Introduce types DeltaUTxO and DeltaWallet

[HeinrichApfelmus]

Issue number

ADP-1434

Overview

Previous work in epic ADP-1043 introduced delta encodings, DBVars, and an embedding of the wallet state and its delta encodings into a database table. It's time to integrate these tools with the wallet code. To facilitate code review, the integration proceeds in a sequence of refactorings that do not change functionality and pass all unit tests.

In this step, we finally introduce the type DeltaUTxO which represents a delta encoding of the UTxO set, i.e. a package of changes that can be applied to one UTxO set in order to obtain another one. The type DeltaUTxO is abstract, but internally, it is implemented as

data DeltaUTxO = DeltaUTxO
    { excluded :: !(Set TxIn) -- ^ First exclude these inputs
    , received :: !UTxO       -- ^ Then receive these additional outputs.
    }

i.e. it stores a set of TxIn that was spent, and a UTxO that was received.

Building on this, we also introduce a delta encoding DeltaWallet for the wallet state Wallet s. The main change is that the applyBlock function now also computes and returns this delta encoding when computing the new Wallet s state.

applyBlock
    :: (…)
    => Block -> Wallet s -> (FilteredBlock, (DeltaWallet s, Wallet s))

However, this pull request does not yet integrate this type into the WalletState checkpoints; this will be the subject of an upcoming pull request.

Details

• DeltaWallet s currently uses the trivial delta encoding Data.Delta.Replace for the address discovery state.
• As indicated, the delta encodings DeltaWallet s are not yet used with the Checkpoint mechanism. Instead, they are currently discarded.
• Even though it is currently discarded, the DeltaUTxO type has suggested an optimization where a transaction that does not change the UTxO set can be discarded more quickly. This may already improve the wallet restoration benchmarks slightly.
• I have smuggled in a small cleanup commit that removes the Dom type class, as it is no longer used.

Comments

• @piotr-iohk I'd be interested in the wallet restoration benchmark for this branch — I expect slight improvements for the 0.01-percent-seq and 0.01-percent-rnd benchmarks.

[paweljakubas]

I like this manual example as it shows the case. One case. But what about also using proper arbitrary instances showing that deltaUTxO ops works properly for different overlapping of deltas? There are many possible cases here, right?

[HeinrichApfelmus]

The beauty of elementary statements about sets is that they can be decided by looking at truth tables, which in turn corresponds to one universal example ("Venn diagram").

For example, in order to show that the equality (A ∩ B) ∪ C = (A ∪ C) ∩ (B ∪ C) holds for all finite sets A,B,C, it is sufficient to show that it holds for the specific choice A = { "100", "110", "101", "111"}, B = { "010", "110", "011", "111"}, and C = { "001", "101", "011", "111"}. Even though the elements like "010" seem very specific, they stand for an entire set of elements; here, "010" stands for all elements that are contained in B, but not in A or C.

The associativity of DeltaUTxO is a statement of this form, assuming that all operations are implemented in terms of set-theoretic operations like Set.intersect, which they essentially are. Hence, we can decide its validity with a single, universal example — the one in the test. This example can be visualize as follows:

In this sense, "a2" represents an entire set of outputs (suggested by the colored dots), namely the set of outputs contained in utxo0, not spent by delta1, but spent by delta2. Likewise, "a0" represents the set of outputs contained in utxo0, not spent by delta1, and not spent by delta2. Due to the "no double-spending" constraint, not all cases are possible; the diagram shows all the possible cases.

EDIT: I have also added a comment in the source code.

[paweljakubas]

lgtm! Maybe better generation of delta overlapping could be useful here

[HeinrichApfelmus]

bors merge

[iohk-bors]

Build failed:

• ci/hydra-build:required

[HeinrichApfelmus]

bors merge

[iohk-bors]

Build succeeded:

• buildkite/cardano-wallet
• ci/hydra-build:required