internal/pkgbits: add Index type

Element indices are very common in the pkgbits API, so introduce a new
defined type to help make that clearer.

Change-Id: I8f9493e2335601c740eb403d1fdcd11183122907
Reviewed-on: https://go-review.googlesource.com/c/go/+/407435
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

3 files changed, 29 insertions, 25 deletions

diff --git a/src/internal/pkgbits/decoder.go b/src/internal/pkgbits/decoder.go
index a2367b7e99..0b5fd9705c 100644
--- a/src/internal/pkgbits/decoder.go
+++ b/src/internal/pkgbits/decoder.go
@@ -108,8 +108,8 @@ func (pr *PkgDecoder) Fingerprint() [8]byte {
 
 // AbsIdx returns the absolute index for the given (section, index)
 // pair.
-func (pr *PkgDecoder) AbsIdx(k RelocKind, idx int) int {
-	absIdx := idx
+func (pr *PkgDecoder) AbsIdx(k RelocKind, idx Index) int {
+	absIdx := int(idx)
 	if k > 0 {
 		absIdx += int(pr.elemEndsEnds[k-1])
 	}
@@ -121,7 +121,7 @@ func (pr *PkgDecoder) AbsIdx(k RelocKind, idx int) int {
 
 // DataIdx returns the raw element bitstream for the given (section,
 // index) pair.
-func (pr *PkgDecoder) DataIdx(k RelocKind, idx int) string {
+func (pr *PkgDecoder) DataIdx(k RelocKind, idx Index) string {
 	absIdx := pr.AbsIdx(k, idx)
 
 	var start uint32
@@ -134,13 +134,13 @@ func (pr *PkgDecoder) DataIdx(k RelocKind, idx int) string {
 }
 
 // StringIdx returns the string value for the given string index.
-func (pr *PkgDecoder) StringIdx(idx int) string {
+func (pr *PkgDecoder) StringIdx(idx Index) string {
 	return pr.DataIdx(RelocString, idx)
 }
 
 // NewDecoder returns a Decoder for the given (section, index) pair,
 // and decodes the given SyncMarker from the element bitstream.
-func (pr *PkgDecoder) NewDecoder(k RelocKind, idx int, marker SyncMarker) Decoder {
+func (pr *PkgDecoder) NewDecoder(k RelocKind, idx Index, marker SyncMarker) Decoder {
 	r := pr.NewDecoderRaw(k, idx)
 	r.Sync(marker)
 	return r
@@ -149,7 +149,7 @@ func (pr *PkgDecoder) NewDecoder(k RelocKind, idx int, marker SyncMarker) Decode
 // NewDecoderRaw returns a Decoder for the given (section, index) pair.
 //
 // Most callers should use NewDecoder instead.
-func (pr *PkgDecoder) NewDecoderRaw(k RelocKind, idx int) Decoder {
+func (pr *PkgDecoder) NewDecoderRaw(k RelocKind, idx Index) Decoder {
 	r := Decoder{
 		common: pr,
 		k:      k,
@@ -163,7 +163,7 @@ func (pr *PkgDecoder) NewDecoderRaw(k RelocKind, idx int) Decoder {
 	r.Relocs = make([]RelocEnt, r.Len())
 	for i := range r.Relocs {
 		r.Sync(SyncReloc)
-		r.Relocs[i] = RelocEnt{RelocKind(r.Len()), r.Len()}
+		r.Relocs[i] = RelocEnt{RelocKind(r.Len()), Index(r.Len())}
 	}
 
 	return r
@@ -178,7 +178,7 @@ type Decoder struct {
 	Data   strings.Reader
 
 	k   RelocKind
-	Idx int
+	Idx Index
 }
 
 func (r *Decoder) checkErr(err error) {
@@ -204,7 +204,7 @@ func (r *Decoder) rawVarint() int64 {
 	return x
 }
 
-func (r *Decoder) rawReloc(k RelocKind, idx int) int {
+func (r *Decoder) rawReloc(k RelocKind, idx int) Index {
 	e := r.Relocs[idx]
 	assert(e.Kind == k)
 	return e.Idx
@@ -313,7 +313,7 @@ func (r *Decoder) Code(mark SyncMarker) int {
 
 // Reloc decodes a relocation of expected section k from the element
 // bitstream and returns an index to the referenced element.
-func (r *Decoder) Reloc(k RelocKind) int {
+func (r *Decoder) Reloc(k RelocKind) Index {
 	r.Sync(SyncUseReloc)
 	return r.rawReloc(k, r.Len())
 }
@@ -390,7 +390,7 @@ func (r *Decoder) bigFloat() *big.Float {
 
 // PeekPkgPath returns the package path for the specified package
 // index.
-func (pr *PkgDecoder) PeekPkgPath(idx int) string {
+func (pr *PkgDecoder) PeekPkgPath(idx Index) string {
 	r := pr.NewDecoder(RelocPkg, idx, SyncPkgDef)
 	path := r.String()
 	if path == "" {
@@ -401,7 +401,7 @@ func (pr *PkgDecoder) PeekPkgPath(idx int) string {
 
