selector: rework to match the spec, cleanup lots of edge cases

- removed field selection against a list
- a selector return exactly one node, or nil
- implemented the full set of slicing, including negative indexes
- plenty of other cleanup and simplification

pkg/policy/selector/parsing.go

@@ -2,6 +2,7 @@ package selector
 
 import (
 	"fmt"
+	"math"
 	"regexp"
 	"strconv"
 	"strings"
@@ -33,54 +34,64 @@ func Parse(str string) (Selector, error) {
 		if opt {
 			seg = tok[0 : len(tok)-1]
 		}
-		switch seg {
-		case ".":
+		switch {
+		case seg == ".":
 			if len(sel) > 0 && sel[len(sel)-1].Identity() {
 				return nil, newParseError("selector contains unsupported recursive descent segment: '..'", str, col, tok)
 			}
 			sel = append(sel, segment{str: ".", identity: true})
-		case "[]":
-			sel = append(sel, segment{str: tok, optional: opt, iterator: true})
-		default:
-			if strings.HasPrefix(seg, "[") && strings.HasSuffix(seg, "]") {
-				lookup := seg[1 : len(seg)-1]
 
-				if indexRegex.MatchString(lookup) { // index
-					idx, err := strconv.Atoi(lookup)
-					if err != nil {
-						return nil, newParseError("invalid index", str, col, tok)
-					}
-					sel = append(sel, segment{str: tok, optional: opt, index: idx})
-				} else if strings.HasPrefix(lookup, "\"") && strings.HasSuffix(lookup, "\"") { // explicit field
-					sel = append(sel, segment{str: tok, optional: opt, field: lookup[1 : len(lookup)-1]})
-				} else if sliceRegex.MatchString(lookup) { // slice [3:5] or [:5] or [3:]
-					var rng []int
-					splt := strings.Split(lookup, ":")
-					if splt[0] == "" {
-						rng = append(rng, 0)
-					} else {
-						i, err := strconv.Atoi(splt[0])
-						if err != nil {
-							return nil, newParseError("invalid slice index", str, col, tok)
-						}
-						rng = append(rng, i)
-					}
-					if splt[1] != "" {
-						i, err := strconv.Atoi(splt[1])
-						if err != nil {
-							return nil, newParseError("invalid slice index", str, col, tok)
-						}
-						rng = append(rng, i)
-					}
-					sel = append(sel, segment{str: tok, optional: opt, slice: rng})
-				} else {
-					return nil, newParseError(fmt.Sprintf("invalid segment: %s", seg), str, col, tok)
+		case seg == "[]":
+			sel = append(sel, segment{str: tok, optional: opt, iterator: true})
+
+		case strings.HasPrefix(seg, "[") && strings.HasSuffix(seg, "]"):
+			lookup := seg[1 : len(seg)-1]
+
+			switch {
+			// index, [123]
+			case indexRegex.MatchString(lookup):
+				idx, err := strconv.Atoi(lookup)
+				if err != nil {
+					return nil, newParseError("invalid index", str, col, tok)
 				}
-			} else if fieldRegex.MatchString(seg) {
-				sel = append(sel, segment{str: tok, optional: opt, field: seg[1:]})
-			} else {
+				sel = append(sel, segment{str: tok, optional: opt, index: idx})
+
+			// explicit field, ["abcd"]
+			case strings.HasPrefix(lookup, "\"") && strings.HasSuffix(lookup, "\""):
+				sel = append(sel, segment{str: tok, optional: opt, field: lookup[1 : len(lookup)-1]})
+
+			// slice [3:5] or [:5] or [3:], also negative numbers
+			case sliceRegex.MatchString(lookup):
+				var rng [2]int64
+				splt := strings.Split(lookup, ":")
+				if splt[0] == "" {
+					rng[0] = math.MinInt
+				} else {
+					i, err := strconv.ParseInt(splt[0], 10, 0)
+					if err != nil {
+						return nil, newParseError("invalid slice index", str, col, tok)
+					}
+					rng[0] = i
+				}
+				if splt[1] == "" {
+					rng[1] = math.MaxInt
+				} else {
+					i, err := strconv.ParseInt(splt[1], 10, 0)
+					if err != nil {
+						return nil, newParseError("invalid slice index", str, col, tok)
+					}
+					rng[1] = i
+				}
+				sel = append(sel, segment{str: tok, optional: opt, slice: rng[:]})
+
+			default:
 				return nil, newParseError(fmt.Sprintf("invalid segment: %s", seg), str, col, tok)
 			}
+
+		case fieldRegex.MatchString(seg):
+			sel = append(sel, segment{str: tok, optional: opt, field: seg[1:]})
+		default:
+			return nil, newParseError(fmt.Sprintf("invalid segment: %s", seg), str, col, tok)
 		}
 		col += len(tok)
 	}

