foundationdb/fdbclient/WriteMap.h

/*
 * WriteMap.h
 *
 * This source file is part of the FoundationDB open source project
 *
 * Copyright 2013-2018 Apple Inc. and the FoundationDB project authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef FDBCLIENT_WRITEMAP_H
#define FDBCLIENT_WRITEMAP_H
#pragma once

#include "fdbclient/FDBTypes.h"
#include "fdbclient/VersionedMap.h"
#include "fdbclient/SnapshotCache.h"
#include "fdbclient/Atomic.h"

struct RYWMutation {
	Optional<ValueRef> value;
	enum MutationRef::Type type;

	RYWMutation(Optional<ValueRef> const& entry, MutationRef::Type type ) : value(entry), type(type) {}
	RYWMutation() : value(), type(MutationRef::NoOp) {}

	bool operator == (const RYWMutation& r) const {
		return value == r.value && type == r.type;
	}
};

class OperationStack {
	private:
		RYWMutation singletonOperation;
		Optional<std::vector<RYWMutation>> optionalOperations;
		bool hasVector() const { return optionalOperations.present(); }
		bool defaultConstructed;
	public:
		OperationStack () { defaultConstructed = true; } // Don't use this!
		explicit OperationStack (RYWMutation initialEntry) { defaultConstructed = false; singletonOperation = initialEntry; }
		void reset(RYWMutation initialEntry) { defaultConstructed = false; singletonOperation = initialEntry; optionalOperations = Optional<std::vector<RYWMutation>>(); }
		void poppush(RYWMutation entry) { if(hasVector()) { optionalOperations.get().pop_back(); optionalOperations.get().push_back(entry); } else singletonOperation = entry; }
		void push(RYWMutation entry) { 
			if(defaultConstructed) { 
				singletonOperation = entry; 
				defaultConstructed = false; 
			} 
			else if(hasVector()) 
				optionalOperations.get().push_back(entry); 
			else { 
				optionalOperations = std::vector<RYWMutation>(); 
				optionalOperations.get().push_back(entry); 
			} 
		}
		bool isDependent() const {
			if( !size() )
				return false;
			return singletonOperation.type != MutationRef::SetValue && singletonOperation.type != MutationRef::ClearRange && singletonOperation.type != MutationRef::SetVersionstampedValue && singletonOperation.type != MutationRef::SetVersionstampedKey;
		}
		const RYWMutation& top() const { return hasVector() ? optionalOperations.get().back() : singletonOperation; }
		RYWMutation& operator[] (int n) { return (n==0) ? singletonOperation : optionalOperations.get()[n-1]; }

		const RYWMutation& at(int n) const { return (n==0) ? singletonOperation : optionalOperations.get()[n-1]; }

		int size() const { return defaultConstructed ? 0 : hasVector() ? optionalOperations.get().size() + 1 : 1; }

		bool operator == (const OperationStack& r) const {
			if (size() != r.size())
				return false;

			if (size() == 0)
				return true;

			if (singletonOperation != r.singletonOperation)
				return false;

			if (size() == 1)
				return true;

			for (int i = 0; i < optionalOperations.get().size(); i++) {
				if (optionalOperations.get()[i] != r.optionalOperations.get()[i])
					return false;
			}

			return true;
		}
};

struct WriteMapEntry {
	KeyRef key;
	OperationStack stack;
	bool following_keys_cleared;
	bool following_keys_conflict;
	bool is_conflict;
	bool following_keys_unreadable;
	bool is_unreadable;