 // PeekObj returns the package path, object name, and CodeObj for the
 // specified object index.
-func (pr *PkgDecoder) PeekObj(idx int) (string, string, CodeObj) {
+func (pr *PkgDecoder) PeekObj(idx Index) (string, string, CodeObj) {
 	r := pr.NewDecoder(RelocName, idx, SyncObject1)
 	r.Sync(SyncSym)
 	r.Sync(SyncPkg)
diff --git a/src/internal/pkgbits/encoder.go b/src/internal/pkgbits/encoder.go
index 9fddb58237..1326a135cf 100644
--- a/src/internal/pkgbits/encoder.go
+++ b/src/internal/pkgbits/encoder.go
@@ -23,7 +23,7 @@ type PkgEncoder struct {
 	// stringsIdx maps previously encoded strings to their index within
 	// the RelocString section, to allow deduplication. That is,
 	// elems[RelocString][stringsIdx[s]] == s (if present).
-	stringsIdx map[string]int
+	stringsIdx map[string]Index
 
 	syncFrames int
 }
@@ -36,7 +36,7 @@ type PkgEncoder struct {
 // higher-level Unified IR reader/writer code.
 func NewPkgEncoder(syncFrames int) PkgEncoder {
 	return PkgEncoder{
-		stringsIdx: make(map[string]int),
+		stringsIdx: make(map[string]Index),
 		syncFrames: syncFrames,
 	}
 }
@@ -87,13 +87,13 @@ func (pw *PkgEncoder) DumpTo(out0 io.Writer) (fingerprint [8]byte) {
 
 // StringIdx adds a string value to the strings section, if not
 // already present, and returns its index.
-func (pw *PkgEncoder) StringIdx(s string) int {
+func (pw *PkgEncoder) StringIdx(s string) Index {
 	if idx, ok := pw.stringsIdx[s]; ok {
 		assert(pw.elems[RelocString][idx] == s)
 		return idx
 	}
 
-	idx := len(pw.elems[RelocString])
+	idx := Index(len(pw.elems[RelocString]))
 	pw.elems[RelocString] = append(pw.elems[RelocString], s)
 	pw.stringsIdx[s] = idx
 	return idx
@@ -113,7 +113,7 @@ func (pw *PkgEncoder) NewEncoder(k RelocKind, marker SyncMarker) Encoder {
 //
 // Most callers should use NewEncoder instead.
 func (pw *PkgEncoder) NewEncoderRaw(k RelocKind) Encoder {
-	idx := len(pw.elems[k])
+	idx := Index(len(pw.elems[k]))
 	pw.elems[k] = append(pw.elems[k], "") // placeholder
 
 	return Encoder{
@@ -134,11 +134,11 @@ type Encoder struct {
 	encodingRelocHeader bool
 
 	k   RelocKind
-	Idx int // index within relocation section
+	Idx Index // index within relocation section
 }
 
 // Flush finalizes the element's bitstream and returns its Index.
-func (w *Encoder) Flush() int {
+func (w *Encoder) Flush() Index {
 	var sb bytes.Buffer // TODO(mdempsky): strings.Builder after #44505 is resolved
 
 	// Backup the data so we write the relocations at the front.
@@ -157,7 +157,7 @@ func (w *Encoder) Flush() int {
 	for _, rEnt := range w.Relocs {
 		w.Sync(SyncReloc)
 		w.Len(int(rEnt.Kind))
-		w.Len(rEnt.Idx)
+		w.Len(int(rEnt.Idx))
 	}
 
 	io.Copy(&sb, &w.Data)
@@ -190,7 +190,7 @@ func (w *Encoder) rawVarint(x int64) {
 	w.rawUvarint(ux)
 }
 
-func (w *Encoder) rawReloc(r RelocKind, idx int) int {
+func (w *Encoder) rawReloc(r RelocKind, idx Index) int {
 	// TODO(mdempsky): Use map for lookup; this takes quadratic time.
 	for i, rEnt := range w.Relocs {
 		if rEnt.Kind == r && rEnt.Idx == idx {
@@ -279,7 +279,7 @@ func (w *Encoder) Uint(x uint) { w.Uint64(uint64(x)) }
 // Note: Only the index is formally written into the element
 // bitstream, so bitstream decoders must know from context which
 // section an encoded relocation refers to.
-func (w *Encoder) Reloc(r RelocKind, idx int) {
+func (w *Encoder) Reloc(r RelocKind, idx Index) {
 	w.Sync(SyncUseReloc)
 	w.Len(w.rawReloc(r, idx))
 }
diff --git a/src/internal/pkgbits/reloc.go b/src/internal/pkgbits/reloc.go
index 84cf03ef98..7a8f04ab3f 100644
--- a/src/internal/pkgbits/reloc.go
+++ b/src/internal/pkgbits/reloc.go
@@ -7,19 +7,23 @@ package pkgbits
 // A RelocKind indicates a particular section within a unified IR export.
 type RelocKind int
 
+// An Index represents a bitstream element index within a particular
+// section.
+type Index int
+
 // A relocEnt (relocation entry) is an entry in an element's local
 // reference table.
 //
 // TODO(mdempsky): Rename this too.
 type RelocEnt struct {
 	Kind RelocKind
-	Idx  int
+	Idx  Index
 }
 
 // Reserved indices within the meta relocation section.
 const (
-	PublicRootIdx  = 0
-	PrivateRootIdx = 1
+	PublicRootIdx  Index = 0
+	PrivateRootIdx Index = 1
 )
 
 const (