selector.go

package selector

import (
	"fmt"

	ld "gitlab.dms3.io/ld/go-ld-prime"
)

// Selector is the programmatic representation of an LD Selector Node
// and can be applied to traverse a given LD DAG
type Selector interface {
	Interests() []ld.PathSegment              // returns the segments we're likely interested in **or nil** if we're a high-cardinality or expression based matcher and need all segments proposed to us.
	Explore(ld.Node, ld.PathSegment) Selector // explore one step -- iteration comes from outside (either whole node, or by following suggestions of Interests).  returns nil if no interest.  you have to traverse to the next node yourself (the selector doesn't do it for you because you might be considering multiple selection reasons at the same time).
	Decide(ld.Node) bool
}

// ParsedParent is created whenever you are parsing a selector node that may have
// child selectors nodes that need to know it
type ParsedParent interface {
	Link(s Selector) bool
}

// ParseContext tracks the progress when parsing a selector
type ParseContext struct {
	parentStack []ParsedParent
}

// ParseSelector creates a Selector that can be traversed from an LD Selector node
func ParseSelector(n ld.Node) (Selector, error) {
	return ParseContext{}.ParseSelector(n)
}

// ParseSelector creates a Selector from an LD Selector Node with the given context
func (pc ParseContext) ParseSelector(n ld.Node) (Selector, error) {
	if n.Kind() != ld.Kind_Map {
		return nil, fmt.Errorf("selector spec parse rejected: selector is a keyed union and thus must be a map")
	}
	if n.Length() != 1 {
		return nil, fmt.Errorf("selector spec parse rejected: selector is a keyed union and thus must be single-entry map")
	}
	kn, v, _ := n.MapIterator().Next()
	kstr, _ := kn.AsString()
	// Switch over the single key to determine which selector body comes next.
	//  (This switch is where the keyed union discriminators concretely happen.)
	switch kstr {
	case SelectorKey_ExploreFields:
		return pc.ParseExploreFields(v)
	case SelectorKey_ExploreAll:
		return pc.ParseExploreAll(v)
	case SelectorKey_ExploreIndex:
		return pc.ParseExploreIndex(v)
	case SelectorKey_ExploreRange:
		return pc.ParseExploreRange(v)
	case SelectorKey_ExploreUnion:
		return pc.ParseExploreUnion(v)
	case SelectorKey_ExploreRecursive:
		return pc.ParseExploreRecursive(v)
	case SelectorKey_ExploreRecursiveEdge:
		return pc.ParseExploreRecursiveEdge(v)
	case SelectorKey_Matcher:
		return pc.ParseMatcher(v)
	default:
		return nil, fmt.Errorf("selector spec parse rejected: %q is not a known member of the selector union", kstr)
	}
}

// PushParent puts a parent onto the stack of parents for a parse context
func (pc ParseContext) PushParent(parent ParsedParent) ParseContext {
	l := len(pc.parentStack)
	parents := make([]ParsedParent, 0, l+1)
	parents = append(parents, parent)
	parents = append(parents, pc.parentStack...)
	return ParseContext{parents}
}

// SegmentIterator iterates either a list or a map, generating PathSegments
// instead of indexes or keys
type SegmentIterator interface {
	Next() (pathSegment ld.PathSegment, value ld.Node, err error)
	Done() bool
}

// NewSegmentIterator generates a new iterator based on the node type
func NewSegmentIterator(n ld.Node) SegmentIterator {
	if n.Kind() == ld.Kind_List {
		return listSegmentIterator{n.ListIterator()}
	}
	return mapSegmentIterator{n.MapIterator()}
}

type listSegmentIterator struct {
	ld.ListIterator
}

func (lsi listSegmentIterator) Next() (pathSegment ld.PathSegment, value ld.Node, err error) {
	i, v, err := lsi.ListIterator.Next()
	return ld.PathSegmentOfInt(i), v, err
}

func (lsi listSegmentIterator) Done() bool {
	return lsi.ListIterator.Done()
}

type mapSegmentIterator struct {
	ld.MapIterator
}

func (msi mapSegmentIterator) Next() (pathSegment ld.PathSegment, value ld.Node, err error) {
	k, v, err := msi.MapIterator.Next()
	if err != nil {
		return ld.PathSegment{}, v, err
	}
	kstr, _ := k.AsString()
	return ld.PathSegmentOfString(kstr), v, err
}

func (msi mapSegmentIterator) Done() bool {
	return msi.MapIterator.Done()
}