	WriteMapEntry( KeyRef const& key, OperationStack && stack, bool following_keys_cleared, bool following_keys_conflict, bool is_conflict, bool following_keys_unreadable, bool is_unreadable ) : key(key), stack(std::move(stack)), following_keys_cleared(following_keys_cleared), following_keys_conflict(following_keys_conflict), is_conflict(is_conflict), following_keys_unreadable(following_keys_unreadable), is_unreadable(is_unreadable) {}

	std::string toString() const { return printable(key); }
};

inline bool operator < ( const WriteMapEntry& lhs, const WriteMapEntry& rhs ) { return lhs.key < rhs.key; }
inline bool operator < ( const WriteMapEntry& lhs, const StringRef& rhs ) { return lhs.key < rhs; }
inline bool operator < ( const StringRef& lhs, const WriteMapEntry& rhs ) { return lhs < rhs.key; }
inline bool operator < ( const WriteMapEntry& lhs, const ExtStringRef& rhs ) { return rhs.cmp(lhs.key)>0; }
inline bool operator < ( const ExtStringRef& lhs, const WriteMapEntry& rhs ) { return lhs.cmp(rhs.key)<0; }

class WriteMap {
private:
	typedef PTreeImpl::PTree< WriteMapEntry > PTreeT;
	typedef Reference<PTreeT> Tree;

public:
	explicit WriteMap(Arena* arena) : arena(arena), ver(-1), scratch_iterator(this), writeMapEmpty(true)  {
		PTreeImpl::insert( writes, ver, WriteMapEntry( allKeys.begin, OperationStack(), false, false, false, false, false ) );
		PTreeImpl::insert( writes, ver, WriteMapEntry( allKeys.end, OperationStack(), false, false, false, false, false ) );
		PTreeImpl::insert( writes, ver, WriteMapEntry( afterAllKeys, OperationStack(), false, false, false, false, false ) );
	}

	WriteMap(WriteMap&& r) BOOST_NOEXCEPT : writeMapEmpty(r.writeMapEmpty), writes(std::move(r.writes)), ver(r.ver), scratch_iterator(std::move(r.scratch_iterator)), arena(r.arena) {}
	WriteMap& operator=(WriteMap&& r) BOOST_NOEXCEPT { writeMapEmpty = r.writeMapEmpty; writes = std::move(r.writes); ver = r.ver; scratch_iterator = std::move(r.scratch_iterator); arena = r.arena; return *this; }

	//a write with addConflict false on top of an existing write with a conflict range will not remove the conflict
	void mutate( KeyRef key, MutationRef::Type operation, ValueRef param, bool addConflict ) {
		writeMapEmpty = false;
		auto& it = scratch_iterator;
		it.reset(writes, ver);
		it.skip( key );
		
		bool is_cleared = it.entry().following_keys_cleared;
		bool following_conflict = it.entry().following_keys_conflict;
		bool is_conflict = addConflict || it.is_conflict_range();
		bool following_unreadable = it.entry().following_keys_unreadable;
		bool is_unreadable = it.is_unreadable() || operation == MutationRef::SetVersionstampedValue ||  operation == MutationRef::SetVersionstampedKey;
		bool is_dependent = operation != MutationRef::SetValue && operation != MutationRef::SetVersionstampedValue && operation != MutationRef::SetVersionstampedKey;

		if (it.entry().key != key) {
			if( it.is_cleared_range() && is_dependent ) {
				it.tree.clear();
				OperationStack op( RYWMutation( Optional<StringRef>(), MutationRef::SetValue ) );
				coalesceOver(op, RYWMutation(param, operation), *arena);
				PTreeImpl::insert( writes, ver, WriteMapEntry( key, std::move(op), true, following_conflict, is_conflict, following_unreadable, is_unreadable ) );
			} else {
				it.tree.clear();
				PTreeImpl::insert( writes, ver, WriteMapEntry( key, OperationStack( RYWMutation( param, operation ) ), is_cleared, following_conflict, is_conflict, following_unreadable, is_unreadable ) );
			}
		} else {
			if( !it.is_unreadable() && operation == MutationRef::SetValue ) {
				it.tree.clear();
				PTreeImpl::remove( writes, ver, key );
				PTreeImpl::insert( writes, ver, WriteMapEntry( key, OperationStack( RYWMutation( param, operation ) ), is_cleared, following_conflict, is_conflict, following_unreadable, is_unreadable ) );
			} else {
				WriteMapEntry e( it.entry() );
				e.is_conflict = is_conflict;
				e.is_unreadable = is_unreadable;
				if (e.stack.size() == 0 && it.is_cleared_range() && is_dependent)  {
					e.stack.push(RYWMutation(Optional<StringRef>(), MutationRef::SetValue));
					coalesceOver(e.stack, RYWMutation(param, operation), *arena);
				} else if( !is_unreadable && e.stack.size() > 0 )
					coalesceOver( e.stack, RYWMutation( param, operation ), *arena );
				else
					e.stack.push( RYWMutation( param, operation ) );

