Understanding skip pointers in BlockIndex in bitcoin sources

From source code:

/** The block chain is a tree shaped structure starting with the
 * genesis block at the root, with each block potentially having multiple
 * candidates to be the next block. A blockindex may have multiple pprev pointing
 * to it, but at most one of them can be part of the currently active branch.
 */
class CBlockIndex




//! pointer to the index of the predecessor of this block
CBlockIndex* pprev;

//! pointer to the index of some further predecessor of this block
CBlockIndex* pskip;

Can anyone help me understand the need for skip pointers, perhaps with a picture? Why should we track this? The comment says some predecessor . When and where are these useful?

Guhan S

Posted 2016-01-05T04:29:30.400

Reputation: 85

Answers

It is implenting a skiplist.

Every index node points to its immediate predecessor (pprev), and to some other predecessor that is further away (pskip). These skip pointers are used when we need to find a far predecessor quickly. One common case is when we need to find the 2016th ancestor of a block to recompute the target difficulty.

Most of the skip pointers only go back a few blocks, but ocassionally one points back very far, allowing many nodes to be skipped.

Pieter Wuille

Posted 2016-01-05T04:29:30.400

Reputation: 54 032