From 95e294bcdfc6192454eee1908b9677c4bba9557c Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 14:51:04 +0100
Subject: [PATCH 01/13] Add partial-match travesal of large bytes

---
 node/basicnode/bytes.go                    | 13 ++-
 node/basicnode/bytes_stream.go             | 81 ++++++++++++++++++
 traversal/selector/builder/builder.go      | 14 +++
 traversal/selector/exploreAll.go           |  5 ++
 traversal/selector/exploreFields.go        |  5 ++
 traversal/selector/exploreIndex.go         |  5 ++
 traversal/selector/exploreInterpretAs.go   |  5 ++
 traversal/selector/exploreRange.go         |  5 ++
 traversal/selector/exploreRecursive.go     |  5 ++
 traversal/selector/exploreRecursiveEdge.go |  5 ++
 traversal/selector/exploreUnion.go         | 12 +++
 traversal/selector/fieldKeys.go            |  3 +
 traversal/selector/matcher.go              | 99 +++++++++++++++++++++-
 traversal/selector/matcher_util.go         | 15 ++++
 traversal/selector/selector.go             |  6 ++
 traversal/walk.go                          |  6 +-
 16 files changed, 278 insertions(+), 6 deletions(-)
 create mode 100644 node/basicnode/bytes_stream.go
 create mode 100644 traversal/selector/matcher_util.go

diff --git a/node/basicnode/bytes.go b/node/basicnode/bytes.go
index 7c45d208..566c32fe 100644
--- a/node/basicnode/bytes.go
+++ b/node/basicnode/bytes.go
@@ -106,7 +106,7 @@ func (nb *plainBytes__Builder) Reset() {
 // -- NodeAssembler -->
 
 type plainBytes__Assembler struct {
-	w *plainBytes
+	w datamodel.Node
 }
 
 func (plainBytes__Assembler) BeginMap(sizeHint int64) (datamodel.MapAssembler, error) {
@@ -131,17 +131,24 @@ func (plainBytes__Assembler) AssignString(string) error {
 	return mixins.BytesAssembler{TypeName: "bytes"}.AssignString("")
 }
 func (na *plainBytes__Assembler) AssignBytes(v []byte) error {
-	*na.w = plainBytes(v)
+	na.w = datamodel.Node(plainBytes(v))
 	return nil
 }
 func (plainBytes__Assembler) AssignLink(datamodel.Link) error {
 	return mixins.BytesAssembler{TypeName: "bytes"}.AssignLink(nil)
 }
 func (na *plainBytes__Assembler) AssignNode(v datamodel.Node) error {
+	if lb, ok := v.(datamodel.LargeBytesNode); ok {
+		lbn, err := lb.AsLargeBytes()
+		if err == nil {
+			na.w = streamBytes{lbn}
+			return nil
+		}
+	}
 	if v2, err := v.AsBytes(); err != nil {
 		return err
 	} else {
-		*na.w = plainBytes(v2)
+		na.w = plainBytes(v2)
 		return nil
 	}
 }
diff --git a/node/basicnode/bytes_stream.go b/node/basicnode/bytes_stream.go
new file mode 100644
index 00000000..ad238bcf
--- /dev/null
+++ b/node/basicnode/bytes_stream.go
@@ -0,0 +1,81 @@
+package basicnode
+
+import (
+	"io"
+
+	"github.com/ipld/go-ipld-prime/datamodel"
+	"github.com/ipld/go-ipld-prime/node/mixins"
+)
+
+var (
+	_ datamodel.Node          = streamBytes{nil}
+	_ datamodel.NodePrototype = Prototype__Bytes{}
+	_ datamodel.NodeBuilder   = &plainBytes__Builder{}
+	_ datamodel.NodeAssembler = &plainBytes__Assembler{}
+)
+
+func NewBytesFromReader(rs io.ReadSeeker) datamodel.Node {
+	return streamBytes{rs}
+}
+
+// streamBytes is a boxed reader that complies with datamodel.Node.
+type streamBytes struct {
+	io.ReadSeeker
+}
+
+// -- Node interface methods -->
+
+func (streamBytes) Kind() datamodel.Kind {
+	return datamodel.Kind_Bytes
+}
+func (streamBytes) LookupByString(string) (datamodel.Node, error) {
+	return mixins.Bytes{TypeName: "bytes"}.LookupByString("")
+}
+func (streamBytes) LookupByNode(key datamodel.Node) (datamodel.Node, error) {
+	return mixins.Bytes{TypeName: "bytes"}.LookupByNode(nil)
+}
+func (streamBytes) LookupByIndex(idx int64) (datamodel.Node, error) {
+	return mixins.Bytes{TypeName: "bytes"}.LookupByIndex(0)
+}
+func (streamBytes) LookupBySegment(seg datamodel.PathSegment) (datamodel.Node, error) {
+	return mixins.Bytes{TypeName: "bytes"}.LookupBySegment(seg)
+}
+func (streamBytes) MapIterator() datamodel.MapIterator {
+	return nil
+}
+func (streamBytes) ListIterator() datamodel.ListIterator {
+	return nil
+}
+func (streamBytes) Length() int64 {
+	return -1
+}
+func (streamBytes) IsAbsent() bool {
+	return false
+}
+func (streamBytes) IsNull() bool {
+	return false
+}
+func (streamBytes) AsBool() (bool, error) {
+	return mixins.Bytes{TypeName: "bytes"}.AsBool()
+}
+func (streamBytes) AsInt() (int64, error) {
+	return mixins.Bytes{TypeName: "bytes"}.AsInt()
+}
+func (streamBytes) AsFloat() (float64, error) {
+	return mixins.Bytes{TypeName: "bytes"}.AsFloat()
+}
+func (streamBytes) AsString() (string, error) {
+	return mixins.Bytes{TypeName: "bytes"}.AsString()
+}
+func (n streamBytes) AsBytes() ([]byte, error) {
+	return io.ReadAll(n)
+}
+func (streamBytes) AsLink() (datamodel.Link, error) {
+	return mixins.Bytes{TypeName: "bytes"}.AsLink()
+}
+func (streamBytes) Prototype() datamodel.NodePrototype {
+	return Prototype__Bytes{}
+}
+func (n streamBytes) AsLargeBytes() (io.ReadSeeker, error) {
+	return n.ReadSeeker, nil
+}
diff --git a/traversal/selector/builder/builder.go b/traversal/selector/builder/builder.go
index 80067758..6cc69e3c 100644
--- a/traversal/selector/builder/builder.go
+++ b/traversal/selector/builder/builder.go
@@ -32,6 +32,7 @@ type SelectorSpecBuilder interface {
 	ExploreFields(ExploreFieldsSpecBuildingClosure) SelectorSpec
 	ExploreInterpretAs(as string, next SelectorSpec) SelectorSpec
 	Matcher() SelectorSpec
+	MatcherSubset(from, to int64) SelectorSpec
 }
 
 // ExploreFieldsSpecBuildingClosure is a function that provided to SelectorSpecBuilder's
@@ -170,6 +171,19 @@ func (ssb *selectorSpecBuilder) Matcher() SelectorSpec {
 	}
 }
 