				it.tree.clear();
				PTreeImpl::remove( writes, ver, e.key ); // FIXME: Make PTreeImpl::insert do this automatically (see also VersionedMap.h FIXME)
				PTreeImpl::insert( writes, ver, std::move(e) );
			}
		}
	}

	void clear( KeyRangeRef keys, bool addConflict ) {
		writeMapEmpty = false;
		if( !addConflict ) {
			clearNoConflict( keys );
			return;
		}
		
		auto& it = scratch_iterator;
		it.reset(writes, ver);
		it.skip( keys.begin );

		bool insert_begin = !it.is_cleared_range() || !it.is_conflict_range() || it.is_unreadable();

		if(it.endKey() == keys.end) {
			++it;
		} else if(it.endKey() < keys.end) {
			it.skip( keys.end );
		}

		bool insert_end = ( it.is_unmodified_range() || !it.is_conflict_range() || it.is_unreadable() ) && ( !it.keyAtBegin() || it.beginKey() != keys.end );
		bool end_coalesce_clear = it.is_cleared_range() && it.beginKey() == keys.end && it.is_conflict_range() && !it.is_unreadable();
		bool end_conflict = it.is_conflict_range();
		bool end_cleared = it.is_cleared_range();
		bool end_unreadable = it.is_unreadable();

		it.tree.clear();
		
		PTreeImpl::remove( writes, ver, ExtStringRef(keys.begin, !insert_begin ? 1 : 0), ExtStringRef(keys.end, end_coalesce_clear ? 1 : 0) );
		
		if (insert_begin)
			PTreeImpl::insert( writes, ver, WriteMapEntry( keys.begin, OperationStack(), true, true, true, false, false ) );

		if (insert_end)
			PTreeImpl::insert( writes, ver, WriteMapEntry( keys.end, OperationStack(), end_cleared, end_conflict, end_conflict, end_unreadable, end_unreadable ) );
	}

	void addUnmodifiedAndUnreadableRange( KeyRangeRef keys ) {
		auto& it = scratch_iterator;
		it.reset(writes, ver);
		it.skip( keys.begin );

		bool insert_begin = !it.is_unmodified_range() || it.is_conflict_range() || !it.is_unreadable();

		if(it.endKey() == keys.end) {
			++it;
		} else if(it.endKey() < keys.end) {
			it.skip( keys.end );
		}

		bool insert_end = ( it.is_cleared_range() || it.is_conflict_range() || !it.is_unreadable() ) && ( !it.keyAtBegin() || it.beginKey() != keys.end );
		bool end_coalesce_unmodified = it.is_unmodified_range() && it.beginKey() == keys.end && !it.is_conflict_range() && it.is_unreadable();
		bool end_conflict = it.is_conflict_range();
		bool end_cleared = it.is_cleared_range();
		bool end_unreadable = it.is_unreadable();

		it.tree.clear();
		
