LedgerTrie.h

//------------------------------------------------------------------------------
/*
    This file is part of rippled: https://github.com/ripple/rippled
    Copyright (c) 2017 Ripple Labs Inc.
 
    Permission to use, copy, modify, and/or distribute this software for any
    purpose  with  or without fee is hereby granted, provided that the above
    copyright notice and this permission notice appear in all copies.
 
    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
//==============================================================================
 
#ifndef RIPPLE_APP_CONSENSUS_LEDGERS_TRIE_H_INCLUDED
#define RIPPLE_APP_CONSENSUS_LEDGERS_TRIE_H_INCLUDED
 
#include <ripple/basics/ToString.h>
#include <ripple/json/json_value.h>
#include <algorithm>
#include <memory>
#include <optional>
#include <sstream>
#include <stack>
#include <vector>
 
namespace ripple {
 
template <class Ledger>
class SpanTip
{
public:
    using Seq = typename Ledger::Seq;
    using ID = typename Ledger::ID;
 
    SpanTip(Seq s, ID i, Ledger const lgr)
        : seq{s}, id{i}, ledger{std::move(lgr)}
    {
    }
 
    // The sequence number of the tip ledger
    Seq seq;
    // The ID of the tip ledger
    ID id;
 
    ID
    ancestor(Seq const& s) const
    {
        assert(s <= seq);
        return ledger[s];
    }
 
private:
    Ledger const ledger;
};
 
namespace ledger_trie_detail {
 
// Represents a span of ancestry of a ledger
template <class Ledger>
class Span
{
    using Seq = typename Ledger::Seq;
    using ID = typename Ledger::ID;
 
    // The span is the half-open interval [start,end) of ledger_
    Seq start_{0};
    Seq end_{1};
    Ledger ledger_;
 
public:
    Span() : ledger_{typename Ledger::MakeGenesis{}}
    {
        // Require default ledger to be genesis seq
        assert(ledger_.seq() == start_);
    }
 
    Span(Ledger ledger)
        : start_{0}, end_{ledger.seq() + Seq{1}}, ledger_{std::move(ledger)}
    {
    }
 
    Span(Span const& s) = default;
    Span(Span&& s) = default;
    Span&
    operator=(Span const&) = default;
    Span&
    operator=(Span&&) = default;
 
    Seq
    start() const
    {
        return start_;
    }
 
    Seq
    end() const
    {
        return end_;
    }
 
    // Return the Span from [spot,end_) or none if no such valid span
    std::optional<Span>
    from(Seq spot) const
    {
        return sub(spot, end_);
    }
 
    // Return the Span from [start_,spot) or none if no such valid span
    std::optional<Span>
    before(Seq spot) const
    {
        return sub(start_, spot);
    }
 
    // Return the ID of the ledger that starts this span
    ID
    startID() const
    {
        return ledger_[start_];
    }
 
    // Return the ledger sequence number of the first possible difference
    // between this span and a given ledger.
    Seq
    diff(Ledger const& o) const
    {
        return clamp(mismatch(ledger_, o));
    }
 
    //  The tip of this span
    SpanTip<Ledger>
    tip() const
    {
        Seq tipSeq{end_ - Seq{1}};
        return SpanTip<Ledger>{tipSeq, ledger_[tipSeq], ledger_};
    }
 
private:
    Span(Seq start, Seq end, Ledger const& l)
        : start_{start}, end_{end}, ledger_{l}
    {
        // Spans cannot be empty
        assert(start < end);
    }
 
    Seq
    clamp(Seq val) const
    {
        return std::min(std::max(start_, val), end_);
    }
 
    // Return a span of this over the half-open interval [from,to)
    std::optional<Span>
    sub(Seq from, Seq to) const
    {
        Seq newFrom = clamp(from);
        Seq newTo = clamp(to);
        if (newFrom < newTo)
            return Span(newFrom, newTo, ledger_);
        return std::nullopt;
    }
 
    friend std::ostream&
    operator<<(std::ostream& o, Span const& s)
    {
        return o << s.tip().id << "[" << s.start_ << "," << s.end_ << ")";
    }
 
    friend Span
    merge(Span const& a, Span const& b)
    {
        // Return combined span, using ledger_ from higher sequence span
        if (a.end_ < b.end_)
            return Span(std::min(a.start_, b.start_), b.end_, b.ledger_);
 
        return Span(std::min(a.start_, b.start_), a.end_, a.ledger_);
    }
};
 