+func (ssb *selectorSpecBuilder) MatcherSubset(from, to int64) SelectorSpec {
+	return selectorSpec{
+		fluent.MustBuildMap(ssb.np, 1, func(na fluent.MapAssembler) {
+			na.AssembleEntry(selector.SelectorKey_Matcher).CreateMap(1, func(na fluent.MapAssembler) {
+				na.AssembleEntry(selector.SelectorKey_Subset).CreateMap(2, func(na fluent.MapAssembler) {
+					na.AssembleEntry(selector.SelectorKey_From).AssignInt(from)
+					na.AssembleEntry(selector.SelectorKey_To).AssignInt(to)
+				})
+			})
+		}),
+	}
+}
+
 type exploreFieldsSpecBuilder struct {
 	na fluent.MapAssembler
 }
diff --git a/traversal/selector/exploreAll.go b/traversal/selector/exploreAll.go
index a7749f8e..7d889d7e 100644
--- a/traversal/selector/exploreAll.go
+++ b/traversal/selector/exploreAll.go
@@ -27,6 +27,11 @@ func (s ExploreAll) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreAll) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // ParseExploreAll assembles a Selector from a ExploreAll selector node
 func (pc ParseContext) ParseExploreAll(n datamodel.Node) (Selector, error) {
 	if n.Kind() != datamodel.Kind_Map {
diff --git a/traversal/selector/exploreFields.go b/traversal/selector/exploreFields.go
index 5d2abdd2..b2796ff2 100644
--- a/traversal/selector/exploreFields.go
+++ b/traversal/selector/exploreFields.go
@@ -38,6 +38,11 @@ func (s ExploreFields) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreFields) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // ParseExploreFields assembles a Selector
 // from a ExploreFields selector node
 func (pc ParseContext) ParseExploreFields(n datamodel.Node) (Selector, error) {
diff --git a/traversal/selector/exploreIndex.go b/traversal/selector/exploreIndex.go
index 0c8edf16..56a2badc 100644
--- a/traversal/selector/exploreIndex.go
+++ b/traversal/selector/exploreIndex.go
@@ -37,6 +37,11 @@ func (s ExploreIndex) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreIndex) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // ParseExploreIndex assembles a Selector
 // from a ExploreIndex selector node
 func (pc ParseContext) ParseExploreIndex(n datamodel.Node) (Selector, error) {
diff --git a/traversal/selector/exploreInterpretAs.go b/traversal/selector/exploreInterpretAs.go
index 25cf045b..6f4aed67 100644
--- a/traversal/selector/exploreInterpretAs.go
+++ b/traversal/selector/exploreInterpretAs.go
@@ -27,6 +27,11 @@ func (s ExploreInterpretAs) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreInterpretAs) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // NamedReifier indicates how this selector expects to Reify the current datamodel.Node.
 func (s ExploreInterpretAs) NamedReifier() string {
 	return s.adl
diff --git a/traversal/selector/exploreRange.go b/traversal/selector/exploreRange.go
index 73e356ba..e9e041bc 100644
--- a/traversal/selector/exploreRange.go
+++ b/traversal/selector/exploreRange.go
@@ -41,6 +41,11 @@ func (s ExploreRange) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreRange) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // ParseExploreRange assembles a Selector
 // from a ExploreRange selector node
 func (pc ParseContext) ParseExploreRange(n datamodel.Node) (Selector, error) {
diff --git a/traversal/selector/exploreRecursive.go b/traversal/selector/exploreRecursive.go
index 17614025..28a9bedc 100644
--- a/traversal/selector/exploreRecursive.go
+++ b/traversal/selector/exploreRecursive.go
@@ -176,6 +176,11 @@ func (s ExploreRecursive) Decide(n datamodel.Node) bool {
 	return s.current.Decide(n)
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreRecursive) Match(node datamodel.Node) (datamodel.Node, error) {
+	return s.current.Match(node)
+}
+
 type exploreRecursiveContext struct {
 	edgesFound int
 }
diff --git a/traversal/selector/exploreRecursiveEdge.go b/traversal/selector/exploreRecursiveEdge.go
index 65438572..534c73b8 100644
--- a/traversal/selector/exploreRecursiveEdge.go
+++ b/traversal/selector/exploreRecursiveEdge.go
@@ -31,6 +31,11 @@ func (s ExploreRecursiveEdge) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreRecursiveEdge) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // ParseExploreRecursiveEdge assembles a Selector
 // from a exploreRecursiveEdge selector node
 func (pc ParseContext) ParseExploreRecursiveEdge(n datamodel.Node) (Selector, error) {
diff --git a/traversal/selector/exploreUnion.go b/traversal/selector/exploreUnion.go
index 0af42cd6..21594954 100644
--- a/traversal/selector/exploreUnion.go
+++ b/traversal/selector/exploreUnion.go
@@ -74,6 +74,18 @@ func (s ExploreUnion) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match returns true for a Union selector based on the matched union.
+func (s ExploreUnion) Match(n datamodel.Node) (datamodel.Node, error) {
+	for _, m := range s.Members {
+		if mn, err := m.Match(n); mn != nil {
+			return mn, nil
+		} else if err != nil {
+			return nil, err
+		}
+	}
+	return nil, nil
+}
+
 // ParseExploreUnion assembles a Selector
 // from an ExploreUnion selector node
 func (pc ParseContext) ParseExploreUnion(n datamodel.Node) (Selector, error) {
diff --git a/traversal/selector/fieldKeys.go b/traversal/selector/fieldKeys.go
index 58024b61..39ec61e6 100644
--- a/traversal/selector/fieldKeys.go
+++ b/traversal/selector/fieldKeys.go
@@ -23,5 +23,8 @@ const (
 	SelectorKey_StopAt               = "!"
 	SelectorKey_Condition            = "&"
 	SelectorKey_As                   = "as"
+	SelectorKey_Subset               = "subset"
+	SelectorKey_From                 = "["
+	SelectorKey_To                   = "]"
 	// not filling conditional keys since it's not complete
 )
diff --git a/traversal/selector/matcher.go b/traversal/selector/matcher.go
index b8f64f68..d9538dc6 100644
--- a/traversal/selector/matcher.go
+++ b/traversal/selector/matcher.go
@@ -2,8 +2,10 @@ package selector
 
 import (
 	"fmt"
+	"io"
 
 	"github.com/ipld/go-ipld-prime/datamodel"
+	"github.com/ipld/go-ipld-prime/node/basicnode"
 )
 
 // Matcher marks a node to be included in the "result" set.
@@ -16,7 +18,61 @@ import (
 // A selector tree with only "explore*"-type selectors and no Matcher selectors
 // is valid; it will just generate a "covered" set of nodes and no "result" set.
 // TODO: From spec: implement conditions and labels
-type Matcher struct{}
+type Matcher struct {
+	*Slice
+}
+
+// Slice limits a result node to a subset of the node.
+// The returned node will be limited based on slicing the specified range of the
+// node into a new node, or making use of the `AsLargeBytes` io.ReadSeeker to
+// restrict response with a SectionReader.
+type Slice struct {
+	From int64
+	To   int64
+}
+
+func (s Slice) Slice(n datamodel.Node) (datamodel.Node, error) {
+	if n.Kind() == datamodel.Kind_String {
+		str, err := n.AsString()
+		if err != nil {
+			return nil, err
+		}
+		to := s.To
+		if len(str) < int(to) {
+			to = int64(len(str))
+		}
+		from := s.From
+		if len(str) < int(from) {
+			from = int64(len(str))
+		}
+		return basicnode.NewString(str[from:to]), nil
+	} else if n.Kind() == datamodel.Kind_Bytes {
+		if lbn, ok := n.(datamodel.LargeBytesNode); ok {
+			rdr, err := lbn.AsLargeBytes()
+			if err != nil {
+				return nil, err
+			}
+
+			sr := io.NewSectionReader(readerat{rdr}, int64(s.From), int64(s.To))
+			return basicnode.NewBytesFromReader(sr), nil
+		}
+		bytes, err := n.AsBytes()
+		if err != nil {
+			return nil, err
+		}
+		to := s.To
+		if len(bytes) < int(to) {
+			to = int64(len(bytes))
+		}
+		from := s.From
+		if len(bytes) < int(from) {
+			from = int64(len(bytes))
+		}
+
+		return basicnode.NewBytes(bytes[from:to]), nil
+	}
+	return nil, fmt.Errorf("selector slice rejected: subset match must be over string or bytes")
+}
 