		PTreeImpl::remove( writes, ver, ExtStringRef(keys.begin, !insert_begin ? 1 : 0), ExtStringRef(keys.end, end_coalesce_unmodified ? 1 : 0) );
		
		if (insert_begin)
			PTreeImpl::insert( writes, ver, WriteMapEntry( keys.begin, OperationStack(), false, false, false, true, true ) );

		if (insert_end)
			PTreeImpl::insert( writes, ver, WriteMapEntry( keys.end, OperationStack(), end_cleared, end_conflict, end_conflict, end_unreadable, end_unreadable ) );
	}

	void addConflictRange( KeyRangeRef keys ) {
		writeMapEmpty = false;
		auto& it = scratch_iterator;
		it.reset(writes, ver);
		it.skip( keys.begin );

		std::vector<ExtStringRef> removals;
		std::vector<WriteMapEntry> insertions;
		
		if( !it.entry().following_keys_conflict || !it.entry().is_conflict ) {
			if( it.keyAtBegin() && it.beginKey() == keys.begin ) {
				removals.push_back( keys.begin );
			}
			insertions.push_back( WriteMapEntry( keys.begin, it.is_operation() ? OperationStack( it.op() ) : OperationStack(), it.entry().following_keys_cleared, true, true, it.entry().following_keys_unreadable, it.entry().is_unreadable ) );
		}

		while ( it.endKey() < keys.end ) {
			++it;
			if (it.keyAtBegin() && (!it.entry().following_keys_conflict || !it.entry().is_conflict) ) {
				WriteMapEntry e( it.entry() );
				e.following_keys_conflict = true;
				e.is_conflict = true;
				removals.push_back( e.key );
				insertions.push_back( std::move(e) );
			}
		}

		ASSERT( it.beginKey() != keys.end );
		if( !it.entry().following_keys_conflict || !it.entry().is_conflict ) {
			bool isCleared = it.entry().following_keys_cleared;
			bool isUnreadable = it.entry().is_unreadable;
			bool followingUnreadable = it.entry().following_keys_unreadable;
			++it;

			if ( !it.keyAtBegin() || it.beginKey() != keys.end ) {
				insertions.push_back(WriteMapEntry(keys.end, OperationStack(), isCleared, false, false, followingUnreadable, isUnreadable));
			}
		}

		it.tree.clear();

		//SOMEDAY: optimize this code by having a PTree removal/insertion that takes and returns an iterator
		for( int i = 0; i < removals.size(); i++ ) {
			PTreeImpl::remove( writes, ver, removals[i] ); // FIXME: Make PTreeImpl::insert do this automatically (see also VersionedMap.h FIXME)
		}

		for( int i = 0; i < insertions.size(); i++ ) {
			PTreeImpl::insert( writes, ver, std::move(insertions[i]) );
		}
	}

	struct iterator {
		// Iterates over three types of segments: unmodified ranges, cleared ranges, and modified keys.
		// Modified keys may be dependent (need to be collapsed with a snapshot value) or independent (value is known regardless of the snapshot value)
		// Every key will belong to exactly one segment.  The first segment begins at "" and the last segment ends at \xff\xff.

		explicit iterator( WriteMap* map ) : tree(map->writes), at( map->ver ), offset(false) { ++map->ver; }
			// Creates an iterator which is conceptually before the beginning of map (you may essentially only call skip() or ++ on it)
			// This iterator also represents a snapshot (will be unaffected by future writes)

		enum SEGMENT_TYPE { UNMODIFIED_RANGE, CLEARED_RANGE, INDEPENDENT_WRITE, DEPENDENT_WRITE };

		SEGMENT_TYPE type() {
			if (offset) 
				return entry().following_keys_cleared ? CLEARED_RANGE : UNMODIFIED_RANGE;
			else
				return entry().stack.isDependent() ? DEPENDENT_WRITE : INDEPENDENT_WRITE;
		}
		bool is_cleared_range() { return offset && entry().following_keys_cleared; }
		bool is_unmodified_range() { return offset && !entry().following_keys_cleared; }
		bool is_operation() { return !offset; }
		bool is_conflict_range() { return offset ? entry().following_keys_conflict : entry().is_conflict; }
		bool is_unreadable() { return offset ? entry().following_keys_unreadable : entry().is_unreadable; }

		bool is_independent() { return entry().following_keys_cleared || !entry().stack.isDependent(); }  // Defined if is_operation()

