cayley/graph/path/path.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 path

import (
	"fmt"

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

type applyMorphism func(graph.QuadStore, graph.Iterator, *context) (graph.Iterator, *context)

type morphism struct {
	Name     string
	Reversal func() morphism
	Apply    applyMorphism
	tags     []string
}

type context struct {
	labelSet Path
}

// Path represents either a morphism (a pre-defined path stored for later use),
// or a concrete path, consisting of a morphism and an underlying QuadStore.
type Path struct {
	stack       []morphism
	qs          graph.QuadStore // Optionally. A nil qs is equivalent to a morphism.
	baseContext *context
}

// IsMorphism returns whether this Path is a morphism.
func (p *Path) IsMorphism() bool { return p.qs == nil }

// StartMorphism creates a new Path with no underlying QuadStore.
func StartMorphism(nodes ...string) *Path {
	return StartPath(nil, nodes...)
}

// StartPath creates a new Path from a set of nodes and an underlying QuadStore.
func StartPath(qs graph.QuadStore, nodes ...string) *Path {
	return &Path{
		stack: []morphism{
			isMorphism(nodes...),
		},
		qs: qs,
	}
}

func PathFromIterator(qs graph.QuadStore, it graph.Iterator) *Path {
	return &Path{
		stack: []morphism{
			iteratorMorphism(it),
		},
		qs: qs,
	}
}

// NewPath creates a new, empty Path.
func NewPath(qs graph.QuadStore) *Path {
	return &Path{
		qs: qs,
	}
}

// Reverse returns a new Path that is the reverse of the current one.
func (p *Path) Reverse() *Path {
	newPath := NewPath(p.qs)
	for i := len(p.stack) - 1; i >= 0; i-- {
		newPath.stack = append(newPath.stack, p.stack[i].Reversal())
	}
	return newPath
}

// Is declares that the current nodes in this path are only the nodes
// passed as arguments.
func (p *Path) Is(nodes ...string) *Path {
	p.stack = append(p.stack, isMorphism(nodes...))
	return p
}

// Tag adds tag strings to the nodes at this point in the path for each result
// path in the set.
func (p *Path) Tag(tags ...string) *Path {
	p.stack = append(p.stack, tagMorphism(tags...))
	return p
}

// Out updates this Path to represent the nodes that are adjacent to the
// current nodes, via the given outbound predicate.
//
// For example:
//  // Returns the list of nodes that "A" follows.
//  //
//  // Will return []string{"B"} if there is a predicate (edge) from "A"
//  // to "B" labelled "follows".
//  StartPath(qs, "A").Out("follows")
func (p *Path) Out(via ...interface{}) *Path {
	p.stack = append(p.stack, outMorphism(via...))
	return p
}

// In updates this Path to represent the nodes that are adjacent to the
// current nodes, via the given inbound predicate.
//
// For example:
//  // Return the list of nodes that follow "B".
//  //
//  // Will return []string{"A", "C", "D"} if there are the appropriate
//  // edges from those nodes to "B" labelled "follows".
//  StartPath(qs, "B").In("follows")
func (p *Path) In(via ...interface{}) *Path {
	p.stack = append(p.stack, inMorphism(via...))
	return p
}

// InPredicates updates this path to represent the nodes of the valid inbound
// predicates from the current nodes.
//
// For example:
// // Returns a list of predicates valid from "bob"
// //
// // Will return []string{"follows"} if there are any things that "follow" Bob
// StartPath(qs, "bob").InPredicates()
func (p *Path) InPredicates() *Path {
	p.stack = append(p.stack, predicatesMorphism(true))
	return p
}

// OutPredicates updates this path to represent the nodes of the valid inbound
// predicates from the current nodes.
//
// For example:
// // Returns a list of predicates valid from "bob"
// //
// // Will return []string{"follows", "status"} if there are edges from "bob"
// // labelled "follows", and edges from "bob" that describe his "status".
// StartPath(qs, "bob").OutPredicates()
func (p *Path) OutPredicates() *Path {
	p.stack = append(p.stack, predicatesMorphism(false))
	return p
}

// And updates the current Path to represent the nodes that match both the
// current Path so far, and the given Path.
func (p *Path) And(path *Path) *Path {
	p.stack = append(p.stack, andMorphism(path))
	return p
}