 // Interests are empty for a matcher (for now) because
 // It is always just there to match, not explore further
@@ -35,6 +91,14 @@ func (s Matcher) Decide(n datamodel.Node) bool {
 	return true
 }
 
+// Match is always true for a match cause it's in the result set
+func (s Matcher) Match(node datamodel.Node) (datamodel.Node, error) {
+	if s.Slice != nil {
+		return s.Slice.Slice(node)
+	}
+	return node, nil
+}
+
 // ParseMatcher assembles a Selector
 // from a matcher selector node
 // TODO: Parse labels and conditions
@@ -42,5 +106,38 @@ func (pc ParseContext) ParseMatcher(n datamodel.Node) (Selector, error) {
 	if n.Kind() != datamodel.Kind_Map {
 		return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map")
 	}
+
+	// check if a slice is specified
+	if subset, err := n.LookupByString("subset"); err == nil {
+		if subset.Kind() != datamodel.Kind_Map {
+			return nil, fmt.Errorf("selector spec parse rejected: subset body must be a map")
+		}
+		from, err := subset.LookupByString("[")
+		if err != nil {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with a from '[' key")
+		}
+		fromN, err := from.AsInt()
+		if err != nil {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with a 'from' key that is a number")
+		}
+		to, err := subset.LookupByString("]")
+		if err != nil {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with a to ']' key")
+		}
+		toN, err := to.AsInt()
+		if err != nil {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with a 'to' key that is a number")
+		}
+		if fromN > toN {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with a 'from' key that is less than or equal to the 'to' key")
+		}
+		if fromN < 0 || toN < 0 {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with keys not less than 0")
+		}
+		return Matcher{&Slice{
+			From: fromN,
+			To:   toN,
+		}}, nil
+	}
 	return Matcher{}, nil
 }
diff --git a/traversal/selector/matcher_util.go b/traversal/selector/matcher_util.go
new file mode 100644
index 00000000..b67cad09
--- /dev/null
+++ b/traversal/selector/matcher_util.go
@@ -0,0 +1,15 @@
+package selector
+
+import "io"
+
+type readerat struct {
+	io.ReadSeeker
+}
+
+func (r readerat) ReadAt(p []byte, off int64) (n int, err error) {
+	_, err = r.Seek(off, 0)
+	if err != nil {
+		return 0, err
+	}
+	return r.Read(p)
+}
diff --git a/traversal/selector/selector.go b/traversal/selector/selector.go
index 4d0a7814..b58fcd86 100644
--- a/traversal/selector/selector.go
+++ b/traversal/selector/selector.go
@@ -106,6 +106,12 @@ type Selector interface {
 	// Only "Matcher" clauses actually implement this in a way that ever returns "true".
 	// See the Selector specs for discussion on "matched" vs "reached"/"visited" nodes.
 	Decide(node datamodel.Node) bool
+
+	// Match is an extension to Decide allowing the matcher to `decide` a transformation of
+	// the matched node. This is used for `Subset` match behavior. If the node is matched,
+	// the first argument will be the matched node. If it is not matched, the first argument
+	// will be null. If there is an error, the first argument will be null.
+	Match(node datamodel.Node) (datamodel.Node, error)
 }
 
 // REVIEW: do ParsedParent and ParseContext need to be exported?  They're mostly used during the compilation process.
diff --git a/traversal/walk.go b/traversal/walk.go
index 80250170..0f659189 100644
--- a/traversal/walk.go
+++ b/traversal/walk.go
@@ -182,10 +182,12 @@ func (prog Progress) walkAdv(n datamodel.Node, s selector.Selector, fn AdvVisitF
 	}
 
 	// Decide if this node is matched -- do callbacks as appropriate.
-	if s.Decide(n) {
-		if err := fn(prog, n, VisitReason_SelectionMatch); err != nil {
+	if match, err := s.Match(n); match != nil {
+		if err := fn(prog, match, VisitReason_SelectionMatch); err != nil {
 			return err
 		}
+	} else if err != nil {
+		return err
 	} else {
 		if err := fn(prog, n, VisitReason_SelectionCandidate); err != nil {
 			return err

From 3f71e8e6e3c932bb6ffb7f47cafbbbb0413b7f0f Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 15:28:23 +0100
Subject: [PATCH 02/13] bytes spec test

---
 node/basicnode/bytes_test.go | 12 ++++++++++++
 node/tests/byteSpecs.go      | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)
 create mode 100644 node/basicnode/bytes_test.go
 create mode 100644 node/tests/byteSpecs.go

diff --git a/node/basicnode/bytes_test.go b/node/basicnode/bytes_test.go
new file mode 100644
index 00000000..ca408484
--- /dev/null
+++ b/node/basicnode/bytes_test.go
@@ -0,0 +1,12 @@
+package basicnode_test
+
+import (
+	"testing"
+
+	"github.com/ipld/go-ipld-prime/node/basicnode"
+	"github.com/ipld/go-ipld-prime/node/tests"
+)
+
+func TestBytes(t *testing.T) {
+	tests.SpecTestBytes(t, basicnode.Prototype__Bytes{})
+}
diff --git a/node/tests/byteSpecs.go b/node/tests/byteSpecs.go
new file mode 100644
index 00000000..69bf494b
--- /dev/null
+++ b/node/tests/byteSpecs.go
@@ -0,0 +1,35 @@
+package tests
+
+import (
+	"io"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+
+	"github.com/ipld/go-ipld-prime/datamodel"
+)
+
+func SpecTestBytes(t *testing.T, np datamodel.NodePrototype) {
+	t.Run("byte node", func(t *testing.T) {
+		nb := np.NewBuilder()
+		err := nb.AssignBytes([]byte("asdf"))
+		qt.Check(t, err, qt.IsNil)
+		n := nb.Build()
+
+		qt.Check(t, n.Kind(), qt.Equals, datamodel.Kind_Bytes)
+		qt.Check(t, n.IsNull(), qt.IsFalse)
+		x, err := n.AsBytes()
+		qt.Check(t, err, qt.IsNil)
+		qt.Check(t, x, qt.DeepEquals, []byte("asdf"))
+
+		lbn, ok := n.(datamodel.LargeBytesNode)
+		if ok {
+			str, err := lbn.AsLargeBytes()
+			qt.Check(t, err, qt.IsNil)
+			bytes, err := io.ReadAll(str)
+			qt.Check(t, err, qt.IsNil)
+			qt.Check(t, bytes, qt.DeepEquals, []byte("asdf"))
+		}
+
+	})
+}

From 981297b3c950ea89788478a0dbcf2c9b06914c0b Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 15:32:55 +0100
Subject: [PATCH 03/13] extend traversal to handle partial byte range match
 access behavior