		ExtStringRef beginKey() { return ExtStringRef( entry().key, offset && entry().stack.size() ); }
		ExtStringRef endKey() { return offset ? nextEntry().key : ExtStringRef( entry().key, 1 ); }

		OperationStack const& op() { return entry().stack; }  // Only if is_operation()
		
		iterator& operator++() {
			if (!offset && !equalsKeyAfter( entry().key, nextEntry().key )) {
				offset = true;
			} else {
				beginLen = endLen;
				finger.resize( beginLen );
				endLen = PTreeImpl::halfNext( at, finger );
				offset = !entry().stack.size();
			}
			return *this;
		}
		iterator& operator--() { 
			if (offset && entry().stack.size() ) {
				offset = false;
			} else {
				endLen = beginLen;
				finger.resize(endLen);
				beginLen = PTreeImpl::halfPrevious( at, finger );
				offset = !entry().stack.size() || !equalsKeyAfter( entry().key, nextEntry().key );
			}
			return *this; 
		}
		bool operator == ( const iterator& r ) const { return offset == r.offset && beginLen == r.beginLen && finger[beginLen-1] == r.finger[beginLen-1]; }

		void skip( KeyRef key ) {      // Changes *this to the segment containing key (so that beginKey()<=key && key < endKey())
			finger.clear();

			if( key == allKeys.end )
				PTreeImpl::last(tree, at, finger);
			else
				PTreeImpl::upper_bound( tree, at, key, finger );
			endLen = finger.size();
			beginLen = PTreeImpl::halfPrevious( at, finger );

			offset = !entry().stack.size() || (entry().key != key);
		}
	private:
		friend class WriteMap;
		void reset( Tree const& tree, Version ver ) { this->tree = tree; this->at = ver; this->finger.clear(); beginLen=endLen=0; offset = false; }

		WriteMapEntry const& entry() { return finger[beginLen-1]->data; }
		WriteMapEntry const& nextEntry() { return finger[endLen-1]->data; }

		bool keyAtBegin() { return !offset || !entry().stack.size(); }

		Tree tree;
		Version at;
		int beginLen, endLen;
		vector< PTreeT const* > finger;
		bool offset;  // false-> the operation stack at entry(); true-> the following cleared or unmodified range
	};

	bool empty() const { return writeMapEmpty; }

	static RYWMutation coalesce(RYWMutation existingEntry, RYWMutation newEntry, Arena& arena) {
		ASSERT(newEntry.value.present());

