iterator.go

// Copyright 2014 The Cayley Authors. All rights reserved.
//
// 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.

package iterator

// Define the general iterator interface.

import (
	"sync/atomic"

	"github.com/cayleygraph/cayley/graph"
)

var nextIteratorID uint64

func init() {
	atomic.StoreUint64(&nextIteratorID, 1)
}

func NextUID() uint64 {
	return atomic.AddUint64(&nextIteratorID, 1) - 1
}

// Here we define the simplest iterator -- the Null iterator. It contains nothing.
// It is the empty set. Often times, queries that contain one of these match nothing,
// so it's important to give it a special iterator.
type Null struct {
	uid  uint64
	tags graph.Tagger
}

// Fairly useless New function.
func NewNull() *Null {
	return &Null{uid: NextUID()}
}

func (it *Null) UID() uint64 {
	return it.uid
}

func (it *Null) Tagger() *graph.Tagger {
	return &it.tags
}

// Fill the map based on the tags assigned to this iterator.
func (it *Null) TagResults(dst map[string]graph.Value) {
	for _, tag := range it.tags.Tags() {
		dst[tag] = it.Result()
	}

	for tag, value := range it.tags.Fixed() {
		dst[tag] = value
	}
}

func (it *Null) Contains(graph.Value) bool {
	return false
}

func (it *Null) Clone() graph.Iterator { return NewNull() }

// Name the null iterator.
func (it *Null) Type() graph.Type { return graph.Null }

// A good iterator will close itself when it returns true.
// Null has nothing it needs to do.
func (it *Null) Optimize() (graph.Iterator, bool) { return it, false }

func (it *Null) Describe() graph.Description {
	return graph.Description{
		UID:  it.UID(),
		Type: it.Type(),
	}
}

func (it *Null) Next() bool {
	return false
}

func (it *Null) Err() error {
	return nil
}

func (it *Null) Result() graph.Value {
	return nil
}

func (it *Null) SubIterators() []graph.Iterator {
	return nil
}

func (it *Null) NextPath() bool {
	return false
}

func (it *Null) Size() (int64, bool) {
	return 0, true
}

func (it *Null) Reset() {}

func (it *Null) Close() error {
	return nil
}

// A null iterator costs nothing. Use it!
func (it *Null) Stats() graph.IteratorStats {
	return graph.IteratorStats{}
}

var _ graph.Iterator = &Null{}