// A node in the trie
template <class Ledger>
struct Node
{
    Node() = default;
 
    explicit Node(Ledger const& l) : span{l}, tipSupport{1}, branchSupport{1}
    {
    }
 
    explicit Node(Span<Ledger> s) : span{std::move(s)}
    {
    }
 
    Span<Ledger> span;
    std::uint32_t tipSupport = 0;
    std::uint32_t branchSupport = 0;
 
    std::vector<std::unique_ptr<Node>> children;
    Node* parent = nullptr;
 
    void
    erase(Node const* child)
    {
        auto it = std::find_if(
            children.begin(),
            children.end(),
            [child](std::unique_ptr<Node> const& curr) {
                return curr.get() == child;
            });
        assert(it != children.end());
        std::swap(*it, children.back());
        children.pop_back();
    }
 
    friend std::ostream&
    operator<<(std::ostream& o, Node const& s)
    {
        return o << s.span << "(T:" << s.tipSupport << ",B:" << s.branchSupport
                 << ")";
    }
 
    Json::Value
    getJson() const
    {
        Json::Value res;
        std::stringstream sps;
        sps << span;
        res["span"] = sps.str();
        res["startID"] = to_string(span.startID());
        res["seq"] = static_cast<std::uint32_t>(span.tip().seq);
        res["tipSupport"] = tipSupport;
        res["branchSupport"] = branchSupport;
        if (!children.empty())
        {
            Json::Value& cs = (res["children"] = Json::arrayValue);
            for (auto const& child : children)
            {
                cs.append(child->getJson());
            }
        }
        return res;
    }
};
}  // namespace ledger_trie_detail
 
template <class Ledger>
class LedgerTrie
{
    using Seq = typename Ledger::Seq;
    using ID = typename Ledger::ID;
 
    using Node = ledger_trie_detail::Node<Ledger>;
    using Span = ledger_trie_detail::Span<Ledger>;
 
    // The root of the trie. The root is allowed to break the no-single child
    // invariant.
    std::unique_ptr<Node> root;
 
    // Count of the tip support for each sequence number
    std::map<Seq, std::uint32_t> seqSupport;
 
    std::pair<Node*, Seq>
    find(Ledger const& ledger) const
    {
        Node* curr = root.get();
 
        // Root is always defined and is in common with all ledgers
        assert(curr);
        Seq pos = curr->span.diff(ledger);
 
        bool done = false;
 
        // Continue searching for a better span as long as the current position
        // matches the entire span
        while (!done && pos == curr->span.end())
        {
            done = true;
            // Find the child with the longest ancestry match
            for (std::unique_ptr<Node> const& child : curr->children)
            {
                auto const childPos = child->span.diff(ledger);
                if (childPos > pos)
                {
                    done = false;
                    pos = childPos;
                    curr = child.get();
                    break;
                }
            }
        }
        return std::make_pair(curr, pos);
    }
 
    Node*
    findByLedgerID(Ledger const& ledger, Node* parent = nullptr) const
    {
        if (!parent)
            parent = root.get();
        if (ledger.id() == parent->span.tip().id)
            return parent;
        for (auto const& child : parent->children)
        {
            auto cl = findByLedgerID(ledger, child.get());
            if (cl)
                return cl;
        }
        return nullptr;
    }
 
    void
    dumpImpl(std::ostream& o, std::unique_ptr<Node> const& curr, int offset)
        const
    {
        if (curr)
        {
            if (offset > 0)
                o << std::setw(offset) << "|-";
 
            std::stringstream ss;
            ss << *curr;
            o << ss.str() << std::endl;
            for (std::unique_ptr<Node> const& child : curr->children)
                dumpImpl(o, child, offset + 1 + ss.str().size() + 2);
        }
    }
 
public:
    LedgerTrie() : root{std::make_unique<Node>()}
    {
    }
 
    void
    insert(Ledger const& ledger, std::uint32_t count = 1)
    {
        auto const [loc, diffSeq] = find(ledger);
 
        // There is always a place to insert
        assert(loc);
 
        // Node from which to start incrementing branchSupport
        Node* incNode = loc;
 