pkg/policy/selector/parsing_test.go

@@ -0,0 +1,427 @@
+package selector
+
+import (
+	"fmt"
+	"math"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestParse(t *testing.T) {
+	t.Run("identity", func(t *testing.T) {
+		sel, err := Parse(".")
+		require.NoError(t, err)
+		require.Equal(t, 1, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+	})
+
+	t.Run("dotted field name", func(t *testing.T) {
+		sel, err := Parse(".foo")
+		require.NoError(t, err)
+		require.Equal(t, 1, len(sel))
+		require.False(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Equal(t, sel[0].Field(), "foo")
+		require.Empty(t, sel[0].Index())
+	})
+
+	t.Run("explicit field", func(t *testing.T) {
+		sel, err := Parse(`.["foo"]`)
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.False(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Empty(t, sel[1].Slice())
+		require.Equal(t, sel[1].Field(), "foo")
+		require.Empty(t, sel[1].Index())
+	})
+
+	t.Run("iterator, collection value", func(t *testing.T) {
+		sel, err := Parse(".[]")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.False(t, sel[1].Optional())
+		require.True(t, sel[1].Iterator())
+		require.Empty(t, sel[1].Slice())
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+	})
+
+	t.Run("index", func(t *testing.T) {
+		sel, err := Parse(".[138]")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.False(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Empty(t, sel[1].Slice())
+		require.Empty(t, sel[1].Field())
+		require.Equal(t, sel[1].Index(), 138)
+	})
+
+	t.Run("negative index", func(t *testing.T) {
+		sel, err := Parse(".[-138]")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.False(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Empty(t, sel[1].Slice())
+		require.Empty(t, sel[1].Field())
+		require.Equal(t, sel[1].Index(), -138)
+	})
+
+	t.Run("List slice", func(t *testing.T) {
+		sel, err := Parse(".[7:11]")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.False(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Equal(t, sel[1].Slice(), []int64{7, 11})
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+
+		sel, err = Parse(".[2:]")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.False(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Equal(t, sel[1].Slice(), []int64{2, math.MaxInt})
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+
+		sel, err = Parse(".[:42]")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.False(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Equal(t, sel[1].Slice(), []int64{math.MinInt, 42})
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+
+		sel, err = Parse(".[0:-2]")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.False(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Equal(t, sel[1].Slice(), []int64{0, -2})
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+	})
+
+	t.Run("optional identity", func(t *testing.T) {
+		sel, err := Parse(".?")
+		require.NoError(t, err)
+		require.Equal(t, 1, len(sel))
+		require.True(t, sel[0].Identity())
+		require.True(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+	})
+
+	t.Run("optional dotted field name", func(t *testing.T) {
+		sel, err := Parse(".foo?")
+		require.NoError(t, err)
+		require.Equal(t, 1, len(sel))
+		require.False(t, sel[0].Identity())
+		require.True(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Equal(t, sel[0].Field(), "foo")
+		require.Empty(t, sel[0].Index())
+	})
+
+	t.Run("optional explicit field", func(t *testing.T) {
+		sel, err := Parse(`.["foo"]?`)
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.True(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Empty(t, sel[1].Slice())
+		require.Equal(t, sel[1].Field(), "foo")
+		require.Empty(t, sel[1].Index())
+	})
+
+	t.Run("optional iterator", func(t *testing.T) {
+		sel, err := Parse(".[]?")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.True(t, sel[1].Optional())
+		require.True(t, sel[1].Iterator())
+		require.Empty(t, sel[1].Slice())
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+	})
+
+	t.Run("optional index", func(t *testing.T) {
+		sel, err := Parse(".[138]?")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.True(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Empty(t, sel[1].Slice())
+		require.Empty(t, sel[1].Field())
+		require.Equal(t, sel[1].Index(), 138)
+	})
+
+	t.Run("optional negative index", func(t *testing.T) {
+		sel, err := Parse(".[-138]?")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.True(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Empty(t, sel[1].Slice())
+		require.Empty(t, sel[1].Field())
+		require.Equal(t, sel[1].Index(), -138)
+	})
+
+	t.Run("optional list slice", func(t *testing.T) {
+		sel, err := Parse(".[7:11]?")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.True(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Equal(t, sel[1].Slice(), []int64{7, 11})
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+
+		sel, err = Parse(".[2:]?")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.True(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Equal(t, sel[1].Slice(), []int64{2, math.MaxInt})
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+
+		sel, err = Parse(".[:42]?")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.True(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Equal(t, sel[1].Slice(), []int64{math.MinInt, 42})
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+
+		sel, err = Parse(".[0:-2]?")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.True(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Equal(t, sel[1].Slice(), []int64{0, -2})
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+	})
+
+	t.Run("idempotent optional", func(t *testing.T) {
+		sel, err := Parse(".foo???")
+		require.NoError(t, err)
+		require.Equal(t, 2, len(sel))
+		require.True(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Empty(t, sel[0].Field())
+		require.Empty(t, sel[0].Index())
+		require.False(t, sel[1].Identity())
+		require.True(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Empty(t, sel[1].Slice())
+		require.Equal(t, sel[1].Field(), "foo")
+		require.Empty(t, sel[1].Index())
+	})
+
+	t.Run("deny multi dot", func(t *testing.T) {
+		_, err := Parse("..")
+		require.Error(t, err)
+	})
+
+	t.Run("nesting", func(t *testing.T) {
+		str := `.foo.["bar"].[138]?.baz[1:]`
+		sel, err := Parse(str)
+		require.NoError(t, err)
+		printSegments(sel)
+		require.Equal(t, str, sel.String())
+		require.Equal(t, 7, len(sel))
+		require.False(t, sel[0].Identity())
+		require.False(t, sel[0].Optional())
+		require.False(t, sel[0].Iterator())
+		require.Empty(t, sel[0].Slice())
+		require.Equal(t, sel[0].Field(), "foo")
+		require.Empty(t, sel[0].Index())
+		require.True(t, sel[1].Identity())
+		require.False(t, sel[1].Optional())
+		require.False(t, sel[1].Iterator())
+		require.Empty(t, sel[1].Slice())
+		require.Empty(t, sel[1].Field())
+		require.Empty(t, sel[1].Index())
+		require.False(t, sel[2].Identity())
+		require.False(t, sel[2].Optional())
+		require.False(t, sel[2].Iterator())
+		require.Empty(t, sel[2].Slice())
+		require.Equal(t, sel[2].Field(), "bar")
+		require.Empty(t, sel[2].Index())
+		require.True(t, sel[3].Identity())
+		require.False(t, sel[3].Optional())
+		require.False(t, sel[3].Iterator())
+		require.Empty(t, sel[3].Slice())
+		require.Empty(t, sel[3].Field())
+		require.Empty(t, sel[3].Index())
+		require.False(t, sel[4].Identity())
+		require.True(t, sel[4].Optional())
+		require.False(t, sel[4].Iterator())
+		require.Empty(t, sel[4].Slice())
+		require.Empty(t, sel[4].Field())
+		require.Equal(t, sel[4].Index(), 138)
+		require.False(t, sel[5].Identity())
+		require.False(t, sel[5].Optional())
+		require.False(t, sel[5].Iterator())
+		require.Empty(t, sel[5].Slice())
+		require.Equal(t, sel[5].Field(), "baz")
+		require.Empty(t, sel[5].Index())
+		require.False(t, sel[6].Identity())
+		require.False(t, sel[6].Optional())
+		require.False(t, sel[6].Iterator())
+		require.Equal(t, sel[6].Slice(), []int64{1, math.MaxInt})
+		require.Empty(t, sel[6].Field())
+		require.Empty(t, sel[6].Index())
+	})
+
+	t.Run("non dotted", func(t *testing.T) {
+		_, err := Parse("foo")
+		require.NotNil(t, err)
+		fmt.Println(err)
+	})
+
+	t.Run("non quoted", func(t *testing.T) {
+		_, err := Parse(".[foo]")
+		require.NotNil(t, err)
+		fmt.Println(err)
+	})
+}
+
+func printSegments(s Selector) {
+	for i, seg := range s {
+		fmt.Printf("%d: %s\n", i, seg.String())
+	}
+}

pkg/policy/selector/selector.go

@@ -1,11 +1,15 @@
 package selector
 
 import (
+	"errors"
 	"fmt"
+	"math"
+	"strconv"
 	"strings"
 
 	"github.com/ipld/go-ipld-prime"
 	"github.com/ipld/go-ipld-prime/datamodel"
+	"github.com/ipld/go-ipld-prime/fluent/qp"
 	"github.com/ipld/go-ipld-prime/node/basicnode"
 	"github.com/ipld/go-ipld-prime/schema"
 )
@@ -15,16 +19,20 @@ import (
 type Selector []segment
 
