packfile: add Index type

Updates #12

CHANGELOG.md

@@ -12,16 +12,22 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 -  A new `packfile/client` package enables downloading commits from and
    uploading commits to remote Git repositories.
    ([#7](https://github.com/gg-scm/gg-git/issues/7))
--  The `githash` package is now the home for the `Ref` type. This permits
-   ref string manipulation without depending on the larger `git` package.
-   `git.Ref` is now a type alias for `githash.Ref`.
 -  `*object.Commit` and `*object.Tag` now implement `BinaryMarshaler` and
    `BinaryUnmarshaler` in addition to `TextMarshaler` and `TextUnmarshaler`.
    This is for symmetry with `object.Tree`.
 -  The `packfile.DeltaReader` type is a flexible way of expanding a deltified
    object from a packfile.
+-  The `packfile.Index` type stores a packfile object ID lookup table that is
+   interoperable with Git packfile index files.
+   ([#12](https://github.com/gg-scm/gg-git/issues/12))
 -  `packfile.ReadHeader` enables random access to a packfile.
 
+### Changed
+
+-  The `githash` package is now the home for the `Ref` type. This permits
+   ref string manipulation without depending on the larger `git` package.
+   `git.Ref` is now a type alias for `githash.Ref`.
+
 ### Removed
 
 -  Removed the `packfile.ApplyDelta` function. The `packfile.DeltaReader` type

misc/genpack.go

@@ -33,13 +33,13 @@ import (
 
 func main() {
 	funcMap := map[string]func() error{
-		"Empty":        empty,
-		"FirstCommit":  firstCommit,
-		"DeltaOffset":  deltaOffset,
-		"ObjectOffset": objectOffset,
-		"EmptyBlob":    emptyBlob,
-		"TooLong":      tooLong,
-		"TooShort":     tooShort,
+		"Empty":       empty,
+		"FirstCommit": firstCommit,
+		"DeltaOffset": deltaOffset,
+		"DeltaObject": deltaObject,
+		"EmptyBlob":   emptyBlob,
+		"TooLong":     tooLong,
+		"TooShort":    tooShort,
 	}
 	var names []string
 	for k := range funcMap {
@@ -187,7 +187,7 @@ func deltaOffset() (err error) {
 	return nil
 }
 
-func objectOffset() (err error) {
+func deltaObject() (err error) {
 	w := packfile.NewWriter(os.Stdout, 2)
 	defer func() {
 		if closeErr := w.Close(); err == nil && closeErr != nil {

misc/indexpack.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+# Copyright 2021 The gg Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+set -euo pipefail
+if [[ $# -eq 0 ]]; then
+  echo "usage: indexpack.sh PACKFILE [...]" 1>&2
+  exit 64
+fi
+for packfile in "$@"; do
+  idx1="${packfile/%.pack/.idx1}"
+  git index-pack --index-version=1 -o "$idx1" "$packfile"
+  idx2="${packfile/%.pack/.idx2}"
+  git index-pack --index-version=2 -o "$idx2" "$packfile"
+  # git-index-pack does not set the writable bit.
+  chmod u+w "$idx1" "$idx2"
+done