        // loc->span has the longest common prefix with Span{ledger} of all
        // existing nodes in the trie. The optional<Span>'s below represent
        // the possible common suffixes between loc->span and Span{ledger}.
        //
        // loc->span
        //  a b c  | d e f
        //  prefix | oldSuffix
        //
        // Span{ledger}
        //  a b c  | g h i
        //  prefix | newSuffix
 
        std::optional<Span> prefix = loc->span.before(diffSeq);
        std::optional<Span> oldSuffix = loc->span.from(diffSeq);
        std::optional<Span> newSuffix = Span{ledger}.from(diffSeq);
 
        if (oldSuffix)
        {
            // Have
            //   abcdef -> ....
            // Inserting
            //   abc
            // Becomes
            //   abc -> def -> ...
 
            // Create oldSuffix node that takes over loc
            auto newNode = std::make_unique<Node>(*oldSuffix);
            newNode->tipSupport = loc->tipSupport;
            newNode->branchSupport = loc->branchSupport;
            newNode->children = std::move(loc->children);
            assert(loc->children.empty());
            for (std::unique_ptr<Node>& child : newNode->children)
                child->parent = newNode.get();
 
            // Loc truncates to prefix and newNode is its child
            assert(prefix);
            loc->span = *prefix;
            newNode->parent = loc;
            loc->children.emplace_back(std::move(newNode));
            loc->tipSupport = 0;
        }
        if (newSuffix)
        {
            // Have
            //  abc -> ...
            // Inserting
            //  abcdef-> ...
            // Becomes
            //  abc -> ...
            //     \-> def
 
            auto newNode = std::make_unique<Node>(*newSuffix);
            newNode->parent = loc;
            // increment support starting from the new node
            incNode = newNode.get();
            loc->children.push_back(std::move(newNode));
        }
 
        incNode->tipSupport += count;
        while (incNode)
        {
            incNode->branchSupport += count;
            incNode = incNode->parent;
        }
 
        seqSupport[ledger.seq()] += count;
    }
 
    bool
    remove(Ledger const& ledger, std::uint32_t count = 1)
    {
        Node* loc = findByLedgerID(ledger);
        // Must be exact match with tip support
        if (!loc || loc->tipSupport == 0)
            return false;
 
        // found our node, remove it
        count = std::min(count, loc->tipSupport);
        loc->tipSupport -= count;
 
        auto const it = seqSupport.find(ledger.seq());
        assert(it != seqSupport.end() && it->second >= count);
        it->second -= count;
        if (it->second == 0)
            seqSupport.erase(it->first);
 
        Node* decNode = loc;
        while (decNode)
        {
            decNode->branchSupport -= count;
            decNode = decNode->parent;
        }
 
        while (loc->tipSupport == 0 && loc != root.get())
        {
            Node* parent = loc->parent;
            if (loc->children.empty())
            {
                // this node can be erased
                parent->erase(loc);
            }
            else if (loc->children.size() == 1)
            {
                // This node can be combined with its child
                std::unique_ptr<Node> child = std::move(loc->children.front());
                child->span = merge(loc->span, child->span);
                child->parent = parent;
                parent->children.emplace_back(std::move(child));
                parent->erase(loc);
            }
            else
                break;
            loc = parent;
        }
        return true;
    }
 
    std::uint32_t
    tipSupport(Ledger const& ledger) const
    {
        if (auto const* loc = findByLedgerID(ledger))
            return loc->tipSupport;
        return 0;
    }
 
    std::uint32_t
    branchSupport(Ledger const& ledger) const
    {
        Node const* loc = findByLedgerID(ledger);
        if (!loc)
        {
            Seq diffSeq;
            std::tie(loc, diffSeq) = find(ledger);
            // Check that ledger is a proper prefix of loc
            if (!(diffSeq > ledger.seq() && ledger.seq() < loc->span.end()))
                loc = nullptr;
        }
        return loc ? loc->branchSupport : 0;
    }
 
    std::optional<SpanTip<Ledger>>
    getPreferred(Seq const largestIssued) const
    {
        if (empty())
            return std::nullopt;
 