---
 traversal/selector/spec_test.go | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/traversal/selector/spec_test.go b/traversal/selector/spec_test.go
index 215bfd6b..7d309d05 100644
--- a/traversal/selector/spec_test.go
+++ b/traversal/selector/spec_test.go
@@ -2,6 +2,7 @@ package selector_test
 
 import (
 	"bytes"
+	"io"
 	"os"
 	"testing"
 
@@ -14,7 +15,7 @@ import (
 	"github.com/ipld/go-ipld-prime/fluent/qp"
 	"github.com/ipld/go-ipld-prime/node/basicnode"
 	"github.com/ipld/go-ipld-prime/traversal"
-	"github.com/ipld/go-ipld-prime/traversal/selector/parse"
+	selectorparse "github.com/ipld/go-ipld-prime/traversal/selector/parse"
 )
 
 func TestSpecFixtures(t *testing.T) {
@@ -81,6 +82,18 @@ func testOneSpecFixtureFile(t *testing.T, filename string) {
 					}))
 					qp.MapEntry(ma, "matched", qp.Bool(reason == traversal.VisitReason_SelectionMatch))
 				})
+				if reason == traversal.VisitReason_SelectionMatch && n.Kind() == datamodel.Kind_Bytes {
+					if lbn, ok := n.(datamodel.LargeBytesNode); ok {
+						rdr, err := lbn.AsLargeBytes()
+						if err == nil {
+							io.Copy(io.Discard, rdr)
+						}
+					}
+					_, err := n.AsBytes()
+					if err != nil {
+						panic("insanity at a deeper level than this test's target")
+					}
+				}
 				if err != nil {
 					panic("insanity at a deeper level than this test's target")
 				}

From 5a41f7fcbac8f7e699b57ca22cce62bc2761d448 Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 15:47:06 +0100
Subject: [PATCH 04/13] code review

---
 traversal/selector/matcher.go      | 19 +++++++++++--------
 traversal/selector/matcher_util.go |  1 +
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/traversal/selector/matcher.go b/traversal/selector/matcher.go
index d9538dc6..38314db4 100644
--- a/traversal/selector/matcher.go
+++ b/traversal/selector/matcher.go
@@ -32,46 +32,49 @@ type Slice struct {
 }
 
 func (s Slice) Slice(n datamodel.Node) (datamodel.Node, error) {
-	if n.Kind() == datamodel.Kind_String {
+	var from, to int64
+	switch n.Kind() {
+	case datamodel.Kind_String:
 		str, err := n.AsString()
 		if err != nil {
 			return nil, err
 		}
-		to := s.To
+		to = s.To
 		if len(str) < int(to) {
 			to = int64(len(str))
 		}
-		from := s.From
+		from = s.From
 		if len(str) < int(from) {
 			from = int64(len(str))
 		}
 		return basicnode.NewString(str[from:to]), nil
-	} else if n.Kind() == datamodel.Kind_Bytes {
+	case datamodel.Kind_Bytes:
 		if lbn, ok := n.(datamodel.LargeBytesNode); ok {
 			rdr, err := lbn.AsLargeBytes()
 			if err != nil {
 				return nil, err
 			}
 
-			sr := io.NewSectionReader(readerat{rdr}, int64(s.From), int64(s.To))
+			sr := io.NewSectionReader(readerat{rdr}, s.From, s.To)
 			return basicnode.NewBytesFromReader(sr), nil
 		}
 		bytes, err := n.AsBytes()
 		if err != nil {
 			return nil, err
 		}
-		to := s.To
+		to = s.To
 		if len(bytes) < int(to) {
 			to = int64(len(bytes))
 		}
-		from := s.From
+		from = s.From
 		if len(bytes) < int(from) {
 			from = int64(len(bytes))
 		}
 
 		return basicnode.NewBytes(bytes[from:to]), nil
+	default:
+		return nil, fmt.Errorf("selector slice rejected: subset match must be over string or bytes")
 	}
-	return nil, fmt.Errorf("selector slice rejected: subset match must be over string or bytes")
 }
 
 // Interests are empty for a matcher (for now) because
diff --git a/traversal/selector/matcher_util.go b/traversal/selector/matcher_util.go
index b67cad09..b60d11c8 100644
--- a/traversal/selector/matcher_util.go
+++ b/traversal/selector/matcher_util.go
@@ -7,6 +7,7 @@ type readerat struct {
 }
 
 func (r readerat) ReadAt(p []byte, off int64) (n int, err error) {
+	// TODO: consider keeping track of current offset.
 	_, err = r.Seek(off, 0)
 	if err != nil {
 		return 0, err

From e0f4c23806e3f65a03530a55db5c4f7040dce062 Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 16:00:28 +0100
Subject: [PATCH 05/13] warning on readerat

---
 traversal/selector/matcher_util.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/traversal/selector/matcher_util.go b/traversal/selector/matcher_util.go
index b60d11c8..237c5e71 100644
--- a/traversal/selector/matcher_util.go
+++ b/traversal/selector/matcher_util.go
@@ -6,6 +6,10 @@ type readerat struct {
 	io.ReadSeeker
 }
 
+// ReadAt provides the io.ReadAt method over a ReadSeeker.
+// This implementation does not support concurrent calls to `ReadAt`,
+// as specified by the ReaderAt interface, and so must only be used
+// in non-concurrent use cases.
 func (r readerat) ReadAt(p []byte, off int64) (n int, err error) {
 	// TODO: consider keeping track of current offset.
 	_, err = r.Seek(off, 0)

From 1a0310848f49d81c24dd293acee95ca254312599 Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 19:06:38 +0100
Subject: [PATCH 06/13] fix section reader

---
 traversal/selector/matcher.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/traversal/selector/matcher.go b/traversal/selector/matcher.go
index 38314db4..4186221f 100644
--- a/traversal/selector/matcher.go
+++ b/traversal/selector/matcher.go
@@ -55,7 +55,7 @@ func (s Slice) Slice(n datamodel.Node) (datamodel.Node, error) {
 				return nil, err
 			}
 
-			sr := io.NewSectionReader(readerat{rdr}, s.From, s.To)
+			sr := io.NewSectionReader(readerat{rdr}, s.From, s.To-s.From)
 			return basicnode.NewBytesFromReader(sr), nil
 		}
 		bytes, err := n.AsBytes()

From 40773754af1c432c5a67599cbab165b57c9253ba Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 14:51:04 +0100
Subject: [PATCH 07/13] Add partial-match travesal of large bytes

---
 node/basicnode/bytes.go                    | 13 ++-
 node/basicnode/bytes_stream.go             | 81 ++++++++++++++++++
 traversal/selector/builder/builder.go      | 14 +++
 traversal/selector/exploreAll.go           |  5 ++
 traversal/selector/exploreFields.go        |  5 ++
 traversal/selector/exploreIndex.go         |  5 ++
 traversal/selector/exploreInterpretAs.go   |  5 ++
 traversal/selector/exploreRange.go         |  5 ++
 traversal/selector/exploreRecursive.go     |  5 ++
 traversal/selector/exploreRecursiveEdge.go |  5 ++
 traversal/selector/exploreUnion.go         | 12 +++
 traversal/selector/fieldKeys.go            |  3 +
 traversal/selector/matcher.go              | 99 +++++++++++++++++++++-
 traversal/selector/matcher_util.go         | 15 ++++
 traversal/selector/selector.go             |  6 ++
 traversal/walk.go                          | 22 ++---
 16 files changed, 286 insertions(+), 14 deletions(-)
 create mode 100644 node/basicnode/bytes_stream.go
 create mode 100644 traversal/selector/matcher_util.go