 // Select perform the selection described by the selector on the input IPLD DAG.
-// If no error, Select returns either one ipld.Node or a []ipld.Node.
-func (s Selector) Select(subject ipld.Node) (ipld.Node, []ipld.Node, error) {
+// Select can return:
+//   - exactly one matched IPLD node
+//   - an error that can be checked with IsResolutionErr(err) to distinguish not being able to resolve to a node,
+//     or another type of error.
+//   - nil and no error, if the selector couldn't match on an optional segment (with ?).
+func (s Selector) Select(subject ipld.Node) (ipld.Node, error) {
 	return resolve(s, subject, nil)
 }
 
-// MatchPath tells if the selector operates on the given (string only) path segments.
-// It returns the segments that didn't get consumed by the matching.
-func (s Selector) MatchPath(pathSegment ...string) (bool, []string) {
-	return matchPath(s, pathSegment)
-}
+// // MatchPath tells if the selector operates on the given (string only) path segments.
+// // It returns the segments that didn't get consumed by the matching.
+// func (s Selector) MatchPath(pathSegment ...string) (bool, []string) {
+// 	return matchPath(s, pathSegment)
+// }
 
 func (s Selector) String() string {
 	var res strings.Builder
@@ -39,7 +47,7 @@ type segment struct {
 	identity bool
 	optional bool
 	iterator bool
-	slice    []int
+	slice    []int64 // either 0-length or 2-length
 	field    string
 	index    int
 }
@@ -65,7 +73,7 @@ func (s segment) Iterator() bool {
 }
 
 // Slice flags that this segment targets a range of a slice.
-func (s segment) Slice() []int {
+func (s segment) Slice() []int64 {
 	return s.slice
 }
 
@@ -79,337 +87,222 @@ func (s segment) Index() int {
 	return s.index
 }
 
-func resolve(sel Selector, subject ipld.Node, at []string) (ipld.Node, []ipld.Node, error) {
-	cur := subject
-	for i, seg := range sel {
-		if seg.Identity() {
-			continue
-		}
-
-		// 1st level: handle the different segment types (iterator, field, slice, index)
-		// 2nd level: handle different node kinds (list, map, string, bytes)
-		switch {
-		case seg.Iterator():
-			if cur == nil || cur.Kind() == datamodel.Kind_Null {
-				if seg.Optional() {
-					// build empty list
-					nb := basicnode.Prototype.List.NewBuilder()
-					assembler, err := nb.BeginList(0)
-					if err != nil {
-						return nil, nil, err
-					}
-
-					if err = assembler.Finish(); err != nil {
-						return nil, nil, err
-					}
-
-					return nb.Build(), nil, nil
-				} else {
-					return nil, nil, newResolutionError(fmt.Sprintf("can not iterate over kind: %s", kindString(cur)), at)
-				}
-			} else {
-				var many []ipld.Node
-				switch cur.Kind() {
-				case datamodel.Kind_List:
-					it := cur.ListIterator()
-					for !it.Done() {
-						_, v, err := it.Next()
-						if err != nil {
-							return nil, nil, err
-						}
-
-						// check if there are more iterator segments
-						if len(sel) > i+1 && sel[i+1].Iterator() {
-							if v.Kind() == datamodel.Kind_List {
-								// recursively resolve the remaining selector segments
-								var o ipld.Node
-								var m []ipld.Node
-								o, m, err = resolve(sel[i+1:], v, at)
-								if err != nil {
-									// if the segment is optional and an error occurs, skip the current iteration.
-									if seg.Optional() {
-										continue
-									} else {
-										return nil, nil, err
-									}
-								}
-								if m != nil {
-									many = append(many, m...)
-								} else if o != nil {
-									many = append(many, o)
-								}
-							} else {
-								// if the current value is not a list and the next segment is optional, skip the current iteration
-								if sel[i+1].Optional() {
-									continue
-								} else {
-									return nil, nil, newResolutionError(fmt.Sprintf("can not iterate over kind: %s", kindString(v)), at)
-								}
-							}
-						} else {
-							// if there are no more iterator segments, append the current value to the result
-							many = append(many, v)
-						}
-					}
-				case datamodel.Kind_Map:
-					it := cur.MapIterator()
-					for !it.Done() {
-						_, v, err := it.Next()
-						if err != nil {
-							return nil, nil, err
-						}
-
-						if len(sel) > i+1 && sel[i+1].Iterator() {
-							if v.Kind() == datamodel.Kind_List {
-								var o ipld.Node
-								var m []ipld.Node
-								o, m, err = resolve(sel[i+1:], v, at)
-								if err != nil {
-									if seg.Optional() {
-										continue
-									} else {
-										return nil, nil, err
-									}
-								}
-								if m != nil {
-									many = append(many, m...)
-								} else if o != nil {
-									many = append(many, o)
-								}
-							} else {
-								if sel[i+1].Optional() {
-									continue
-								} else {
-									return nil, nil, newResolutionError(fmt.Sprintf("can not iterate over kind: %s", kindString(v)), at)
-								}
-							}
-						} else {
-							many = append(many, v)
-						}
-					}
-				default:
-					return nil, nil, newResolutionError(fmt.Sprintf("can not iterate over kind: %s", kindString(cur)), at)
-				}
-
-				return nil, many, nil
-			}
-
-		case seg.Field() != "":
-			at = append(at, seg.Field())
-			if cur == nil {
-				if seg.Optional() {
-					cur = nil
-				} else {
-					return nil, nil, newResolutionError(fmt.Sprintf("can not access field: %s on kind: %s", seg.Field(), kindString(cur)), at)
-				}
-			} else {
-				switch cur.Kind() {
-				case datamodel.Kind_Map:
-					n, err := cur.LookupByString(seg.Field())
-					if err != nil {
-						if isMissing(err) {
-							if seg.Optional() {
-								cur = nil
-							} else {
-								return nil, nil, newResolutionError(fmt.Sprintf("object has no field named: %s", seg.Field()), at)
-							}
-						} else {
-							return nil, nil, err
-						}
-					} else {
-						cur = n
-					}
-				case datamodel.Kind_List:
-					var many []ipld.Node
-					it := cur.ListIterator()
-					for !it.Done() {
-						_, v, err := it.Next()
-						if err != nil {
-							return nil, nil, err
-						}
-						if v.Kind() == datamodel.Kind_Map {
-							n, err := v.LookupByString(seg.Field())
-							if err == nil {
-								many = append(many, n)
-							}
-						}
-					}
-					if len(many) > 0 {
-						cur = nil
-						return nil, many, nil
-					} else if seg.Optional() {
-						cur = nil
-					} else {
-						return nil, nil, newResolutionError(fmt.Sprintf("no elements in list have field named: %s", seg.Field()), at)
-					}
-				default:
-					if seg.Optional() {
-						cur = nil
-					} else {
-						return nil, nil, newResolutionError(fmt.Sprintf("can not access field: %s on kind: %s", seg.Field(), kindString(cur)), at)
-					}
-				}
-			}
-
-		case seg.Slice() != nil:
-			if cur == nil {
-				if seg.Optional() {
-					cur = nil
-				} else {
-					return nil, nil, newResolutionError(fmt.Sprintf("can not slice on kind: %s", kindString(cur)), at)
-				}
-			} else {
-				slice := seg.Slice()
-				var start, end, length int64
-				switch cur.Kind() {
-				case datamodel.Kind_List:
-					length = cur.Length()
-					start, end = resolveSliceIndices(slice, length)
-				case datamodel.Kind_Bytes:
-					b, _ := cur.AsBytes()
-					length = int64(len(b))
-					start, end = resolveSliceIndices(slice, length)
-				case datamodel.Kind_String:
-					str, _ := cur.AsString()
-					length = int64(len(str))
-					start, end = resolveSliceIndices(slice, length)
-				default:
-					return nil, nil, newResolutionError(fmt.Sprintf("can not slice on kind: %s", kindString(cur)), at)
-				}
-
-				if start < 0 || end < start || end > length {
-					if seg.Optional() {
-						cur = nil
-					} else {
-						return nil, nil, newResolutionError(fmt.Sprintf("slice out of bounds: [%d:%d]", start, end), at)
-					}
-				} else {
-					switch cur.Kind() {
-					case datamodel.Kind_List:
-						if end > cur.Length() {
-							end = cur.Length()
-						}
-						nb := basicnode.Prototype.List.NewBuilder()
-						assembler, _ := nb.BeginList(int64(end - start))
-						for i := start; i < end; i++ {
-							item, _ := cur.LookupByIndex(int64(i))
-							assembler.AssembleValue().AssignNode(item)
-						}
-						assembler.Finish()
-						cur = nb.Build()
-					case datamodel.Kind_Bytes:
-						b, _ := cur.AsBytes()
-						l := int64(len(b))
-						if end > l {
-							end = l
-						}
-						cur = basicnode.NewBytes(b[start:end])
-					case datamodel.Kind_String:
-						str, _ := cur.AsString()
-						l := int64(len(str))
-						if end > l {
-							end = l
-						}
-						cur = basicnode.NewString(str[start:end])
-					}
-				}
-			}
-
-		default: // Index()
-			at = append(at, fmt.Sprintf("%d", seg.Index()))
-			if cur == nil {
-				if seg.Optional() {
-					cur = nil
-				} else {
-					return nil, nil, newResolutionError(fmt.Sprintf("can not access index: %d on kind: %s", seg.Index(), kindString(cur)), at)
-				}
-			} else {
-				idx := seg.Index()
-				switch cur.Kind() {
-				case datamodel.Kind_List:
-					if idx < 0 {
-						idx = int(cur.Length()) + idx
-					}
-					if idx < 0 || idx >= int(cur.Length()) {
-						if seg.Optional() {
-							cur = nil
-						} else {
-							return nil, nil, newResolutionError(fmt.Sprintf("index out of bounds: %d", seg.Index()), at)
-						}
-					} else {
-						cur, _ = cur.LookupByIndex(int64(idx))
-					}
-				case datamodel.Kind_String:
-					str, _ := cur.AsString()
-					if idx < 0 {
-						idx = len(str) + idx
-					}
-					if idx < 0 || idx >= len(str) {
-						if seg.Optional() {
-							cur = nil
-						} else {
-							return nil, nil, newResolutionError(fmt.Sprintf("index out of bounds: %d", seg.Index()), at)
-						}
-					} else {
-						cur = basicnode.NewString(string(str[idx]))
-					}
-				case datamodel.Kind_Bytes:
-					b, _ := cur.AsBytes()
-					if idx < 0 {
-						idx = len(b) + idx
-					}
-					if idx < 0 || idx >= len(b) {
-						if seg.Optional() {
-							cur = nil
-						} else {
-							return nil, nil, newResolutionError(fmt.Sprintf("index out of bounds: %d", seg.Index()), at)
-						}
-					} else {
-						cur = basicnode.NewInt(int64(b[idx]))
-					}
-				default:
-					return nil, nil, newResolutionError(fmt.Sprintf("can not access index: %d on kind: %s", seg.Index(), kindString(cur)), at)
-				}
-			}
+func resolve(sel Selector, subject ipld.Node, at []string) (ipld.Node, error) {
+	errIfNotOptional := func(s segment, err error) error {
+		if !s.Optional() {
+			return err
 		}
+		return nil
 	}
 
