怎么理解下面这段代码，是关于Splaytrees的

不太理解这段代码中treeLink()函数的操作，请大神们指点，那个*hook到底起了什么作用？

/* link operations for top-down splay */
/* this pastes a node in as !d-most node in subtree on side d */
static inline void
treeLink(struct tree ***hook, int d, struct tree *node)
{
    *hook[d] = node;
    /* strictly speaking we don't need to do this, but it allows printing the partial trees */
    node->child[!d] = 0;
    hook[d] = &node->child[!d];
}

/* splay last element on path to target to root */
static void
treeSplay(struct tree **root, int target)
{
    struct tree *t;
    struct tree *child;
    struct tree *grandchild;
    struct tree *top[TREE_NUM_CHILDREN];   /* accumulator trees that will become subtrees of new root */
    struct tree **hook[TREE_NUM_CHILDREN]; /* where to link new elements into accumulator trees */
    int d;
    int dChild;        /* direction of child */                   
    int dGrandchild;   /* direction of grandchild */

    /* we don't need to keep following this pointer, we'll just fix it at the end */
    t = *root;

    /* don't do anything to an empty tree */
    if(t == 0) { return; }

    /* ok, tree is not empty, start chopping it up */
    for(d = 0; d < TREE_NUM_CHILDREN; d++) {
        top[d] = 0;
        hook[d] = &top[d];
    }

    /* keep going until we hit the key or we would hit a null pointer in the child */
    while(t->key != target && (child = t->child[dChild = t->key < target]) != 0) {
        /* child is not null */
        grandchild = child->child[dGrandchild = child->key < target];

        treePrint(top[0]);
        puts("---");
        treePrint(t);
        puts("---");
        treePrint(top[1]);
        puts("===");

        if(grandchild == 0 || child->key == target) {
            /* zig case; paste root into opposite-side hook */
            treeLink(hook, !dChild, t);
            t = child;
            /* we can break because we know we will hit child == 0 next */
            break;
        } else if(dChild == dGrandchild) {
            /* zig-zig case */
            /* rotate and then hook up child */
            /* grandChild becomes new root */
            treeRotate(&t, dChild);
            treeLink(hook, !dChild, child);
            t = grandchild;
        } else {
            /* zig-zag case */
            /* root goes to !dChild, child goes to dChild, grandchild goes to root */
            treeLink(hook, !dChild, t);
            treeLink(hook, dChild, child);
            t = grandchild;
        }
    }

    /* now reassemble the tree */
    /* t's children go in hooks, top nodes become t's new children */
    for(d = 0; d < TREE_NUM_CHILDREN; d++) {
        *hook[d] = t->child[d];
        t->child[d] = top[d];
    }

    /* and put t back in *root */
    *root = t;
}