diff --git a/node/basicnode/bytes.go b/node/basicnode/bytes.go
index 7c45d208..566c32fe 100644
--- a/node/basicnode/bytes.go
+++ b/node/basicnode/bytes.go
@@ -106,7 +106,7 @@ func (nb *plainBytes__Builder) Reset() {
 // -- NodeAssembler -->
 
 type plainBytes__Assembler struct {
-	w *plainBytes
+	w datamodel.Node
 }
 
 func (plainBytes__Assembler) BeginMap(sizeHint int64) (datamodel.MapAssembler, error) {
@@ -131,17 +131,24 @@ func (plainBytes__Assembler) AssignString(string) error {
 	return mixins.BytesAssembler{TypeName: "bytes"}.AssignString("")
 }
 func (na *plainBytes__Assembler) AssignBytes(v []byte) error {
-	*na.w = plainBytes(v)
+	na.w = datamodel.Node(plainBytes(v))
 	return nil
 }
 func (plainBytes__Assembler) AssignLink(datamodel.Link) error {
 	return mixins.BytesAssembler{TypeName: "bytes"}.AssignLink(nil)
 }
 func (na *plainBytes__Assembler) AssignNode(v datamodel.Node) error {
+	if lb, ok := v.(datamodel.LargeBytesNode); ok {
+		lbn, err := lb.AsLargeBytes()
+		if err == nil {
+			na.w = streamBytes{lbn}
+			return nil
+		}
+	}
 	if v2, err := v.AsBytes(); err != nil {
 		return err
 	} else {
-		*na.w = plainBytes(v2)
+		na.w = plainBytes(v2)
 		return nil
 	}
 }
diff --git a/node/basicnode/bytes_stream.go b/node/basicnode/bytes_stream.go
new file mode 100644
index 00000000..ad238bcf
--- /dev/null
+++ b/node/basicnode/bytes_stream.go
@@ -0,0 +1,81 @@
+package basicnode
+
+import (
+	"io"
+
+	"github.com/ipld/go-ipld-prime/datamodel"
+	"github.com/ipld/go-ipld-prime/node/mixins"
+)
+
+var (
+	_ datamodel.Node          = streamBytes{nil}
+	_ datamodel.NodePrototype = Prototype__Bytes{}
+	_ datamodel.NodeBuilder   = &plainBytes__Builder{}
+	_ datamodel.NodeAssembler = &plainBytes__Assembler{}
+)
+
+func NewBytesFromReader(rs io.ReadSeeker) datamodel.Node {
+	return streamBytes{rs}
+}
+
+// streamBytes is a boxed reader that complies with datamodel.Node.
+type streamBytes struct {
+	io.ReadSeeker
+}
+
+// -- Node interface methods -->
+
+func (streamBytes) Kind() datamodel.Kind {
+	return datamodel.Kind_Bytes
+}
+func (streamBytes) LookupByString(string) (datamodel.Node, error) {
+	return mixins.Bytes{TypeName: "bytes"}.LookupByString("")
+}
+func (streamBytes) LookupByNode(key datamodel.Node) (datamodel.Node, error) {
+	return mixins.Bytes{TypeName: "bytes"}.LookupByNode(nil)
+}
+func (streamBytes) LookupByIndex(idx int64) (datamodel.Node, error) {
+	return mixins.Bytes{TypeName: "bytes"}.LookupByIndex(0)
+}
+func (streamBytes) LookupBySegment(seg datamodel.PathSegment) (datamodel.Node, error) {
+	return mixins.Bytes{TypeName: "bytes"}.LookupBySegment(seg)
+}
+func (streamBytes) MapIterator() datamodel.MapIterator {
+	return nil
+}
+func (streamBytes) ListIterator() datamodel.ListIterator {
+	return nil
+}
+func (streamBytes) Length() int64 {
+	return -1
+}
+func (streamBytes) IsAbsent() bool {
+	return false
+}
+func (streamBytes) IsNull() bool {
+	return false
+}
+func (streamBytes) AsBool() (bool, error) {
+	return mixins.Bytes{TypeName: "bytes"}.AsBool()
+}
+func (streamBytes) AsInt() (int64, error) {
+	return mixins.Bytes{TypeName: "bytes"}.AsInt()
+}
+func (streamBytes) AsFloat() (float64, error) {
+	return mixins.Bytes{TypeName: "bytes"}.AsFloat()
+}
+func (streamBytes) AsString() (string, error) {
+	return mixins.Bytes{TypeName: "bytes"}.AsString()
+}
+func (n streamBytes) AsBytes() ([]byte, error) {
+	return io.ReadAll(n)
+}
+func (streamBytes) AsLink() (datamodel.Link, error) {
+	return mixins.Bytes{TypeName: "bytes"}.AsLink()
+}
+func (streamBytes) Prototype() datamodel.NodePrototype {
+	return Prototype__Bytes{}
+}
+func (n streamBytes) AsLargeBytes() (io.ReadSeeker, error) {
+	return n.ReadSeeker, nil
+}
diff --git a/traversal/selector/builder/builder.go b/traversal/selector/builder/builder.go
index 80067758..6cc69e3c 100644
--- a/traversal/selector/builder/builder.go
+++ b/traversal/selector/builder/builder.go
@@ -32,6 +32,7 @@ type SelectorSpecBuilder interface {
 	ExploreFields(ExploreFieldsSpecBuildingClosure) SelectorSpec
 	ExploreInterpretAs(as string, next SelectorSpec) SelectorSpec
 	Matcher() SelectorSpec
+	MatcherSubset(from, to int64) SelectorSpec
 }
 
 // ExploreFieldsSpecBuildingClosure is a function that provided to SelectorSpecBuilder's
@@ -170,6 +171,19 @@ func (ssb *selectorSpecBuilder) Matcher() SelectorSpec {
 	}
 }
 
