These proofs encode large directories into a tree-like structure.
Invariants are dependent on the backend.
length is the total number of entries in the children of the inode. It's the size of the "flattened" version of that inode. length can be used to prove the correctness of operations such as Tree.length and Tree.list ~offset ~length in an efficient way.
proofs contains the children proofs. It is a sparse list of 'a values. These values are associated to their index in the list, and the list is kept sorted in increasing order of indices. 'a can be a concrete proof or a hash of that proof.
For irmin-pack: proofs have a length of at most Conf.entries entries. For binary trees, this boolean index is a step of the left-right sequence / decision proof corresponding to the path in that binary tree.
An extender is a compact representation of a sequence of inode which contain only one child. As for inodes, the 'a parameter can be a concrete proof or a hash of that proof.
If an inode proof contains singleton children i_0, ..., i_n such as: {length=l; proofs = [ (i_0, {proofs = ... { proofs = [ (i_n, p) ] }})]}, then it is compressed into the inode extender {length=l; segment = [i_0;..;i_n]; proof=p} sharing the same length l and final proof p.
The type for compressed and partial Merkle tree proofs.
Tree proofs do not provide any guarantee with the ordering of computations. For instance, if two effects commute, they won't be distinguishable by this kind of proof.
Value v proves that a value v exists in the store.
Blinded_value h proves a value with hash h exists in the store.
Node ls proves that a a "flat" node containing the list of files ls exists in the store. For irmin-pack: the length of ls is at most Conf.stable_hash;
Blinded_node h proves that a node with hash h exists in the store.
Inode i proves that an inode i exists in the store.
Extender e proves that an inode extender e exist in the store.
Stream proofs represent an explicit traversal of a Merle tree proof. Every element (a node, a value, or a shallow pointer) met is first "compressed" by shallowing its children and then recorded in the proof.
As stream proofs directly encode the recursive construction of the Merkle root hash is slightly simpler to implement: the verifier simply needs to hash the compressed elements lazily, without any memory or choice.
Moreover, the minimality of stream proofs is trivial to check. Once the computation has consumed the compressed elements required, it is sufficient to check that no more compressed elements remain in the proof.
However, as the compressed elements contain all the hashes of their shallow children, the size of stream proofs is larger (at least double in size in practice) than tree proofs, which only contains the hash for intermediate shallow pointers.
The type for proofs of kind 'a (i.e. stream or proof).
A proof p proves that the state advanced from before p to after p. state p's hash is before p, and state p contains the minimal information for the computation to reach after p.
v ~before ~after p proves that the state advanced from before to after. p's hash is before, and p contains the minimal information for the computation to reach after.