		if (newEntry.type == MutationRef::SetValue)
			return newEntry;
		else if (newEntry.type == MutationRef::AddValue) {
			switch(existingEntry.type) {
				case MutationRef::SetValue:
					return RYWMutation(doLittleEndianAdd(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
				case MutationRef::AddValue:
					return RYWMutation(doLittleEndianAdd(existingEntry.value, newEntry.value.get(), arena), MutationRef::AddValue);
				default:
					throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::CompareAndClear) {
			switch (existingEntry.type) {
			case MutationRef::SetValue:
				if (doCompareAndClear(existingEntry.value, newEntry.value.get(), arena).present()) {
					return existingEntry;
				} else {
					return RYWMutation(Optional<ValueRef>(), MutationRef::SetValue);
				}
			default:
				throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::AppendIfFits) {
			switch(existingEntry.type) {
				case MutationRef::SetValue:
					return RYWMutation(doAppendIfFits(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
				case MutationRef::AppendIfFits:
					return RYWMutation(doAppendIfFits(existingEntry.value, newEntry.value.get(), arena), MutationRef::AppendIfFits);
				default:
					throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::And) {
			switch(existingEntry.type) {
				case MutationRef::SetValue:
					return RYWMutation(doAnd(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
				case MutationRef::And:
					return RYWMutation(doAnd(existingEntry.value, newEntry.value.get(), arena), MutationRef::And);
				default:
					throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::Or) {
			switch(existingEntry.type) {
				case MutationRef::SetValue:
					return RYWMutation(doOr(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
				case MutationRef::Or:
					return RYWMutation(doOr(existingEntry.value, newEntry.value.get(), arena), MutationRef::Or);
				default:
					throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::Xor) {
			switch(existingEntry.type) {
				case MutationRef::SetValue:
					return RYWMutation(doXor(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
				case MutationRef::Xor:
					return RYWMutation(doXor(existingEntry.value, newEntry.value.get(), arena), MutationRef::Xor);
				default:
					throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::Max) {
			switch (existingEntry.type) {
			case MutationRef::SetValue:
				return RYWMutation(doMax(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
			case MutationRef::Max:
				return RYWMutation(doMax(existingEntry.value, newEntry.value.get(), arena), MutationRef::Max);
			default:
				throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::Min) {
			switch (existingEntry.type) {
				case MutationRef::SetValue:
					return RYWMutation(doMin(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
				case MutationRef::Min:
					return RYWMutation(doMin(existingEntry.value, newEntry.value.get(), arena), MutationRef::Min);
				default:
					throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::ByteMin) {
			switch (existingEntry.type) {
			case MutationRef::SetValue:
				return RYWMutation(doByteMin(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
			case MutationRef::ByteMin:
				return RYWMutation(doByteMin(existingEntry.value, newEntry.value.get(), arena), MutationRef::ByteMin);
			default:
				throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::ByteMax) {
			switch (existingEntry.type) {
			case MutationRef::SetValue:
				return RYWMutation(doByteMax(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
			case MutationRef::ByteMax:
				return RYWMutation(doByteMax(existingEntry.value, newEntry.value.get(), arena), MutationRef::ByteMax);
			default:
				throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::MinV2) {
			switch (existingEntry.type) {
			case MutationRef::SetValue:
				return RYWMutation(doMinV2(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
			case MutationRef::MinV2:
				return RYWMutation(doMinV2(existingEntry.value, newEntry.value.get(), arena), MutationRef::MinV2);
			default:
				throw operation_failed();
			}
		} else if (newEntry.type == MutationRef::AndV2) {
			switch (existingEntry.type) {
			case MutationRef::SetValue:
				return RYWMutation(doAndV2(existingEntry.value, newEntry.value.get(), arena), MutationRef::SetValue);
			case MutationRef::AndV2:
				return RYWMutation(doAndV2(existingEntry.value, newEntry.value.get(), arena), MutationRef::AndV2);
			default:
				throw operation_failed();
			}
		} else
			throw operation_failed();
	}
		
	static void coalesceOver(OperationStack& stack, RYWMutation newEntry, Arena& arena) {
		RYWMutation existingEntry = stack.top();
		if (existingEntry.type == newEntry.type && newEntry.type != MutationRef::CompareAndClear) {
			if (isNonAssociativeOp(existingEntry.type) && existingEntry.value.present() && existingEntry.value.get().size() != newEntry.value.get().size()) {
				stack.push(newEntry);
			}
			else {
				stack.poppush(coalesce(existingEntry, newEntry, arena));
			}
		}
		else {
			if (isAtomicOp(newEntry.type) && isAtomicOp(existingEntry.type)) {
				stack.push(newEntry);
			}
			else {
				stack.poppush(coalesce(existingEntry, newEntry, arena));
			}
		}
	}

	static RYWMutation coalesceUnder(OperationStack const& stack, Optional<ValueRef> const& value, Arena& arena) {
		if( !stack.isDependent() && stack.size() == 1 )
			return stack.at(0);