-	return cur, nil, nil
-}
-
-func matchPath(sel Selector, path []string) (bool, []string) {
+	cur := subject
 	for _, seg := range sel {
-		if len(path) == 0 {
-			return true, path
-		}
+		// 1st level: handle the different segment types (iterator, field, slice, index)
+		// 2nd level: handle different node kinds (list, map, string, bytes)
 		switch {
 		case seg.Identity():
 			continue
 
 		case seg.Iterator():
-			// we have reached a [] iterator, it should have matched earlier
-			return false, nil
+			switch {
+			case cur == nil || cur.Kind() == datamodel.Kind_Null:
+				if seg.Optional() {
+					// build an empty list
+					n, err := qp.BuildList(basicnode.Prototype.Any, 0, func(_ datamodel.ListAssembler) {})
+					return n, err
+				}
+				return nil, newResolutionError(fmt.Sprintf("can not iterate over kind: %s", kindString(cur)), at)
+
+			case cur.Kind() == datamodel.Kind_List:
+				// iterators are no-op on list
+				continue
+
+			case cur.Kind() == datamodel.Kind_Map:
+				// iterators on maps collect the values
+				return qp.BuildList(basicnode.Prototype.Any, cur.Length(), func(l datamodel.ListAssembler) {
+					it := cur.MapIterator()
+					for !it.Done() {
+						_, v, err := it.Next()
+						if err != nil {
+							panic(err) // recovered by BuildList
+						}
+						qp.ListEntry(l, qp.Node(v))
+					}
+				})
+
+			default:
+				return nil, newResolutionError(fmt.Sprintf("can not iterate over kind: %s", kindString(cur)), at)
+			}
 
 		case seg.Field() != "":
-			// if exact match on the segment, we continue
-			if path[0] == seg.Field() {
-				path = path[1:]
-				continue
-			}
-			return false, nil
+			at = append(at, seg.Field())
+			switch {
+			case cur == nil:
+				err := newResolutionError(fmt.Sprintf("can not access field: %s on kind: %s", seg.Field(), kindString(cur)), at)
+				return nil, errIfNotOptional(seg, err)
 
