Fantom-foundation/go-lachesis

package vector

import (
    "github.com/Fantom-foundation/go-lachesis/hash"
    "github.com/Fantom-foundation/go-lachesis/inter/idx"
)

type kv struct {
    a, b hash.Event
}

// ForklessCause calculates "sufficient coherence" between the events.
// The A.HighestBefore array remembers the sequence number of the last
// event by each validator that is an ancestor of A. The array for
// B.LowestAfter remembers the sequence number of the earliest
// event by each validator that is a descendant of B. Compare the two arrays,
// and find how many elements in the A.HighestBefore array are greater
// than or equal to the corresponding element of the B.LowestAfter
// array. If there are more than 2n/3 such matches, then the A and B
// have achieved sufficient coherency.
//
// If B1 and B2 are forks, then they cannot BOTH forkless-cause any specific event A,
// unless more than 1/3W are Byzantine.
// This great property is the reason why this function exists,
// providing the base for the BFT algorithm.
func (vi *Index) ForklessCause(aID, bID hash.Event) bool {
    if res, ok := vi.cache.ForklessCause.Get(kv{aID, bID}); ok {
        return res.(bool)
    }

    res := vi.forklessCause(aID, bID)

    vi.cache.ForklessCause.Add(kv{aID, bID}, res)
    return res
}

func (vi *Index) forklessCause(aID, bID hash.Event) bool {
    vi.initBranchesInfo()

    // get events by hash
    a := vi.GetHighestBeforeSeq(aID)
    if a == nil {
        vi.Log.Crit("Event A not found", "event", aID.String())
        return false
    }

    // check A doesn't observe any forks from B
    if vi.atLeastOneFork() {
        bBranchID := vi.getEventBranchID(bID)
        if a.Get(bBranchID).IsForkDetected() { // B is observed as cheater by A
            return false
        }
    }

    // check A observes that {QUORUM} non-cheater-validators observe B
    b := vi.GetLowestAfterSeq(bID)
    if b == nil {
        vi.Log.Crit("Event B not found", "event", bID.String())
        return false
    }

    yes := vi.validators.NewCounter()
    // calculate forkless causing using the indexes
    for branchIDint, creatorIdx := range vi.bi.BranchIDCreatorIdxs {
        branchID := idx.Validator(branchIDint)

        // bLowestAfter := vi.GetLowestAfterSeq_(bID, branchID)   // lowest event from creator on branchID, which observes B
        bLowestAfter := b.Get(branchID)   // lowest event from creator on branchID, which observes B
        aHighestBefore := a.Get(branchID) // highest event from creator, observed by A

        // if lowest event from branchID which observes B <= highest from branchID observed by A
        // then {highest from branchID observed by A} observes B
        if bLowestAfter <= aHighestBefore.Seq && bLowestAfter != 0 && !aHighestBefore.IsForkDetected() {
            // we may count the same creator multiple times (on different branches)!
            // so not every call increases the counter
            yes.CountByIdx(creatorIdx)
        }
    }
    return yes.HasQuorum()
}

// NoCheaters excludes events which are observed by selfParents as cheaters.
// Called by emitter to exclude cheater's events from potential parents list.
func (vi *Index) NoCheaters(selfParent *hash.Event, options hash.Events) hash.Events {
    if selfParent == nil {
        return options
    }
    vi.initBranchesInfo()

    // no need to merge, because every branch is marked by IsForkDetected if fork is observed
    highest := vi.GetHighestBeforeSeq(*selfParent)
    filtered := make(hash.Events, 0, len(options))
    for _, id := range options {
        header := vi.getEvent(id)
        if header == nil {
            vi.Log.Crit("Event not found", "id", id.String())
        }
        if !highest.Get(vi.validatorIdxs[header.Creator]).IsForkDetected() {
            filtered.Add(id)
        }
    }
    return filtered
}