        Node* curr = root.get();
 
        bool done = false;
 
        std::uint32_t uncommitted = 0;
        auto uncommittedIt = seqSupport.begin();
 
        while (curr && !done)
        {
            // Within a single span, the preferred by branch strategy is simply
            // to continue along the span as long as the branch support of
            // the next ledger exceeds the uncommitted support for that ledger.
            {
                // Add any initial uncommitted support prior for ledgers
                // earlier than nextSeq or earlier than largestIssued
                Seq nextSeq = curr->span.start() + Seq{1};
                while (uncommittedIt != seqSupport.end() &&
                       uncommittedIt->first < std::max(nextSeq, largestIssued))
                {
                    uncommitted += uncommittedIt->second;
                    uncommittedIt++;
                }
 
                // Advance nextSeq along the span
                while (nextSeq < curr->span.end() &&
                       curr->branchSupport > uncommitted)
                {
                    // Jump to the next seqSupport change
                    if (uncommittedIt != seqSupport.end() &&
                        uncommittedIt->first < curr->span.end())
                    {
                        nextSeq = uncommittedIt->first + Seq{1};
                        uncommitted += uncommittedIt->second;
                        uncommittedIt++;
                    }
                    else  // otherwise we jump to the end of the span
                        nextSeq = curr->span.end();
                }
                // We did not consume the entire span, so we have found the
                // preferred ledger
                if (nextSeq < curr->span.end())
                    return curr->span.before(nextSeq)->tip();
            }
 
            // We have reached the end of the current span, so we need to
            // find the best child
            Node* best = nullptr;
            std::uint32_t margin = 0;
            if (curr->children.size() == 1)
            {
                best = curr->children[0].get();
                margin = best->branchSupport;
            }
            else if (!curr->children.empty())
            {
                // Sort placing children with largest branch support in the
                // front, breaking ties with the span's starting ID
                std::partial_sort(
                    curr->children.begin(),
                    curr->children.begin() + 2,
                    curr->children.end(),
                    [](std::unique_ptr<Node> const& a,
                       std::unique_ptr<Node> const& b) {
                        return std::make_tuple(
                                   a->branchSupport, a->span.startID()) >
                            std::make_tuple(
                                   b->branchSupport, b->span.startID());
                    });
 
                best = curr->children[0].get();
                margin = curr->children[0]->branchSupport -
                    curr->children[1]->branchSupport;
 
                // If best holds the tie-breaker, gets one larger margin
                // since the second best needs additional branchSupport
                // to overcome the tie
                if (best->span.startID() > curr->children[1]->span.startID())
                    margin++;
            }
 
            // If the best child has margin exceeding the uncommitted support,
            // continue from that child, otherwise we are done
            if (best && ((margin > uncommitted) || (uncommitted == 0)))
                curr = best;
            else  // current is the best
                done = true;
        }
        return curr->span.tip();
    }
 
    bool
    empty() const
    {
        return !root || root->branchSupport == 0;
    }
 
    void
    dump(std::ostream& o) const
    {
        dumpImpl(o, root, 0);
    }
 
    Json::Value
    getJson() const
    {
        Json::Value res;
        res["trie"] = root->getJson();
        res["seq_support"] = Json::objectValue;
        for (auto const& [seq, sup] : seqSupport)
            res["seq_support"][to_string(seq)] = sup;
        return res;
    }
 
    bool
    checkInvariants() const
    {
        std::map<Seq, std::uint32_t> expectedSeqSupport;
 
        std::stack<Node const*> nodes;
        nodes.push(root.get());
        while (!nodes.empty())
        {
            Node const* curr = nodes.top();
            nodes.pop();
            if (!curr)
                continue;
 
            // Node with 0 tip support must have multiple children
            // unless it is the root node
            if (curr != root.get() && curr->tipSupport == 0 &&
                curr->children.size() < 2)
                return false;
 
            // branchSupport = tipSupport + sum(child->branchSupport)
            std::size_t support = curr->tipSupport;
            if (curr->tipSupport != 0)
                expectedSeqSupport[curr->span.end() - Seq{1}] +=
                    curr->tipSupport;
 
            for (auto const& child : curr->children)
            {
                if (child->parent != curr)
                    return false;
 
                support += child->branchSupport;
                nodes.push(child.get());
            }
            if (support != curr->branchSupport)
                return false;
        }
        return expectedSeqSupport == seqSupport;
    }
};
 
}  // namespace ripple
#endif