-		case seg.Slice() != nil:
-			// we have reached a [<int>:<int>] slicing, it should have matched earlier
-			return false, nil
+			case cur.Kind() == datamodel.Kind_Map:
+				n, err := cur.LookupByString(seg.Field())
+				if err != nil {
+					if !isMissing(err) {
+						return nil, err
+					}
+					if seg.Optional() {
+						cur = nil
+					} else {
+						return nil, newResolutionError(fmt.Sprintf("object has no field named: %s", seg.Field()), at)
+					}
+				} else {
+					cur = n
+				}
+
+			default:
+				err := newResolutionError(fmt.Sprintf("can not access field: %s on kind: %s", seg.Field(), kindString(cur)), at)
+				return nil, errIfNotOptional(seg, err)
+			}
+
+		case len(seg.Slice()) > 0:
+			if cur == nil {
+				err := newResolutionError(fmt.Sprintf("can not slice on kind: %s", kindString(cur)), at)
+				return nil, errIfNotOptional(seg, err)
+			}
+
+			slice := seg.Slice()
+			var start, end, length int64
+			switch cur.Kind() {
+			case datamodel.Kind_List:
+				length = cur.Length()
+				start, end = resolveSliceIndices(slice, length)
+			case datamodel.Kind_Bytes:
+				b, _ := cur.AsBytes()
+				length = int64(len(b))
+				start, end = resolveSliceIndices(slice, length)
+
+			// TODO: cleanup if confirmed that slicing/indexing on string is not legal
+			// case datamodel.Kind_String:
+			// 	str, _ := cur.AsString()
+			// 	length = int64(len(str))
+			// 	start, end = resolveSliceIndices(slice, length)
+			default:
+				return nil, newResolutionError(fmt.Sprintf("can not slice on kind: %s", kindString(cur)), at)
+			}
+
+			if start < 0 || end < start || end > length {
+				err := newResolutionError(fmt.Sprintf("slice out of bounds: [%d:%d]", start, end), at)
+				return nil, errIfNotOptional(seg, err)
+			}
+
+			switch cur.Kind() {
+			case datamodel.Kind_List:
+				sliced, err := qp.BuildList(basicnode.Prototype.Any, end-start, func(l datamodel.ListAssembler) {
+					for i := start; i <= end; i++ {
+						item, err := cur.LookupByIndex(i)
+						if err != nil {
+							panic(err) // recovered by BuildList
+						}
+						qp.ListEntry(l, qp.Node(item))
+					}
+				})
+				if err != nil {
+					return nil, errIfNotOptional(seg, err)
+				}
+				cur = sliced
+			case datamodel.Kind_Bytes:
+				b, _ := cur.AsBytes()
+				cur = basicnode.NewBytes(b[start:end])
+				// TODO: cleanup if confirmed that slicing/indexing on string is not legal
+				// case datamodel.Kind_String:
+				// 	str, _ := cur.AsString()
+				// 	cur = basicnode.NewString(str[start:end])
+			}
+			return cur, nil
 
 		default: // Index()
-			// we have reached a [<int>] indexing, it should have matched earlier
-			return false, nil
+			at = append(at, strconv.Itoa(seg.Index()))
+
+			if cur == nil {
+				err := newResolutionError(fmt.Sprintf("can not access index: %d on kind: %s", seg.Index(), kindString(cur)), at)
+				return nil, errIfNotOptional(seg, err)
+			}
+
+			idx := seg.Index()
+			switch cur.Kind() {
+			case datamodel.Kind_List:
+				if idx < 0 {
+					idx = int(cur.Length()) + idx
+				}
+				if idx < 0 || idx >= int(cur.Length()) {
+					err := newResolutionError(fmt.Sprintf("index out of bounds: %d", seg.Index()), at)
+					return nil, errIfNotOptional(seg, err)
+				}
+				cur, _ = cur.LookupByIndex(int64(idx))
+
+			// TODO: cleanup if confirmed that slicing/indexing on string is not legal
+			// case datamodel.Kind_String:
+			// 	str, _ := cur.AsString()
+			// 	if idx < 0 {
+			// 		idx = len(str) + idx
+			// 	}
+			// 	if idx < 0 || idx >= len(str) {
+			// 		err := newResolutionError(fmt.Sprintf("index out of bounds: %d", seg.Index()), at)
+			// 		return nil, errIfNotOptional(seg, err)
+			// 	}
+			// 	cur = basicnode.NewString(string(str[idx]))
+
+			case datamodel.Kind_Bytes:
+				b, _ := cur.AsBytes()
+				if idx < 0 {
+					idx = len(b) + idx
+				}
+				if idx < 0 || idx >= len(b) {
+					err := newResolutionError(fmt.Sprintf("index out of bounds: %d", seg.Index()), at)
+					return nil, errIfNotOptional(seg, err)
+				}
+				cur = basicnode.NewInt(int64(b[idx]))
+
+			default:
+				return nil, newResolutionError(fmt.Sprintf("can not access index: %d on kind: %s", seg.Index(), kindString(cur)), at)
+			}
 		}
 	}
-	return true, path
+
+	// segment exhausted, we return where we are
+	return cur, nil
 }
 
+// func matchPath(sel Selector, path []string) (bool, []string) {
+// 	for _, seg := range sel {
+// 		if len(path) == 0 {
+// 			return true, path
+// 		}
+// 		switch {
+// 		case seg.Identity():
+// 			continue
+//
+// 		case seg.Iterator():
+// 			// we have reached a [] iterator, it should have matched earlier
+// 			return false, nil
+//
+// 		case seg.Field() != "":
+// 			// if exact match on the segment, we continue
+// 			if path[0] == seg.Field() {
+// 				path = path[1:]
+// 				continue
+// 			}
+// 			return false, nil
+//
+// 		case seg.Slice() != nil:
+// 			// we have reached a [<int>:<int>] slicing, it should have matched earlier
+// 			return false, nil
+//
+// 		default: // Index()
+// 			// we have reached a [<int>] indexing, it should have matched earlier
+// 			return false, nil
+// 		}
+// 	}
+// 	return true, path
+// }
+
 // resolveSliceIndices resolves the start and end indices for slicing a list or byte array.
 //
 // It takes the slice indices from the selector segment and the length of the list or byte array,
@@ -422,26 +315,30 @@ func matchPath(sel Selector, path []string) (bool, []string) {
 // Returns:
 //   - start: The resolved start index for slicing.
 //   - end: The resolved end index for slicing.
-func resolveSliceIndices(slice []int, length int64) (int64, int64) {
-	start, end := int64(0), length
-	if len(slice) > 0 {
-		start = int64(slice[0])
-		if start < 0 {
-			start = length + start
-			if start < 0 {
-				start = 0
-			}
-		}
+func resolveSliceIndices(slice []int64, length int64) (start int64, end int64) {
+	if len(slice) != 2 {
+		panic("should always be 2-length")
 	}
-	if len(slice) > 1 {
-		end = int64(slice[1])
-		if end <= 0 {
-			end = length + end
-			if end < start {
-				end = start
-			}
-		}
+
+	start, end = slice[0], slice[1]
+
+	// adjust boundaries
+	switch {
+	case slice[0] == math.MinInt:
+		start = 0
+	case slice[0] < 0:
+		start = length + slice[0]
 	}
+	switch {
+	case slice[1] == math.MaxInt:
+		end = length - 1
+	case slice[1] < 0:
+		end = length + slice[1]
+	}
+
+	// TODO: is backward iteration allowed?
+	// if yes, we need to return a +1 or -1 "step"
+	// if no, we need to error
 
 	return start, end
 }
@@ -466,6 +363,10 @@ func isMissing(err error) bool {
 	return false
 }
 
