truncate_rec()

SRDef syten::T3N::State::truncate_rec	(	Ptr	ptr,
		Truncation const &	trunc,
		State::DropNorm const	dropN
	)

Recursive helper for truncation.

The parent of ptr is assumed to be normalised in the direction of ptr.

Assuming that both subtrees exist, the right subtree is then root-normalised without truncation first, followed by a a-normalisation of the current node without truncation. We can then descend into the left subtree to truncate there (its node is properly left-normalised). Once that subtree is normalised, we can left-normalised the node again with truncation, followed by second root-normalisation of the top of the left subtree. This is necessary to truncate “in both directions”.

Following this, we right-normalise the node, descend into the right subtree and again normalise the node with truncation and the top of the right subtree with truncation.

Finally, the node is root-normalised, as expected from a possible parent caller.