		RYWMutation currentEntry = RYWMutation( value, MutationRef::SetValue);
		for(int i = 0; i < stack.size(); ++i) {
			currentEntry = coalesce(currentEntry, stack.at(i), arena);
		}

		return currentEntry;
	}

private:
	friend class ReadYourWritesTransaction;
	Arena* arena;
	bool writeMapEmpty;
	Tree writes;
	Version ver;  // an internal version number for the tree - no connection to database versions!  Currently this is incremented after reads, so that consecutive writes have the same version and those separated by reads have different versions.
	iterator scratch_iterator;   // Avoid unnecessary memory allocation in write operations

	void dump() {
		iterator it( this );
		it.skip(allKeys.begin);
		while( it.beginKey() < allKeys.end ) {
			TraceEvent("WriteMapDump").detail("Begin", it.beginKey().toStandaloneStringRef())
				.detail("End", it.endKey().toStandaloneStringRef())
				.detail("Cleared", it.is_cleared_range())
				.detail("Conflicted", it.is_conflict_range())
				.detail("Operation", it.is_operation())
				.detail("Unmodified", it.is_unmodified_range())
				.detail("Independent", it.is_operation() && it.is_independent())
				.detail("StackSize", it.is_operation() ? it.op().size() : 0);
			++it;
		}
	}

	//SOMEDAY: clearNoConflict replaces cleared sets with two map entries for everyone one item cleared
	void clearNoConflict( KeyRangeRef keys ) {
		auto& it = scratch_iterator;
		it.reset(writes, ver);

		//Find all write conflict ranges within the cleared range
		it.skip( keys.begin );

		bool insert_begin = !it.is_cleared_range() || it.is_unreadable();

		bool lastConflicted = it.is_conflict_range();
		
		bool conflicted = lastConflicted;
		std::vector<ExtStringRef> conflict_ranges;
		
		if( insert_begin ) {
			conflict_ranges.push_back( keys.begin );
		} else {
			conflicted = !conflicted;
		}

		while( it.endKey() < keys.end ) {
			++it;
			if ( lastConflicted != it.is_conflict_range() ) {
				conflict_ranges.push_back( it.beginKey() );
				lastConflicted = it.is_conflict_range();
			}
		}

		if( it.endKey() == keys.end )
			++it;

		ASSERT( it.beginKey() <= keys.end && keys.end < it.endKey() );

		bool insert_end = ( ( it.is_unmodified_range() || it.is_unreadable() ) && ( !it.keyAtBegin() || it.beginKey() != keys.end ) ) || ( it.entry().is_conflict && !it.entry().following_keys_conflict && it.beginKey() == keys.end && !it.keyAtBegin() );
		bool end_cleared = it.is_cleared_range();
		bool end_coalesce_clear = it.is_cleared_range() && it.beginKey() == keys.end && it.is_conflict_range()==lastConflicted && !it.is_unreadable();
		bool end_conflict = it.is_conflict_range();
		bool end_unreadable = it.is_unreadable();

		TEST( it.is_conflict_range() != lastConflicted );

		it.tree.clear();
		
		PTreeImpl::remove( writes, ver, ExtStringRef(keys.begin, !insert_begin ? 1 : 0), ExtStringRef(keys.end, end_coalesce_clear ? 1 : 0) );
		
		for( int i = 0; i < conflict_ranges.size(); i++ ) {
			PTreeImpl::insert( writes, ver, WriteMapEntry( conflict_ranges[i].toArenaOrRef(*arena), OperationStack(), true, conflicted, conflicted, false, false ) );
			conflicted = !conflicted;
		}

		ASSERT( conflicted != lastConflicted );

		if (insert_end)
			PTreeImpl::insert( writes, ver, WriteMapEntry( keys.end, OperationStack(), end_cleared, end_conflict, end_conflict, end_unreadable, end_unreadable ) );
	}
};

	/*

		for write in writes:   # write.type in [ 'none', 'clear', 'independent', 'dependent' ]
			for read in reads[ write.begin : write.end ]:   # read.type in [ 'unknown', 'empty', 'value' ]
				if write.type == "none":
					yield read
				elif write.type == "clear":
					yield empty()
				elif write.type == "independent":
					yield value( write )
				else:  # Dependent write
					if read.type == "unknown":
						yield read
					else:
						yield value( collapse( read, write ) )

	*/

#endif