+func IsResolutionErr(err error) bool {
+	return errors.As(err, &resolutionerr{})
+}
+
 type resolutionerr struct {
 	msg string
 	at  []string

pkg/policy/selector/selector_test.go

@@ -14,239 +14,6 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func TestParse(t *testing.T) {
-	t.Run("identity", func(t *testing.T) {
-		sel, err := Parse(".")
-		require.NoError(t, err)
-		require.Equal(t, 1, len(sel))
-		require.True(t, sel[0].Identity())
-		require.False(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Empty(t, sel[0].Field())
-		require.Empty(t, sel[0].Index())
-	})
-
-	t.Run("field", func(t *testing.T) {
-		sel, err := Parse(".foo")
-		require.NoError(t, err)
-		require.Equal(t, 1, len(sel))
-		require.False(t, sel[0].Identity())
-		require.False(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Equal(t, sel[0].Field(), "foo")
-		require.Empty(t, sel[0].Index())
-	})
-
-	t.Run("explicit field", func(t *testing.T) {
-		sel, err := Parse(`.["foo"]`)
-		require.NoError(t, err)
-		require.Equal(t, 2, len(sel))
-		require.True(t, sel[0].Identity())
-		require.False(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Empty(t, sel[0].Field())
-		require.Empty(t, sel[0].Index())
-		require.False(t, sel[1].Identity())
-		require.False(t, sel[1].Optional())
-		require.False(t, sel[1].Iterator())
-		require.Empty(t, sel[1].Slice())
-		require.Equal(t, sel[1].Field(), "foo")
-		require.Empty(t, sel[1].Index())
-	})
-
-	t.Run("index", func(t *testing.T) {
-		sel, err := Parse(".[138]")
-		require.NoError(t, err)
-		require.Equal(t, 2, len(sel))
-		require.True(t, sel[0].Identity())
-		require.False(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Empty(t, sel[0].Field())
-		require.Empty(t, sel[0].Index())
-		require.False(t, sel[1].Identity())
-		require.False(t, sel[1].Optional())
-		require.False(t, sel[1].Iterator())
-		require.Empty(t, sel[1].Slice())
-		require.Empty(t, sel[1].Field())
-		require.Equal(t, sel[1].Index(), 138)
-	})
-
-	t.Run("negative index", func(t *testing.T) {
-		sel, err := Parse(".[-138]")
-		require.NoError(t, err)
-		require.Equal(t, 2, len(sel))
-		require.True(t, sel[0].Identity())
-		require.False(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Empty(t, sel[0].Field())
-		require.Empty(t, sel[0].Index())
-		require.False(t, sel[1].Identity())
-		require.False(t, sel[1].Optional())
-		require.False(t, sel[1].Iterator())
-		require.Empty(t, sel[1].Slice())
-		require.Empty(t, sel[1].Field())
-		require.Equal(t, sel[1].Index(), -138)
-	})
-
-	t.Run("iterator", func(t *testing.T) {
-		sel, err := Parse(".[]")
-		require.NoError(t, err)
-		require.Equal(t, 2, len(sel))
-		require.True(t, sel[0].Identity())
-		require.False(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Empty(t, sel[0].Field())
-		require.Empty(t, sel[0].Index())
-		require.False(t, sel[1].Identity())
-		require.False(t, sel[1].Optional())
-		require.True(t, sel[1].Iterator())
-		require.Empty(t, sel[1].Slice())
-		require.Empty(t, sel[1].Field())
-		require.Empty(t, sel[1].Index())
-	})
-
-	t.Run("optional field", func(t *testing.T) {
-		sel, err := Parse(".foo?")
-		require.NoError(t, err)
-		require.Equal(t, 1, len(sel))
-		require.False(t, sel[0].Identity())
-		require.True(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Equal(t, sel[0].Field(), "foo")
-		require.Empty(t, sel[0].Index())
-	})
-
-	t.Run("optional explicit field", func(t *testing.T) {
-		sel, err := Parse(`.["foo"]?`)
-		require.NoError(t, err)
-		require.Equal(t, 2, len(sel))
-		require.True(t, sel[0].Identity())
-		require.False(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Empty(t, sel[0].Field())
-		require.Empty(t, sel[0].Index())
-		require.False(t, sel[1].Identity())
-		require.True(t, sel[1].Optional())
-		require.False(t, sel[1].Iterator())
-		require.Empty(t, sel[1].Slice())
-		require.Equal(t, sel[1].Field(), "foo")
-		require.Empty(t, sel[1].Index())
-	})
-
-	t.Run("optional index", func(t *testing.T) {
-		sel, err := Parse(".[138]?")
-		require.NoError(t, err)
-		require.Equal(t, 2, len(sel))
-		require.True(t, sel[0].Identity())
-		require.False(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Empty(t, sel[0].Field())
-		require.Empty(t, sel[0].Index())
-		require.False(t, sel[1].Identity())
-		require.True(t, sel[1].Optional())
-		require.False(t, sel[1].Iterator())
-		require.Empty(t, sel[1].Slice())
-		require.Empty(t, sel[1].Field())
-		require.Equal(t, sel[1].Index(), 138)
-	})
-
-	t.Run("optional iterator", func(t *testing.T) {
-		sel, err := Parse(".[]?")
-		require.NoError(t, err)
-		require.Equal(t, 2, len(sel))
-		require.True(t, sel[0].Identity())
-		require.False(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Empty(t, sel[0].Field())
-		require.Empty(t, sel[0].Index())
-		require.False(t, sel[1].Identity())
-		require.True(t, sel[1].Optional())
-		require.True(t, sel[1].Iterator())
-		require.Empty(t, sel[1].Slice())
-		require.Empty(t, sel[1].Field())
-		require.Empty(t, sel[1].Index())
-	})
-
-	t.Run("nesting", func(t *testing.T) {
-		str := `.foo.["bar"].[138]?.baz[1:]`
-		sel, err := Parse(str)
-		require.NoError(t, err)
-		printSegments(sel)
-		require.Equal(t, str, sel.String())
-		require.Equal(t, 7, len(sel))
-		require.False(t, sel[0].Identity())
-		require.False(t, sel[0].Optional())
-		require.False(t, sel[0].Iterator())
-		require.Empty(t, sel[0].Slice())
-		require.Equal(t, sel[0].Field(), "foo")
-		require.Empty(t, sel[0].Index())
-		require.True(t, sel[1].Identity())
-		require.False(t, sel[1].Optional())
-		require.False(t, sel[1].Iterator())
-		require.Empty(t, sel[1].Slice())
-		require.Empty(t, sel[1].Field())
-		require.Empty(t, sel[1].Index())
-		require.False(t, sel[2].Identity())
-		require.False(t, sel[2].Optional())
-		require.False(t, sel[2].Iterator())
-		require.Empty(t, sel[2].Slice())
-		require.Equal(t, sel[2].Field(), "bar")
-		require.Empty(t, sel[2].Index())
-		require.True(t, sel[3].Identity())
-		require.False(t, sel[3].Optional())
-		require.False(t, sel[3].Iterator())
-		require.Empty(t, sel[3].Slice())
-		require.Empty(t, sel[3].Field())
-		require.Empty(t, sel[3].Index())
-		require.False(t, sel[4].Identity())
-		require.True(t, sel[4].Optional())
-		require.False(t, sel[4].Iterator())
-		require.Empty(t, sel[4].Slice())
-		require.Empty(t, sel[4].Field())
-		require.Equal(t, sel[4].Index(), 138)
-		require.False(t, sel[5].Identity())
-		require.False(t, sel[5].Optional())
-		require.False(t, sel[5].Iterator())
-		require.Empty(t, sel[5].Slice())
-		require.Equal(t, sel[5].Field(), "baz")
-		require.Empty(t, sel[5].Index())
-		require.False(t, sel[6].Identity())
-		require.False(t, sel[6].Optional())
-		require.False(t, sel[6].Iterator())
-		require.Equal(t, sel[6].Slice(), []int{1})
-		require.Empty(t, sel[6].Field())
-		require.Empty(t, sel[6].Index())
-	})
-
-	t.Run("non dotted", func(t *testing.T) {
-		_, err := Parse("foo")
-		require.NotNil(t, err)
-		fmt.Println(err)
-	})
-
-	t.Run("non quoted", func(t *testing.T) {
-		_, err := Parse(".[foo]")
-		require.NotNil(t, err)
-		fmt.Println(err)
-	})
-}
-
-func printSegments(s Selector) {
-	for i, seg := range s {
-		fmt.Printf("%d: %s\n", i, seg.String())
-	}
-}
-
 func TestSelect(t *testing.T) {
 	type name struct {
 		First  string
@@ -313,14 +80,13 @@ func TestSelect(t *testing.T) {
 		sel, err := Parse(".")
 		require.NoError(t, err)
 
