6.4.5 Aside: Initial Black Insertion |

The traditional algorithm for insertion in an RB tree colors new nodes red. This is a good choice, because it often means that no rebalancing is necessary, but it is not the only possible choice. This section implements an alternate algorithm for insertion into an RB tree that colors new nodes black.

The outline is the same as for initial-red insertion. We change the newly inserted node from red to black and replace the rebalancing algorithm:

212. <RB item insertion function, initial black 212> =void**rb_probe(structrb_table*tree,void*item)

{ <rb_probe() local variables 200> <Step 1: Search RB tree for insertion point 201> <Step 2: Insert RB node; RB_RED => RB_BLACK 202> <Step 3: Rebalance after initial-black RB insertion 213>return&n->rb_data; }

The remaining task is to devise the rebalancing algorithm. Rebalancing is always necessary, unless the tree was empty before insertion, because insertion of a black node into a nonempty tree always violates rule 2. Thus, our invariant is that we have a rule 2 violation to fix.

More specifically, the invariant, as implemented, is that at the top
of each trip through the loop, stack *pa*[] contains the chain of
ancestors of a node that is the black root of a subtree whose
black-height is 1 more than it should be. We give that node the name
*q*. There is one easy rebalancing special case: if node *q* has a
black parent, we can just recolor *q* as red, and we're done. Here's
the loop:

213. <Step 3: Rebalance after initial-black RB insertion 213> =while(k>= 2)

{structrb_node*q=pa[k- 1]->rb_link[da[k- 1]];if(pa[k- 1]->rb_color==RB_BLACK)

{q->rb_color=RB_RED;break; }if(da[k- 2] == 0) {

<Left-side rebalancing after initial-black RB insertion 214>

}else

{

<Right-side rebalancing after initial-black RB insertion 218>

} }

This code is included in 212.

Consider rebalancing where insertion was on the left side of *q*'s
grandparent. We know that *q* is black and its parent *pa*[*k* - 1] is
red. Then, we can divide rebalancing into three cases, described
below in detail. (For additional insight, compare these cases to the
corresponding cases for initial-red insertion.)

214. <Left-side rebalancing after initial-black RB insertion 214> =structrb_node*y=pa[k- 2]->rb_link[1];if(y!=NULL&&y->rb_color==RB_RED) {

<Case 1 in left-side initial-black RB insertion rebalancing 215>

}else

{structrb_node*x;if(da[k- 1] == 0)y=pa[k- 1];else

{

<Case 3 in left-side initial-black RB insertion rebalancing 217>

} <Case 2 in left-side initial-black RB insertion rebalancing 216> }

This code is included in 213.

If *q* has an red “uncle” *y*, then we recolor *q* red and *pa*[*k* - 1] and *y* black. This fixes the immediate problem, making the
black-height of *q* equal to its sibling's, but increases the
black-height of *pa*[*k* - 2], so we must repeat the rebalancing process
farther up the tree:

215. <Case 1 in left-side initial-black RB insertion rebalancing 215> =pa[k- 1]->rb_color=y->rb_color=RB_BLACK;q->rb_color=RB_RED;k-= 2;

This code is included in 214 and 219.

If *q* is a left child, then call *q*'s parent *y* and its grandparent
*x*, rotate right at *x*, and recolor *q*, *y*, and *x*. The effect
is that the black-heights of all three subtrees is the same as before
*q* was inserted, so we're done, and **break** out of the loop.

216. <Case 2 in left-side initial-black RB insertion rebalancing 216> =x=pa[k- 2];x->rb_color=q->rb_color=RB_RED;y->rb_color=RB_BLACK;x->rb_link[0] =y->rb_link[1];y->rb_link[1] =x;pa[k- 3]->rb_link[da[k- 3]] =y;break;

This code is included in 214.

If *q* is a right child, then we rotate left at its parent, which we
here call *x*. The result is in the form for application of case 2,
so after the rotation, we relabel the nodes to be consistent with that
case.

217. <Case 3 in left-side initial-black RB insertion rebalancing 217> =x=pa[k- 1];y=pa[k- 2]->rb_link[0] =q;x->rb_link[1] =y->rb_link[0];q=y->rb_link[0] =x;

This code is included in 214.

218. <Right-side rebalancing after initial-black RB insertion 218> =structrb_node*y=pa[k- 2]->rb_link[0];if(y!=NULL&&y->rb_color==RB_RED) {

<Case 1 in right-side initial-black RB insertion rebalancing 219>

}else

{structrb_node*x;if(da[k- 1] == 1)y=pa[k- 1];else

{

<Case 3 in right-side initial-black RB insertion rebalancing 221>

} <Case 2 in right-side initial-black RB insertion rebalancing 220> }

This code is included in 213.

219. <Case 1 in right-side initial-black RB insertion rebalancing 219> = <Case 1 in left-side initial-black RB insertion rebalancing 215>

This code is included in 218.

220. <Case 2 in right-side initial-black RB insertion rebalancing 220> =x=pa[k- 2];x->rb_color=q->rb_color=RB_RED;y->rb_color=RB_BLACK;x->rb_link[1] =y->rb_link[0];y->rb_link[0] =x;pa[k- 3]->rb_link[da[k- 3]] =y;break;

This code is included in 218.

221. <Case 3 in right-side initial-black RB insertion rebalancing 221> =x=pa[k- 1];y=pa[k- 2]->rb_link[1] =q;x->rb_link[0] =y->rb_link[1];q=y->rb_link[1] =x;

This code is included in 218.