// Or updates the current Path to represent the nodes that match either the
// current Path so far, or the given Path.
func (p *Path) Or(path *Path) *Path {
	p.stack = append(p.stack, orMorphism(path))
	return p
}

// Except updates the current Path to represent the all of the current nodes
// except those in the supplied Path.
//
// For example:
//  // Will return []string{"B"}
//  StartPath(qs, "A", "B").Except(StartPath(qs, "A"))
func (p *Path) Except(path *Path) *Path {
	p.stack = append(p.stack, exceptMorphism(path))
	return p
}

// Follow allows you to stitch two paths together. The resulting path will start
// from where the first path left off and continue iterating down the path given.
func (p *Path) Follow(path *Path) *Path {
	p.stack = append(p.stack, followMorphism(path))
	return p
}

// FollowReverse is the same as follow, except it will iterate backwards up the
// path given as argument.
func (p *Path) FollowReverse(path *Path) *Path {
	p.stack = append(p.stack, followMorphism(path.Reverse()))
	return p
}

// Save will, from the current nodes in the path, retrieve the node
// one linkage away (given by either a path or a predicate), add the given
// tag, and propagate that to the result set.
//
// For example:
// // Will return []map[string]string{{"social_status: "cool"}}
// StartPath(qs, "B").Save("status", "social_status"
func (p *Path) Save(via interface{}, tag string) *Path {
	p.stack = append(p.stack, saveMorphism(via, tag))
	return p
}

// SaveReverse is the same as Save, only in the reverse direction
// (the subject of the linkage should be tagged, instead of the object).
func (p *Path) SaveReverse(via interface{}, tag string) *Path {
	p.stack = append(p.stack, saveReverseMorphism(via, tag))
	return p
}

// Has limits the paths to be ones where the current nodes have some linkage
// to some known node.
func (p *Path) Has(via interface{}, nodes ...string) *Path {
	p.stack = append(p.stack, hasMorphism(via, nodes...))
	return p
}

// Back returns to a previously tagged place in the path. Any constraints applied after the Tag will remain in effect, but traversal continues from the tagged point instead, not from the end of the chain.
//
// For example:
// // Will return "bob" iff "bob" is cool
// StartPath(qs, "bob").Tag("person_tag").Out("status").Is("cool").Back("person_tag")
func (p *Path) Back(tag string) *Path {
	newPath := NewPath(p.qs)
	i := len(p.stack) - 1

	for {
		if i < 0 {
			return p.Reverse()
		}
		if p.stack[i].Name == "tag" {
			for _, x := range p.stack[i].tags {
				if x == tag {
					// Found what we're looking for.
					p.stack = p.stack[:i+1]
					return p.And(newPath)
				}
			}
		}
		newPath.stack = append(newPath.stack, p.stack[i].Reversal())
		i--
	}
}

// BuildIterator returns an iterator from this given Path.  Note that you must
// call this with a full path (not a morphism), since a morphism does not have
// the ability to fetch the underlying quads.  This function will panic if
// called with a morphism (i.e. if p.IsMorphism() is true).
func (p *Path) BuildIterator() graph.Iterator {
	if p.IsMorphism() {
		panic("Building an iterator from a morphism. Bind a QuadStore with BuildIteratorOn(qs)")
	}
	return p.BuildIteratorOn(p.qs)
}

// BuildIteratorOn will return an iterator for this path on the given QuadStore.
func (p *Path) BuildIteratorOn(qs graph.QuadStore) graph.Iterator {
	return p.Morphism()(qs, qs.NodesAllIterator())
}

// Morphism returns the morphism of this path.  The returned value is a
// function that, when given a QuadStore and an existing Iterator, will
// return a new Iterator that yields the subset of values from the existing
// iterator matched by the current Path.
func (p *Path) Morphism() graph.ApplyMorphism {
	return func(qs graph.QuadStore, it graph.Iterator) graph.Iterator {
		i := it.Clone()
		ctx := p.baseContext
		for _, m := range p.stack {
			i, ctx = m.Apply(qs, i, ctx)
		}
		return i
	}
}

func (p *Path) debugPrint() {
	var strs []string
	for _, x := range p.stack {
		strs = append(strs, x.Name)
	}
	fmt.Println("stack:", strs)
	fmt.Println("ctx:", p.baseContext)
}