-		one, many, err := sel.Select(anode)
+		res, err := sel.Select(anode)
 		require.NoError(t, err)
-		require.NotEmpty(t, one)
-		require.Empty(t, many)
+		require.NotEmpty(t, res)
 
-		fmt.Println(printer.Sprint(one))
+		fmt.Println(printer.Sprint(res))
 
-		age := must.Int(must.Node(one.LookupByString("age")))
+		age := must.Int(must.Node(res.LookupByString("age")))
 		require.Equal(t, int64(alice.Age), age)
 	})
 
@@ -328,24 +94,22 @@ func TestSelect(t *testing.T) {
 		sel, err := Parse(".name.first")
 		require.NoError(t, err)
 
-		one, many, err := sel.Select(anode)
+		res, err := sel.Select(anode)
 		require.NoError(t, err)
-		require.NotEmpty(t, one)
-		require.Empty(t, many)
+		require.NotEmpty(t, res)
 
-		fmt.Println(printer.Sprint(one))
+		fmt.Println(printer.Sprint(res))
 
-		name := must.String(one)
+		name := must.String(res)
 		require.Equal(t, alice.Name.First, name)
 
-		one, many, err = sel.Select(bnode)
+		res, err = sel.Select(bnode)
 		require.NoError(t, err)
-		require.NotEmpty(t, one)
-		require.Empty(t, many)
+		require.NotEmpty(t, res)
 
-		fmt.Println(printer.Sprint(one))
+		fmt.Println(printer.Sprint(res))
 
-		name = must.String(one)
+		name = must.String(res)
 		require.Equal(t, bob.Name.First, name)
 	})
 
@@ -353,109 +117,88 @@ func TestSelect(t *testing.T) {
 		sel, err := Parse(".name.middle?")
 		require.NoError(t, err)
 
-		one, many, err := sel.Select(anode)
+		res, err := sel.Select(anode)
 		require.NoError(t, err)
-		require.NotEmpty(t, one)
-		require.Empty(t, many)
+		require.NotEmpty(t, res)
 
-		fmt.Println(printer.Sprint(one))
+		fmt.Println(printer.Sprint(res))
 
-		name := must.String(one)
+		name := must.String(res)
 		require.Equal(t, *alice.Name.Middle, name)
 
-		one, many, err = sel.Select(bnode)
+		res, err = sel.Select(bnode)
 		require.NoError(t, err)
-		require.Empty(t, one)
-		require.Empty(t, many)
+		require.Empty(t, res)
 	})
 
 	t.Run("not exists", func(t *testing.T) {
 		sel, err := Parse(".name.foo")
 		require.NoError(t, err)
 
-		one, many, err := sel.Select(anode)
+		res, err := sel.Select(anode)
 		require.Error(t, err)
-		require.Empty(t, one)
-		require.Empty(t, many)
+		require.Empty(t, res)
 
 		fmt.Println(err)
 
-		require.ErrorAs(t, err, &resolutionerr{}, "error was not a resolution error")
+		require.ErrorAs(t, err, &resolutionerr{}, "error should be a resolution error")
+		require.True(t, IsResolutionErr(err))
 	})
 
 	t.Run("optional not exists", func(t *testing.T) {
 		sel, err := Parse(".name.foo?")
 		require.NoError(t, err)
 
-		one, many, err := sel.Select(anode)
+		one, err := sel.Select(anode)
 		require.NoError(t, err)
 		require.Empty(t, one)
-		require.Empty(t, many)
 	})
 
 	t.Run("iterator", func(t *testing.T) {
 		sel, err := Parse(".interests[]")
 		require.NoError(t, err)
 
-		one, many, err := sel.Select(anode)
+		res, err := sel.Select(anode)
 		require.NoError(t, err)
-		require.Empty(t, one)
-		require.NotEmpty(t, many)
+		require.NotEmpty(t, res)
 
-		for _, n := range many {
-			fmt.Println(printer.Sprint(n))
-		}
+		fmt.Println(printer.Sprint(res))
 
-		iname := must.String(must.Node(many[0].LookupByString("name")))
+		iname := must.String(must.Node(must.Node(res.LookupByIndex(0)).LookupByString("name")))
 		require.Equal(t, alice.Interests[0].Name, iname)
 
-		iname = must.String(must.Node(many[1].LookupByString("name")))
+		iname = must.String(must.Node(must.Node(res.LookupByIndex(1)).LookupByString("name")))
 		require.Equal(t, alice.Interests[1].Name, iname)
 	})
 