+func (ssb *selectorSpecBuilder) MatcherSubset(from, to int64) SelectorSpec {
+	return selectorSpec{
+		fluent.MustBuildMap(ssb.np, 1, func(na fluent.MapAssembler) {
+			na.AssembleEntry(selector.SelectorKey_Matcher).CreateMap(1, func(na fluent.MapAssembler) {
+				na.AssembleEntry(selector.SelectorKey_Subset).CreateMap(2, func(na fluent.MapAssembler) {
+					na.AssembleEntry(selector.SelectorKey_From).AssignInt(from)
+					na.AssembleEntry(selector.SelectorKey_To).AssignInt(to)
+				})
+			})
+		}),
+	}
+}
+
 type exploreFieldsSpecBuilder struct {
 	na fluent.MapAssembler
 }
diff --git a/traversal/selector/exploreAll.go b/traversal/selector/exploreAll.go
index a7749f8e..7d889d7e 100644
--- a/traversal/selector/exploreAll.go
+++ b/traversal/selector/exploreAll.go
@@ -27,6 +27,11 @@ func (s ExploreAll) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreAll) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // ParseExploreAll assembles a Selector from a ExploreAll selector node
 func (pc ParseContext) ParseExploreAll(n datamodel.Node) (Selector, error) {
 	if n.Kind() != datamodel.Kind_Map {
diff --git a/traversal/selector/exploreFields.go b/traversal/selector/exploreFields.go
index 5d2abdd2..b2796ff2 100644
--- a/traversal/selector/exploreFields.go
+++ b/traversal/selector/exploreFields.go
@@ -38,6 +38,11 @@ func (s ExploreFields) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreFields) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // ParseExploreFields assembles a Selector
 // from a ExploreFields selector node
 func (pc ParseContext) ParseExploreFields(n datamodel.Node) (Selector, error) {
diff --git a/traversal/selector/exploreIndex.go b/traversal/selector/exploreIndex.go
index 0c8edf16..56a2badc 100644
--- a/traversal/selector/exploreIndex.go
+++ b/traversal/selector/exploreIndex.go
@@ -37,6 +37,11 @@ func (s ExploreIndex) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreIndex) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // ParseExploreIndex assembles a Selector
 // from a ExploreIndex selector node
 func (pc ParseContext) ParseExploreIndex(n datamodel.Node) (Selector, error) {
diff --git a/traversal/selector/exploreInterpretAs.go b/traversal/selector/exploreInterpretAs.go
index 25cf045b..6f4aed67 100644
--- a/traversal/selector/exploreInterpretAs.go
+++ b/traversal/selector/exploreInterpretAs.go
@@ -27,6 +27,11 @@ func (s ExploreInterpretAs) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreInterpretAs) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // NamedReifier indicates how this selector expects to Reify the current datamodel.Node.
 func (s ExploreInterpretAs) NamedReifier() string {
 	return s.adl
diff --git a/traversal/selector/exploreRange.go b/traversal/selector/exploreRange.go
index 73e356ba..e9e041bc 100644
--- a/traversal/selector/exploreRange.go
+++ b/traversal/selector/exploreRange.go
@@ -41,6 +41,11 @@ func (s ExploreRange) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreRange) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // ParseExploreRange assembles a Selector
 // from a ExploreRange selector node
 func (pc ParseContext) ParseExploreRange(n datamodel.Node) (Selector, error) {
diff --git a/traversal/selector/exploreRecursive.go b/traversal/selector/exploreRecursive.go
index 17614025..28a9bedc 100644
--- a/traversal/selector/exploreRecursive.go
+++ b/traversal/selector/exploreRecursive.go
@@ -176,6 +176,11 @@ func (s ExploreRecursive) Decide(n datamodel.Node) bool {
 	return s.current.Decide(n)
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreRecursive) Match(node datamodel.Node) (datamodel.Node, error) {
+	return s.current.Match(node)
+}
+
 type exploreRecursiveContext struct {
 	edgesFound int
 }
diff --git a/traversal/selector/exploreRecursiveEdge.go b/traversal/selector/exploreRecursiveEdge.go
index 65438572..534c73b8 100644
--- a/traversal/selector/exploreRecursiveEdge.go
+++ b/traversal/selector/exploreRecursiveEdge.go
@@ -31,6 +31,11 @@ func (s ExploreRecursiveEdge) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match always returns false because this is not a matcher
+func (s ExploreRecursiveEdge) Match(node datamodel.Node) (datamodel.Node, error) {
+	return nil, nil
+}
+
 // ParseExploreRecursiveEdge assembles a Selector
 // from a exploreRecursiveEdge selector node
 func (pc ParseContext) ParseExploreRecursiveEdge(n datamodel.Node) (Selector, error) {
diff --git a/traversal/selector/exploreUnion.go b/traversal/selector/exploreUnion.go
index 0af42cd6..21594954 100644
--- a/traversal/selector/exploreUnion.go
+++ b/traversal/selector/exploreUnion.go
@@ -74,6 +74,18 @@ func (s ExploreUnion) Decide(n datamodel.Node) bool {
 	return false
 }
 
+// Match returns true for a Union selector based on the matched union.
+func (s ExploreUnion) Match(n datamodel.Node) (datamodel.Node, error) {
+	for _, m := range s.Members {
+		if mn, err := m.Match(n); mn != nil {
+			return mn, nil
+		} else if err != nil {
+			return nil, err
+		}
+	}
+	return nil, nil
+}
+
 // ParseExploreUnion assembles a Selector
 // from an ExploreUnion selector node
 func (pc ParseContext) ParseExploreUnion(n datamodel.Node) (Selector, error) {
diff --git a/traversal/selector/fieldKeys.go b/traversal/selector/fieldKeys.go
index 58024b61..39ec61e6 100644
--- a/traversal/selector/fieldKeys.go
+++ b/traversal/selector/fieldKeys.go
@@ -23,5 +23,8 @@ const (
 	SelectorKey_StopAt               = "!"
 	SelectorKey_Condition            = "&"
 	SelectorKey_As                   = "as"
+	SelectorKey_Subset               = "subset"
+	SelectorKey_From                 = "["
+	SelectorKey_To                   = "]"
 	// not filling conditional keys since it's not complete
 )
diff --git a/traversal/selector/matcher.go b/traversal/selector/matcher.go
index b8f64f68..d9538dc6 100644
--- a/traversal/selector/matcher.go
+++ b/traversal/selector/matcher.go
@@ -2,8 +2,10 @@ package selector
 
 import (
 	"fmt"
+	"io"
 
 	"github.com/ipld/go-ipld-prime/datamodel"
+	"github.com/ipld/go-ipld-prime/node/basicnode"
 )
 
 // Matcher marks a node to be included in the "result" set.
@@ -16,7 +18,61 @@ import (
 // A selector tree with only "explore*"-type selectors and no Matcher selectors
 // is valid; it will just generate a "covered" set of nodes and no "result" set.
 // TODO: From spec: implement conditions and labels
-type Matcher struct{}
+type Matcher struct {
+	*Slice
+}
+
+// Slice limits a result node to a subset of the node.
+// The returned node will be limited based on slicing the specified range of the
+// node into a new node, or making use of the `AsLargeBytes` io.ReadSeeker to
+// restrict response with a SectionReader.
+type Slice struct {
+	From int64
+	To   int64
+}
+
+func (s Slice) Slice(n datamodel.Node) (datamodel.Node, error) {
+	if n.Kind() == datamodel.Kind_String {
+		str, err := n.AsString()
+		if err != nil {
+			return nil, err
+		}
+		to := s.To
+		if len(str) < int(to) {
+			to = int64(len(str))
+		}
+		from := s.From
+		if len(str) < int(from) {
+			from = int64(len(str))
+		}
+		return basicnode.NewString(str[from:to]), nil
+	} else if n.Kind() == datamodel.Kind_Bytes {
+		if lbn, ok := n.(datamodel.LargeBytesNode); ok {
+			rdr, err := lbn.AsLargeBytes()
+			if err != nil {
+				return nil, err
+			}
+
+			sr := io.NewSectionReader(readerat{rdr}, int64(s.From), int64(s.To))
+			return basicnode.NewBytesFromReader(sr), nil
+		}
+		bytes, err := n.AsBytes()
+		if err != nil {
+			return nil, err
+		}
+		to := s.To
+		if len(bytes) < int(to) {
+			to = int64(len(bytes))
+		}
+		from := s.From
+		if len(bytes) < int(from) {
+			from = int64(len(bytes))
+		}
+
+		return basicnode.NewBytes(bytes[from:to]), nil
+	}
+	return nil, fmt.Errorf("selector slice rejected: subset match must be over string or bytes")
+}
 
 // Interests are empty for a matcher (for now) because
 // It is always just there to match, not explore further
@@ -35,6 +91,14 @@ func (s Matcher) Decide(n datamodel.Node) bool {
 	return true
 }
 
