Revision 48ad042083b8bca89eec4dae96dd78416d8c8776 authored by Iddan Aaronsohn on 22 October 2019, 21:50:39 UTC, committed by Iddan Aaronsohn on 22 October 2019, 22:17:08 UTC
1 parent 9c79218
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 (
"context"
"fmt"
"github.com/cayleygraph/cayley/graph/refs"
)
var (
_ Shape = &Null{}
_ Shape = &Error{}
)
// TaggerBase is a base interface for Tagger and TaggerShape.
type TaggerBase interface {
Tags() []string
FixedTags() map[string]refs.Ref
AddTags(tag ...string)
AddFixedTag(tag string, value refs.Ref)
}
// Base is a set of common methods for Scanner and Index iterators.
type Base interface {
// String returns a short textual representation of an iterator.
String() string
// Fills a tag-to-result-value map.
TagResults(map[string]refs.Ref)
// Returns the current result.
Result() refs.Ref
// These methods are the heart and soul of the iterator, as they constitute
// the iteration interface.
//
// To get the full results of iteration, do the following:
//
// for it.Next(ctx) {
// val := it.Result()
// ... do things with val.
// for it.NextPath(ctx) {
// ... find other paths to iterate
// }
// }
//
// All of them should set iterator.result to be the last returned value, to
// make results work.
//
// NextPath() advances iterators that may have more than one valid result,
// from the bottom up.
NextPath(ctx context.Context) bool
// Err returns any error that was encountered by the Iterator.
Err() error
// TODO: make a requirement that Err should return ErrClosed after Close is called
// Close the iterator and do internal cleanup.
Close() error
}
// Scanner is an iterator that lists all results sequentially, but not necessarily in a sorted order.
type Scanner interface {
Base
// Next advances the iterator to the next value, which will then be available through
// the Result method. It returns false if no further advancement is possible, or if an
// error was encountered during iteration. Err should be consulted to distinguish
// between the two cases.
Next(ctx context.Context) bool
}
// Index is an index lookup iterator. It allows to check if an index contains a specific value.
type Index interface {
Base
// Contains returns whether the value is within the set held by the iterator.
//
// It will set Result to the matching subtree. TagResults can be used to collect values from tree branches.
Contains(ctx context.Context, v refs.Ref) bool
}
// Tagger is an interface for iterators that can tag values. Tags are returned as a part of TagResults call.
type TaggerShape interface {
Shape
TaggerBase
CopyFromTagger(st TaggerBase)
}
type Costs struct {
ContainsCost int64
NextCost int64
Size refs.Size
}
// Shape is an iterator shape, similar to a query plan. But the plan is not specific in this
// case - it is used to reorder query branches, and the decide what branches will be scanned
// and what branches will lookup values (hopefully from the index, but not necessarily).
type Shape interface {
// TODO(dennwc): merge with shape.Shape
// String returns a short textual representation of an iterator.
String() string
// Iterate starts this iterator in scanning mode. Resulting iterator will list all
// results sequentially, but not necessary in the sorted order. Caller must close
// the iterator.
Iterate() Scanner
// Lookup starts this iterator in an index lookup mode. Depending on the iterator type,
// this may still involve database scans. Resulting iterator allows to check an index
// contains a specified value. Caller must close the iterator.
Lookup() Index
// These methods relate to choosing the right iterator, or optimizing an
// iterator tree
//
// Stats() returns the relative costs of calling the iteration methods for
// this iterator, as well as the size. Roughly, it will take NextCost * Size
// "cost units" to get everything out of the iterator. This is a wibbly-wobbly
// thing, and not exact, but a useful heuristic.
Stats(ctx context.Context) (Costs, error)
// Optimizes an iterator. Can replace the iterator, or merely move things
// around internally. if it chooses to replace it with a better iterator,
// returns (the new iterator, true), if not, it returns (self, false).
Optimize(ctx context.Context) (Shape, bool)
// Return a slice of the subiterators for this iterator.
SubIterators() []Shape
}
type Morphism func(Shape) Shape
func IsNull(it Shape) bool {
if _, ok := it.(*Null); ok {
return true
}
return false
}
// Height is a convienence function to measure the height of an iterator tree.
func Height(it Shape, filter func(Shape) bool) int {
if filter != nil && !filter(it) {
return 1
}
subs := it.SubIterators()
maxDepth := 0
for _, sub := range subs {
h := Height(sub, filter)
if h > maxDepth {
maxDepth = h
}
}
return maxDepth + 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{}
// Fairly useless New function.
func NewNull() *Null {
return &Null{}
}
func (it *Null) Iterate() Scanner {
return it
}
func (it *Null) Lookup() Index {
return it
}
// Fill the map based on the tags assigned to this iterator.
func (it *Null) TagResults(dst map[string]refs.Ref) {}
func (it *Null) Contains(ctx context.Context, v refs.Ref) bool {
return false
}
// A good iterator will close itself when it returns true.
// Null has nothing it needs to do.
func (it *Null) Optimize(ctx context.Context) (Shape, bool) { return it, false }
func (it *Null) String() string {
return "Null"
}
func (it *Null) Next(ctx context.Context) bool {
return false
}
func (it *Null) Err() error {
return nil
}
func (it *Null) Result() refs.Ref {
return nil
}
func (it *Null) SubIterators() []Shape {
return nil
}
func (it *Null) NextPath(ctx context.Context) bool {
return false
}
func (it *Null) Reset() {}
func (it *Null) Close() error {
return nil
}
// A null iterator costs nothing. Use it!
func (it *Null) Stats(ctx context.Context) (Costs, error) {
return Costs{}, nil
}
// Error iterator always returns a single error with no other results.
type Error struct {
err error
}
func NewError(err error) *Error {
return &Error{err: err}
}
func (it *Error) Iterate() Scanner {
return it
}
func (it *Error) Lookup() Index {
return it
}
// Fill the map based on the tags assigned to this iterator.
func (it *Error) TagResults(dst map[string]refs.Ref) {}
func (it *Error) Contains(ctx context.Context, v refs.Ref) bool {
return false
}
func (it *Error) Optimize(ctx context.Context) (Shape, bool) { return it, false }
func (it *Error) String() string {
return fmt.Sprintf("Error(%v)", it.err)
}
func (it *Error) Next(ctx context.Context) bool {
return false
}
func (it *Error) Err() error {
return it.err
}
func (it *Error) Result() refs.Ref {
return nil
}
func (it *Error) SubIterators() []Shape {
return nil
}
func (it *Error) NextPath(ctx context.Context) bool {
return false
}
func (it *Error) Reset() {}
func (it *Error) Close() error {
return it.err
}
func (it *Error) Stats(ctx context.Context) (Costs, error) {
return Costs{}, nil
}
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