-	t.Run("map iterator", func(t *testing.T) {
-		sel, err := Parse(".interests[0][]")
-		require.NoError(t, err)
-
-		one, many, err := sel.Select(anode)
-		require.NoError(t, err)
-		require.Empty(t, one)
-		require.NotEmpty(t, many)
-
-		for _, n := range many {
-			fmt.Println(printer.Sprint(n))
-		}
-
-		require.Equal(t, alice.Interests[0].Name, must.String(many[0]))
-		require.Equal(t, alice.Interests[0].Experience, int(must.Int(many[2])))
-	})
+	// TODO: fully test slicing + negative numbers
 }
 
-func TestMatch(t *testing.T) {
-	for _, tc := range []struct {
-		sel       string
-		path      []string
-		want      bool
-		remaining []string
-	}{
-		{sel: ".foo.bar", path: []string{"foo", "bar"}, want: true, remaining: []string{}},
-		{sel: ".foo.bar", path: []string{"foo"}, want: true, remaining: []string{}},
-		{sel: ".foo.bar", path: []string{"foo", "bar", "baz"}, want: true, remaining: []string{"baz"}},
-		{sel: ".foo.bar", path: []string{"foo", "faa"}, want: false},
-		{sel: ".foo.[]", path: []string{"foo", "faa"}, want: false},
-		{sel: ".foo.[]", path: []string{"foo"}, want: true, remaining: []string{}},
-		{sel: ".foo.bar?", path: []string{"foo"}, want: true, remaining: []string{}},
-		{sel: ".foo.bar?", path: []string{"foo", "bar"}, want: true, remaining: []string{}},
-		{sel: ".foo.bar?", path: []string{"foo", "baz"}, want: false},
-	} {
-		t.Run(tc.sel, func(t *testing.T) {
-			sel := MustParse(tc.sel)
-			res, remain := sel.MatchPath(tc.path...)
-			require.Equal(t, tc.want, res)
-			require.EqualValues(t, tc.remaining, remain)
-		})
-	}
-}
+// func TestMatch(t *testing.T) {
+// 	for _, tc := range []struct {
+// 		sel       string
+// 		path      []string
+// 		want      bool
+// 		remaining []string
+// 	}{
+// 		{sel: ".foo.bar", path: []string{"foo", "bar"}, want: true, remaining: []string{}},
+// 		{sel: ".foo.bar", path: []string{"foo"}, want: true, remaining: []string{}},
+// 		{sel: ".foo.bar", path: []string{"foo", "bar", "baz"}, want: true, remaining: []string{"baz"}},
+// 		{sel: ".foo.bar", path: []string{"foo", "faa"}, want: false},
+// 		{sel: ".foo.[]", path: []string{"foo", "faa"}, want: false},
+// 		{sel: ".foo.[]", path: []string{"foo"}, want: true, remaining: []string{}},
+// 		{sel: ".foo.bar?", path: []string{"foo"}, want: true, remaining: []string{}},
+// 		{sel: ".foo.bar?", path: []string{"foo", "bar"}, want: true, remaining: []string{}},
+// 		{sel: ".foo.bar?", path: []string{"foo", "baz"}, want: false},
+// 	} {
+// 		t.Run(tc.sel, func(t *testing.T) {
+// 			sel := MustParse(tc.sel)
+// 			res, remain := sel.MatchPath(tc.path...)
+// 			require.Equal(t, tc.want, res)
+// 			require.EqualValues(t, tc.remaining, remain)
+// 		})
+// 	}
+// }
 
 func FuzzParse(f *testing.F) {
 	selectorCorpus := []string{
@@ -520,6 +263,6 @@ func FuzzParseAndSelect(f *testing.F) {
 		}
 
 		// look for panic()
-		_, _, _ = sel.Select(node)
+		_, _ = sel.Select(node)
 	})
 }

pkg/policy/selector/supported_test.go

@@ -26,7 +26,7 @@ func TestSupportedForms(t *testing.T) {
 		Output   string
 	}
 
-	// Pass
+	// Pass and return a node
 	for _, testcase := range []Testcase{
 		{Name: "Identity", Selector: `.`, Input: `{"x":1}`, Output: `{"x":1}`},
 		{Name: "Iterator", Selector: `.[]`, Input: `[1, 2]`, Output: `[1, 2]`},
@@ -52,35 +52,16 @@ func TestSupportedForms(t *testing.T) {
 			require.NoError(t, err)
 
 			// attempt to select
-			node, nodes, err := sel.Select(makeNode(t, tc.Input))
+			res, err := sel.Select(makeNode(t, tc.Input))
 			require.NoError(t, err)
-			require.NotEqual(t, node != nil, len(nodes) > 0) // XOR (only one of node or nodes should be set)
-
-			// make an IPLD List node from a []datamodel.Node
-			if node == nil {
-				nb := basicnode.Prototype.List.NewBuilder()
-				la, err := nb.BeginList(int64(len(nodes)))
-				require.NoError(t, err)
-
-				for _, n := range nodes {
-					// TODO: This code is probably not needed if the Select operation properly prunes nil values - e.g.: Optional Iterator
-					if n == nil {
-						n = datamodel.Null
-					}
-
-					require.NoError(t, la.AssembleValue().AssignNode(n))
-				}
-				require.NoError(t, la.Finish())
-
-				node = nb.Build()
-			}
+			require.NotNil(t, res)
 
 			exp := makeNode(t, tc.Output)
-			equalIPLD(t, exp, node)
+			equalIPLD(t, exp, res)
 		})
 	}
 
-	// null
+	// No error and return null, as optional
 	for _, testcase := range []Testcase{
 		{Name: "Optional Missing Key", Selector: `.x?`, Input: `{}`},
 		{Name: "Optional Null Key", Selector: `.x?`, Input: `null`},
@@ -97,16 +78,13 @@ func TestSupportedForms(t *testing.T) {
 			require.NoError(t, err)
 
 			// attempt to select
-			node, nodes, err := sel.Select(makeNode(t, tc.Input))
+			res, err := sel.Select(makeNode(t, tc.Input))
 			require.NoError(t, err)
-			// TODO: should Select return a single node which is sometimes a list or null?
-			// require.Equal(t, datamodel.Null, node)
-			assert.Nil(t, node)
-			assert.Empty(t, nodes)
+			require.Nil(t, res)
 		})
 	}
 
-	// error
+	// fail to select and return an error
 	for _, testcase := range []Testcase{
 		{Name: "Null Iterator", Selector: `.[]`, Input: `null`},
 		{Name: "Nested Iterator", Selector: `.[][]`, Input: `[[1], 2, [3]]`},
@@ -124,10 +102,10 @@ func TestSupportedForms(t *testing.T) {
 			require.NoError(t, err)
 
 			// attempt to select
-			node, nodes, err := sel.Select(makeNode(t, tc.Input))
+			res, err := sel.Select(makeNode(t, tc.Input))
 			require.Error(t, err)
-			assert.Nil(t, node)
-			assert.Empty(t, nodes)
+			require.True(t, selector.IsResolutionErr(err))
+			require.Nil(t, res)
 		})
 	}
 }