxNode **childptr, raxNode ***parentlink) {
    assert(n->iscompr == 0);

    size_t curlen = raxNodeCurrentLength(n);
    n->size++;
    size_t newlen = raxNodeCurrentLength(n);
    n->size--; /* For now restore the orignal size. We'll update it only on
                  success at the end. */

    /* Alloc the new child we will link to 'n'. */
    raxNode *child = raxNewNode(0,0);
    if (child == NULL) return NULL;

    /* Make space in the original node. */
    raxNode *newn = rax_realloc(n,newlen);
    if (newn == NULL) {
        rax_free(child);
        return NULL;
    }
    n = newn;

    /* After the reallocation, we have up to 8/16 (depending on the system
     * pointer size, and the required node padding) bytes at the end, that is,
     * the additional char in the 'data' section, plus one pointer to the new
     * child, plus the padding needed in order to store addresses into aligned
     * locations.
     *
     * So if we start with the following node, having "abde" edges.
     *
     * Note:
     * - We assume 4 bytes pointer for simplicity.
     * - Each space below corresponds to one byte
     *
     * [HDR*][abde][Aptr][Bptr][Dptr][Eptr]|AUXP|
     *
     * After the reallocation we need: 1 byte for the new edge character
     * plus 4 bytes for a new child pointer (assuming 32 bit machine).
     * However after adding 1 byte to the edge char, the header + the edge
     * characters are no longer aligned, so we also need 3 bytes of padding.
     * In total the reallocation will add 1+4+3 bytes = 8 bytes:
     *
     * (Blank bytes are represented by ".")
     *
     * [HDR*][abde][Aptr][Bptr][Dptr][Eptr]|AUXP|[....][....]
     *
     * Let's find where to insert the new child in order to make sure
     * it is inserted in-place lexicographically. Assuming we are adding
     * a child "c" in our case pos will be = 2 after the end of the following
     * loop. */
    int pos;
    for (pos = 0; pos < n->size; pos++) {
        if (n->data[pos] > c) break;
    }

    /* Now, if present, move auxiliary data pointer at the end
     * so that we can mess with the other data without overwriting it.
     * We will obtain something like that:
     *