+// Match is always true for a match cause it's in the result set
+func (s Matcher) Match(node datamodel.Node) (datamodel.Node, error) {
+	if s.Slice != nil {
+		return s.Slice.Slice(node)
+	}
+	return node, nil
+}
+
 // ParseMatcher assembles a Selector
 // from a matcher selector node
 // TODO: Parse labels and conditions
@@ -42,5 +106,38 @@ func (pc ParseContext) ParseMatcher(n datamodel.Node) (Selector, error) {
 	if n.Kind() != datamodel.Kind_Map {
 		return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map")
 	}
+
+	// check if a slice is specified
+	if subset, err := n.LookupByString("subset"); err == nil {
+		if subset.Kind() != datamodel.Kind_Map {
+			return nil, fmt.Errorf("selector spec parse rejected: subset body must be a map")
+		}
+		from, err := subset.LookupByString("[")
+		if err != nil {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with a from '[' key")
+		}
+		fromN, err := from.AsInt()
+		if err != nil {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with a 'from' key that is a number")
+		}
+		to, err := subset.LookupByString("]")
+		if err != nil {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with a to ']' key")
+		}
+		toN, err := to.AsInt()
+		if err != nil {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with a 'to' key that is a number")
+		}
+		if fromN > toN {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with a 'from' key that is less than or equal to the 'to' key")
+		}
+		if fromN < 0 || toN < 0 {
+			return nil, fmt.Errorf("selector spec parse rejected: selector body must be a map with keys not less than 0")
+		}
+		return Matcher{&Slice{
+			From: fromN,
+			To:   toN,
+		}}, nil
+	}
 	return Matcher{}, nil
 }
diff --git a/traversal/selector/matcher_util.go b/traversal/selector/matcher_util.go
new file mode 100644
index 00000000..b67cad09
--- /dev/null
+++ b/traversal/selector/matcher_util.go
@@ -0,0 +1,15 @@
+package selector
+
+import "io"
+
+type readerat struct {
+	io.ReadSeeker
+}
+
+func (r readerat) ReadAt(p []byte, off int64) (n int, err error) {
+	_, err = r.Seek(off, 0)
+	if err != nil {
+		return 0, err
+	}
+	return r.Read(p)
+}
diff --git a/traversal/selector/selector.go b/traversal/selector/selector.go
index 4d0a7814..b58fcd86 100644
--- a/traversal/selector/selector.go
+++ b/traversal/selector/selector.go
@@ -106,6 +106,12 @@ type Selector interface {
 	// Only "Matcher" clauses actually implement this in a way that ever returns "true".
 	// See the Selector specs for discussion on "matched" vs "reached"/"visited" nodes.
 	Decide(node datamodel.Node) bool
+
+	// Match is an extension to Decide allowing the matcher to `decide` a transformation of
+	// the matched node. This is used for `Subset` match behavior. If the node is matched,
+	// the first argument will be the matched node. If it is not matched, the first argument
+	// will be null. If there is an error, the first argument will be null.
+	Match(node datamodel.Node) (datamodel.Node, error)
 }
 
 // REVIEW: do ParsedParent and ParseContext need to be exported?  They're mostly used during the compilation process.
diff --git a/traversal/walk.go b/traversal/walk.go
index 1656de15..a794b301 100644
--- a/traversal/walk.go
+++ b/traversal/walk.go
@@ -182,16 +182,18 @@ func (prog Progress) walkAdv(n datamodel.Node, s selector.Selector, fn AdvVisitF
 	}
 
 	if prog.Path.Len() >= prog.Cfg.StartAtPath.Len() || !prog.PastStartAtPath {
-		// Decide if this node is matched -- do callbacks as appropriate.
-		if s.Decide(n) {
-			if err := fn(prog, n, VisitReason_SelectionMatch); err != nil {
-				return err
-			}
-		} else {
-			if err := fn(prog, n, VisitReason_SelectionCandidate); err != nil {
-				return err
-			}
-		}
+        // Decide if this node is matched -- do callbacks as appropriate.
+        if match, err := s.Match(n); match != nil {
+            if err := fn(prog, match, VisitReason_SelectionMatch); err != nil {
+                return err
+            }
+        } else if err != nil {
+            return err
+        } else {
+            if err := fn(prog, n, VisitReason_SelectionCandidate); err != nil {
+                return err
+            }
+        }
 	}
 	// If we're handling scalars (e.g. not maps and lists) we can return now.
 	nk := n.Kind()

From 671b1bee1257c30d7791c3acd35db6e488476701 Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 15:28:23 +0100
Subject: [PATCH 08/13] bytes spec test

---
 node/basicnode/bytes_test.go | 12 ++++++++++++
 node/tests/byteSpecs.go      | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)
 create mode 100644 node/basicnode/bytes_test.go
 create mode 100644 node/tests/byteSpecs.go

diff --git a/node/basicnode/bytes_test.go b/node/basicnode/bytes_test.go
new file mode 100644
index 00000000..ca408484
--- /dev/null
+++ b/node/basicnode/bytes_test.go
@@ -0,0 +1,12 @@
+package basicnode_test
+
+import (
+	"testing"
+
+	"github.com/ipld/go-ipld-prime/node/basicnode"
+	"github.com/ipld/go-ipld-prime/node/tests"
+)
+
+func TestBytes(t *testing.T) {
+	tests.SpecTestBytes(t, basicnode.Prototype__Bytes{})
+}
diff --git a/node/tests/byteSpecs.go b/node/tests/byteSpecs.go
new file mode 100644
index 00000000..69bf494b
--- /dev/null
+++ b/node/tests/byteSpecs.go
@@ -0,0 +1,35 @@
+package tests
+
+import (
+	"io"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+
+	"github.com/ipld/go-ipld-prime/datamodel"
+)
+
+func SpecTestBytes(t *testing.T, np datamodel.NodePrototype) {
+	t.Run("byte node", func(t *testing.T) {
+		nb := np.NewBuilder()
+		err := nb.AssignBytes([]byte("asdf"))
+		qt.Check(t, err, qt.IsNil)
+		n := nb.Build()
+
+		qt.Check(t, n.Kind(), qt.Equals, datamodel.Kind_Bytes)
+		qt.Check(t, n.IsNull(), qt.IsFalse)
+		x, err := n.AsBytes()
+		qt.Check(t, err, qt.IsNil)
+		qt.Check(t, x, qt.DeepEquals, []byte("asdf"))
+
+		lbn, ok := n.(datamodel.LargeBytesNode)
+		if ok {
+			str, err := lbn.AsLargeBytes()
+			qt.Check(t, err, qt.IsNil)
+			bytes, err := io.ReadAll(str)
+			qt.Check(t, err, qt.IsNil)
+			qt.Check(t, bytes, qt.DeepEquals, []byte("asdf"))
+		}
+
+	})
+}

From 918b60840333d728beec996b93a39a0586337728 Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 15:32:55 +0100
Subject: [PATCH 09/13] extend traversal to handle partial byte range match
 access behavior

---
 traversal/selector/spec_test.go | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/traversal/selector/spec_test.go b/traversal/selector/spec_test.go
index 215bfd6b..7d309d05 100644
--- a/traversal/selector/spec_test.go
+++ b/traversal/selector/spec_test.go
@@ -2,6 +2,7 @@ package selector_test
 
 import (
 	"bytes"
+	"io"
 	"os"
 	"testing"
 
