source: src/Common/PersistentMap.h @ 42f1279c

//
// Cforall Version 1.0.0 Copyright (C) 2015 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// PersistentMap.h --
//
// Author           : Aaron B. Moss
// Created On       : Thu Mar  7 15:50:00 2019
// Last Modified By : Aaron B. Moss
// Last Modified On : Thu Mar  7 15:50:00 2019
// Update Count     : 1
//

#pragma once

#include <cassert>        // for assertf
#include <cstddef>        // for size_t
#include <functional>     // for hash, equal_to
#include <memory>         // for shared_ptr, enable_shared_from_this, make_shared
#include <unordered_map>  // for unordered_map
#include <utility>        // for forward, move

/// Wraps a hash table in a persistent data structure, using a technique based 
/// on the persistent array in Conchon & Filliatre "A Persistent Union-Find 
/// Data Structure"

template<typename Key, typename Val,
         typename Hash = std::hash<Key>, typename Eq = std::equal_to<Key>>
class PersistentMap 
        : public std::enable_shared_from_this<PersistentMap<Key, Val, Hash, Eq>> {
public:
        /// Type of this class
        using Self = PersistentMap<Key, Val, Hash, Eq>;
        /// Type of pointer to this class
        using Ptr = std::shared_ptr<Self>;

        /// Types of version nodes
        enum Mode { 
                BASE,  ///< Root node of version tree
                REM,   ///< Key removal node
                INS,   ///< Key update node
                UPD    ///< Key update node
        };

private:
        using Base = std::unordered_map<Key, Val, Hash, Eq>;

        /// Insertion/update node
        struct Ins {
                Ptr base;  ///< Modified map
                Key key;   ///< Key inserted
                Val val;   ///< Value stored

                template<typename P, typename K, typename V>
                Ins(P&& p, K&& k, V&& v)
                : base(std::forward<P>(p)), key(std::forward<K>(k)), val(std::forward<V>(v)) {}
        };

        /// Removal node
        struct Rem {
                Ptr base;  ///< Modified map
                Key key;   ///< Key removed
                
                template<typename P, typename K>
                Rem(P&& p, K&& k) : base(std::forward<P>(p)), key(std::forward<K>(k)) {}
        };

        /// Underlying storage
        union Data {
                char def;
                Base base;
                Ins ins;
                Rem rem;

                Data() : def('\0') {}
                ~Data() {}
        } data;

        /// Type of node
        mutable Mode mode;

        /// get mutable reference as T
        template<typename T>
        T& as() { return reinterpret_cast<T&>(data); }

        /// get const reference as T
        template<typename T>
        const T& as() const { return reinterpret_cast<const T&>(data); }

        /// get rvalue reference as T
        template<typename T>
        T&& take_as() { return std::move(as<T>()); }

        /// initialize as T
        template<typename T, typename... Args>
        void init( Args&&... args ) {
                new( &as<T>() ) T { std::forward<Args>(args)... };
        }

        /// reset as current mode
        void reset() {
                switch( mode ) {
                        case BASE:          as<Base>().~Base(); break;
                        case REM:           as<Rem>().~Rem();   break;
                        case INS: case UPD: as<Ins>().~Ins();   break;
                }
        }

        /// reset as base
        void reset_as_base() {
                as<Base>().~Base();
        }

        /// check if this is the only reference to itself
        /// NOTE: optimizations employing this function are not thread-safe
        bool is_shared(long local_ptrs = 0) const {
                // 2 accounts for both the pointer which owns "this" and the one created for its use_count
                return (this->shared_from_this().use_count() - local_ptrs) > 2;
        }

public:
        using key_type = typename Base::key_type;
        using mapped_type = typename Base::mapped_type;
        using value_type = typename Base::value_type;
        using size_type = typename Base::size_type;
        using difference_type = typename Base::difference_type;
        using iterator = typename Base::const_iterator;

        PersistentMap() : data(), mode(BASE) { init<Base>(); }

        PersistentMap( Base&& b ) : data(), mode(BASE) { init<Base>(std::move(b)); }

        PersistentMap( const Self& o ) = delete;

        Self& operator= ( const Self& o ) = delete;

        ~PersistentMap() { reset(); }

        /// Create a pointer to a new, empty persistent map
        static Ptr new_ptr() { return std::make_shared<Self>(); }

        /// reroot persistent map at current node
        void reroot() const {
                // recursive base case
                if ( mode == BASE ) return;

