// processPredicate updates the working graph clause if there is an available
// predicate.
func processPredicate(c *GraphClause, ce ConsumedElement, lastNopToken *lexer.Token) (*predicate.Predicate, string, string, bool, error) {
	var (
		nP             *predicate.Predicate
		pID            string
		pAnchorBinding string
		temporal       bool
	)
	raw := ce.Token().Text
	p, err := predicate.Parse(raw)
	if err == nil {
		// A fully specified predicate was provided.
		nP = p
		return nP, pID, pAnchorBinding, nP.Type() == predicate.Temporal, nil
	}
	// The predicate may have a binding on the anchor.
	cmps := predicateRegexp.FindAllStringSubmatch(raw, 2)
	if len(cmps) != 1 || (len(cmps) == 1 && len(cmps[0]) != 3) {
		return nil, "", "", false, fmt.Errorf("failed to extract partialy defined predicate %q, got %v instead", raw, cmps)
	}
	id, ta := cmps[0][1], cmps[0][2]
	pID = id
	if ta != "" {
		pAnchorBinding = ta
		temporal = true
	}
	return nil, pID, pAnchorBinding, temporal, nil
}

// TriplesForPredicate returns all triples available for the given predicate.
func (m *memory) TriplesForPredicate(p *predicate.Predicate, lo *storage.LookupOptions) (storage.Triples, error) {
	pGUID := p.GUID()
	m.rwmu.RLock()
	triples := make(chan *triple.Triple, len(m.idxP[pGUID]))
	go func() {
		ckr := newChecker(lo)
		for _, t := range m.idxP[pGUID] {
			if ckr.CheckAndUpdate(t.Predicate()) {
				triples <- t
			}
		}
		m.rwmu.RUnlock()
		close(triples)
	}()
	return triples, nil
}

// TriplesForPredicateAndObject returns all triples available for the given
// predicate and object.
func (m *memory) TriplesForPredicateAndObject(p *predicate.Predicate, o *triple.Object, lo *storage.LookupOptions) (storage.Triples, error) {
	pGUID := p.GUID()
	oGUID := o.GUID()
	poIdx := strings.Join([]string{pGUID, oGUID}, ":")
	m.rwmu.RLock()
	triples := make(chan *triple.Triple, len(m.idxPO[poIdx]))
	go func() {
		ckr := newChecker(lo)
		for _, t := range m.idxPO[poIdx] {
			if ckr.CheckAndUpdate(t.P()) {
				triples <- t
			}
		}
		m.rwmu.RUnlock()
		close(triples)
	}()
	return triples, nil
}

// TriplesForSubjectAndPredicate returns all triples available for the given
// subject and predicate.
func (m *memory) TriplesForSubjectAndPredicate(s *node.Node, p *predicate.Predicate, lo *storage.LookupOptions) (storage.Triples, error) {
	sGUID := s.GUID()
	pGUID := p.GUID()
	spIdx := strings.Join([]string{sGUID, pGUID}, ":")
	m.rwmu.RLock()
	triples := make(chan *triple.Triple, len(m.idxSP[spIdx]))
	go func() {
		ckr := newChecker(lo)
		for _, t := range m.idxSP[spIdx] {
			if ckr.CheckAndUpdate(t.P()) {
				triples <- t
			}
		}
		m.rwmu.RUnlock()
		close(triples)
	}()
	return triples, nil
}

// Subject returns the subjects for the give predicate and object.
func (m *memory) Subjects(p *predicate.Predicate, o *triple.Object, lo *storage.LookupOptions) (storage.Nodes, error) {
	pGUID := p.GUID()
	oGUID := o.GUID()
	poIdx := strings.Join([]string{pGUID, oGUID}, ":")
	m.rwmu.RLock()
	subs := make(chan *node.Node, len(m.idxPO[poIdx]))
	go func() {
		ckr := newChecker(lo)
		for _, t := range m.idxPO[poIdx] {
			if ckr.CheckAndUpdate(t.P()) {
				subs <- t.S()
			}
		}
		m.rwmu.RUnlock()
		close(subs)
	}()
	return subs, nil
}

// Objects returns the objects for the give object and predicate.
func (m *memory) Objects(ctx context.Context, s *node.Node, p *predicate.Predicate, lo *storage.LookupOptions) (storage.Objects, error) {
	sGUID := s.GUID()
	pGUID := p.GUID()
	spIdx := strings.Join([]string{sGUID, pGUID}, ":")
	m.rwmu.RLock()
	objs := make(chan *triple.Object, len(m.idxSP[spIdx]))
	go func() {
		ckr := newChecker(lo)
		for _, t := range m.idxSP[spIdx] {
			if ckr.CheckAndUpdate(t.Predicate()) {
				objs <- t.Object()
			}
		}
		m.rwmu.RUnlock()
		close(objs)
	}()
	return objs, nil
}

// CheckAndUpdate checks if a predicate should be considered and it also updates
// the internal state in case counts are needed.
func (c *checker) CheckAndUpdate(p *predicate.Predicate) bool {
	if c.max {
		if c.c <= 0 {
			return false
		}
		c.c--
	}
	if p.Type() == predicate.Immutable {
		return true
	}
	t, _ := p.TimeAnchor()
	if c.o.LowerAnchor != nil && t.Before(*c.o.LowerAnchor) {
		return false
	}
	if c.o.UpperAnchor != nil && t.After(*c.o.UpperAnchor) {
		return false
	}
	return true
}