@@ -14,7 +15,7 @@ import (
 	"github.com/ipld/go-ipld-prime/fluent/qp"
 	"github.com/ipld/go-ipld-prime/node/basicnode"
 	"github.com/ipld/go-ipld-prime/traversal"
-	"github.com/ipld/go-ipld-prime/traversal/selector/parse"
+	selectorparse "github.com/ipld/go-ipld-prime/traversal/selector/parse"
 )
 
 func TestSpecFixtures(t *testing.T) {
@@ -81,6 +82,18 @@ func testOneSpecFixtureFile(t *testing.T, filename string) {
 					}))
 					qp.MapEntry(ma, "matched", qp.Bool(reason == traversal.VisitReason_SelectionMatch))
 				})
+				if reason == traversal.VisitReason_SelectionMatch && n.Kind() == datamodel.Kind_Bytes {
+					if lbn, ok := n.(datamodel.LargeBytesNode); ok {
+						rdr, err := lbn.AsLargeBytes()
+						if err == nil {
+							io.Copy(io.Discard, rdr)
+						}
+					}
+					_, err := n.AsBytes()
+					if err != nil {
+						panic("insanity at a deeper level than this test's target")
+					}
+				}
 				if err != nil {
 					panic("insanity at a deeper level than this test's target")
 				}

From 5c15a9efaeaab9ebda4c085b4088101bbd911f15 Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 15:47:06 +0100
Subject: [PATCH 10/13] code review

---
 traversal/selector/matcher.go      | 19 +++++++++++--------
 traversal/selector/matcher_util.go |  1 +
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/traversal/selector/matcher.go b/traversal/selector/matcher.go
index d9538dc6..38314db4 100644
--- a/traversal/selector/matcher.go
+++ b/traversal/selector/matcher.go
@@ -32,46 +32,49 @@ type Slice struct {
 }
 
 func (s Slice) Slice(n datamodel.Node) (datamodel.Node, error) {
-	if n.Kind() == datamodel.Kind_String {
+	var from, to int64
+	switch n.Kind() {
+	case datamodel.Kind_String:
 		str, err := n.AsString()
 		if err != nil {
 			return nil, err
 		}
-		to := s.To
+		to = s.To
 		if len(str) < int(to) {
 			to = int64(len(str))
 		}
-		from := s.From
+		from = s.From
 		if len(str) < int(from) {
 			from = int64(len(str))
 		}
 		return basicnode.NewString(str[from:to]), nil
-	} else if n.Kind() == datamodel.Kind_Bytes {
+	case datamodel.Kind_Bytes:
 		if lbn, ok := n.(datamodel.LargeBytesNode); ok {
 			rdr, err := lbn.AsLargeBytes()
 			if err != nil {
 				return nil, err
 			}
 
-			sr := io.NewSectionReader(readerat{rdr}, int64(s.From), int64(s.To))
+			sr := io.NewSectionReader(readerat{rdr}, s.From, s.To)
 			return basicnode.NewBytesFromReader(sr), nil
 		}
 		bytes, err := n.AsBytes()
 		if err != nil {
 			return nil, err
 		}
-		to := s.To
+		to = s.To
 		if len(bytes) < int(to) {
 			to = int64(len(bytes))
 		}
-		from := s.From
+		from = s.From
 		if len(bytes) < int(from) {
 			from = int64(len(bytes))
 		}
 
 		return basicnode.NewBytes(bytes[from:to]), nil
+	default:
+		return nil, fmt.Errorf("selector slice rejected: subset match must be over string or bytes")
 	}
-	return nil, fmt.Errorf("selector slice rejected: subset match must be over string or bytes")
 }
 
 // Interests are empty for a matcher (for now) because
diff --git a/traversal/selector/matcher_util.go b/traversal/selector/matcher_util.go
index b67cad09..b60d11c8 100644
--- a/traversal/selector/matcher_util.go
+++ b/traversal/selector/matcher_util.go
@@ -7,6 +7,7 @@ type readerat struct {
 }
 
 func (r readerat) ReadAt(p []byte, off int64) (n int, err error) {
+	// TODO: consider keeping track of current offset.
 	_, err = r.Seek(off, 0)
 	if err != nil {
 		return 0, err

From 598b16a02fc0e1464279faaf40bbe020690669a0 Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 16:00:28 +0100
Subject: [PATCH 11/13] warning on readerat

---
 traversal/selector/matcher_util.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/traversal/selector/matcher_util.go b/traversal/selector/matcher_util.go
index b60d11c8..237c5e71 100644
--- a/traversal/selector/matcher_util.go
+++ b/traversal/selector/matcher_util.go
@@ -6,6 +6,10 @@ type readerat struct {
 	io.ReadSeeker
 }
 
+// ReadAt provides the io.ReadAt method over a ReadSeeker.
+// This implementation does not support concurrent calls to `ReadAt`,
+// as specified by the ReaderAt interface, and so must only be used
+// in non-concurrent use cases.
 func (r readerat) ReadAt(p []byte, off int64) (n int, err error) {
 	// TODO: consider keeping track of current offset.
 	_, err = r.Seek(off, 0)

From c3d07b4530078699dce5b6ac3e9532a40fd1c440 Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Thu, 3 Mar 2022 19:06:38 +0100
Subject: [PATCH 12/13] fix section reader

---
 traversal/selector/matcher.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/traversal/selector/matcher.go b/traversal/selector/matcher.go
index 38314db4..4186221f 100644
--- a/traversal/selector/matcher.go
+++ b/traversal/selector/matcher.go
@@ -55,7 +55,7 @@ func (s Slice) Slice(n datamodel.Node) (datamodel.Node, error) {
 				return nil, err
 			}
 
-			sr := io.NewSectionReader(readerat{rdr}, s.From, s.To)
+			sr := io.NewSectionReader(readerat{rdr}, s.From, s.To-s.From)
 			return basicnode.NewBytesFromReader(sr), nil
 		}
 		bytes, err := n.AsBytes()

From 777d78d9625238dca7e706845465f338a4a50580 Mon Sep 17 00:00:00 2001
From: Will Scott <will.scott@protocol.ai>
Date: Mon, 7 Mar 2022 09:01:04 +0100
Subject: [PATCH 13/13] clearer comments

---
 traversal/selector/matcher.go | 4 ++--
 traversal/walk.go             | 1 +
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/traversal/selector/matcher.go b/traversal/selector/matcher.go
index 4186221f..ff2596c1 100644
--- a/traversal/selector/matcher.go
+++ b/traversal/selector/matcher.go
@@ -73,7 +73,7 @@ func (s Slice) Slice(n datamodel.Node) (datamodel.Node, error) {
 
 		return basicnode.NewBytes(bytes[from:to]), nil
 	default:
-		return nil, fmt.Errorf("selector slice rejected: subset match must be over string or bytes")
+		return nil, fmt.Errorf("selector slice rejected on %s: subset match must be over string or bytes", n.Kind())
 	}
 }
 
@@ -89,7 +89,7 @@ func (s Matcher) Explore(n datamodel.Node, p datamodel.PathSegment) (Selector, e
 }
 
 // Decide is always true for a match cause it's in the result set
-// TODO: Implement boolean logic for conditionals
+// Deprecated: use Match instead
 func (s Matcher) Decide(n datamodel.Node) bool {
 	return true
 }
diff --git a/traversal/walk.go b/traversal/walk.go
index 0f659189..c2d5b35e 100644
--- a/traversal/walk.go
+++ b/traversal/walk.go
@@ -174,6 +174,7 @@ func (prog Progress) walkAdv(n datamodel.Node, s selector.Selector, fn AdvVisitF
 		if err != nil {
 			return fmt.Errorf("failed to reify node as %q: %w", adl, err)
 		}
+		// explore into the `InterpretAs` clause to the child selector.
 		s, err = s.Explore(n, datamodel.PathSegment{})
 		if err != nil {
 			return err