                // reroot base
                Self* mut_this = const_cast<Self*>(this);
                Ptr base = ( mode == REM ) ? mut_this->as<Rem>().base : mut_this->as<Ins>().base;
                base->reroot();

                // remove map from base
                Base base_map = base->take_as<Base>();

                // switch base to inverse of self and mutate base map
                switch ( mode ) {
                        case REM: {
                                Rem& self = mut_this->as<Rem>();
                                auto it = base_map.find( self.key );

                                if ( base->is_shared( 1 ) ) {
                                        base->reset_as_base();
                                        base->init<Ins>( 
                                                mut_this->shared_from_this(), std::move(self.key), std::move(it->second) );
                                        base->mode = INS;
                                }

                                base_map.erase( it );
                                break;
                        }
                        case INS: {
                                Ins& self = mut_this->as<Ins>();

                                if ( base->is_shared( 1 ) ) {
                                        base->reset_as_base();
                                        base->init<Rem>( mut_this->shared_from_this(), self.key );
                                        base->mode = REM;
                                }

                                base_map.emplace( std::move(self.key), std::move(self.val) );
                                break;
                        }
                        case UPD: {
                                Ins& self = mut_this->as<Ins>();
                                auto it = base_map.find( self.key );

                                if ( base->is_shared( 1 ) ) {
                                        base->reset_as_base();
                                        base->init<Ins>( 
                                                mut_this->shared_from_this(), std::move(self.key), std::move(it->second) );
                                        base->mode = UPD;
                                }

                                it->second = std::move(self.val);
                                break;
                        }
                        case BASE: assertf(false, "unreachable"); break;
                }

                // set base map into self
                mut_this->reset();
                mut_this->init<Base>( std::move(base_map) );
                mode = BASE;
        }

private:
        /// the base after rerooting at the current node
        const Base& rerooted() const {
                reroot();
                return as<Base>();
        }

public:
        /// true iff the map is empty
        bool empty() const { return rerooted().empty(); }

        /// number of entries in map
        size_type size() const { return rerooted().size(); }

        /// begin iterator for map; may be invalidated by calls to non-iteration functions
        /// or functions on other maps in the same tree
        iterator begin() const { return rerooted().begin(); }

        /// end iterator for map; may be invalidated by calls to non-iteration functions
        /// or functions on other maps in the same tree
        iterator end() const { return rerooted().end(); }

        /// underlying map iterator for value
        iterator find(const Key& k) const { return rerooted().find( k ); }

        /// check if value is present
        size_type count(const Key& k) const { return rerooted().count( k ); }

        /// get value; undefined behaviour if not present
        const Val& get(const Key& k) const {
                const Base& self = rerooted();
                auto it = self.find( k );
                return it->second;
        }

        /// get value; returns default if not present
        template<typename V>
        Val get_or_default(const Key& k, V&& d) const {
                const Base& self = rerooted();
                auto it = self.find( k );
                if ( it == self.end() ) return d;
                else return it->second;
        }

        /// set value, storing new map in output variable
        template<typename K, typename V>
        Ptr set(K&& k, V&& v) {
                reroot();

                // transfer map to new node
                Ptr ret = std::make_shared<Self>( take_as<Base>() );
                Base& base_map = ret->as<Base>();

                // check if this is update or insert
                auto it = base_map.find( k );
                if ( it == base_map.end() ) {
                        if ( is_shared() ) {
                                // set self to REM node and insert into base
                                reset_as_base();
                                init<Rem>( ret, k );
                                mode = REM;
                        }

                        base_map.emplace_hint( it, std::forward<K>(k), std::forward<V>(v) );
                } else {
                        if ( is_shared() ) {
                                // set self to UPD node and modify base
                                reset_as_base();
                                init<Ins>( ret, std::forward<K>(k), std::move(it->second) );
                                mode = UPD;
                        }

                        it->second = std::forward<V>(v);
                }

                return ret;
        }

        /// remove value, storing new map in output variable; does nothing if key not in map
        Ptr erase(const Key& k) {
                reroot();
                
                // exit early if key does not exist in map
                if ( ! as<Base>().count( k ) ) return this->shared_from_this();

                // transfer map to new node
                Ptr ret = std::make_shared<Self>( take_as<Base>() );
                Base& base_map = ret->as<Base>();

                if ( is_shared() ) {
                        // set self to INS node and remove from base
                        reset_as_base();
                        init<Ins>( ret, k, base_map[k] );
                        mode = INS;
                }

                base_map.erase( k );

                return ret;
        }
};

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