Support partial hashes in Repository.ResolveRevision.

Like `git rev-parse <prefix>`, this enumerates the hashes of objects
with the given prefix and adds them to the list of candidates for
resolution.

This has an exhaustive slow path, which requires enumerating all objects
and filtering each one, but also a couple of fast paths for common
cases. There's room for future work to make this faster; TODOs have been
left for that.

Fixes #135.

1 files changed, 74 insertions, 6 deletions

diff --git a/repository.go b/repository.go
index 172029c..66c0918 100644
--- a/repository.go
+++ b/repository.go
@@ -3,6 +3,7 @@ package git
 import (
 	"bytes"
 	"context"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
@@ -1426,7 +1427,7 @@ func (r *Repository) Worktree() (*Worktree, error) {
 // resolve to a commit hash, not a tree or annotated tag.
 //
 // Implemented resolvers : HEAD, branch, tag, heads/branch, refs/heads/branch,
-// refs/tags/tag, refs/remotes/origin/branch, refs/remotes/origin/HEAD, tilde and caret (HEAD~1, master~^, tag~2, ref/heads/master~1, ...), selection by text (HEAD^{/fix nasty bug})
+// refs/tags/tag, refs/remotes/origin/branch, refs/remotes/origin/HEAD, tilde and caret (HEAD~1, master~^, tag~2, ref/heads/master~1, ...), selection by text (HEAD^{/fix nasty bug}), hash (prefix and full)
 func (r *Repository) ResolveRevision(rev plumbing.Revision) (*plumbing.Hash, error) {
 	p := revision.NewParserFromString(string(rev))
 
@@ -1445,11 +1446,7 @@ func (r *Repository) ResolveRevision(rev plumbing.Revision) (*plumbing.Hash, err
 
 			var tryHashes []plumbing.Hash
 
-			maybeHash := plumbing.NewHash(string(revisionRef))
-
-			if !maybeHash.IsZero() {
-				tryHashes = append(tryHashes, maybeHash)
-			}
+			tryHashes = append(tryHashes, r.resolveHashPrefix(string(revisionRef))...)
 
 			for _, rule := range append([]string{"%s"}, plumbing.RefRevParseRules...) {
 				ref, err := storer.ResolveReference(r.Storer, plumbing.ReferenceName(fmt.Sprintf(rule, revisionRef)))
@@ -1567,6 +1564,49 @@ func (r *Repository) ResolveRevision(rev plumbing.Revision) (*plumbing.Hash, err
 	return &commit.Hash, nil
 }
 
+// resolveHashPrefix returns a list of potential hashes that the given string
+// is a prefix of. It quietly swallows errors, returning nil.
+func (r *Repository) resolveHashPrefix(hashStr string) []plumbing.Hash {
+	// Handle complete and partial hashes.
+	// plumbing.NewHash forces args into a full 20 byte hash, which isn't suitable
+	// for partial hashes since they will become zero-filled.
+
+	if hashStr == "" {
+		return nil
+	}
+	if len(hashStr) == len(plumbing.ZeroHash)*2 {
+		// Only a full hash is possible.
+		hexb, err := hex.DecodeString(hashStr)
+		if err != nil {
+			return nil
+		}
+		var h plumbing.Hash
+		copy(h[:], hexb)
+		return []plumbing.Hash{h}
+	}
+
+	// Partial hash.
+	// hex.DecodeString only decodes to complete bytes, so only works with pairs of hex digits.
+	evenHex := hashStr[:len(hashStr)&^1]
+	hexb, err := hex.DecodeString(evenHex)
+	if err != nil {
+		return nil
+	}
+	candidates := expandPartialHash(r.Storer, hexb)
+	if len(evenHex) == len(hashStr) {
+		// The prefix was an exact number of bytes.
+		return candidates
+	}
+	// Do another prefix check to ensure the dangling nybble is correct.
+	var hashes []plumbing.Hash
+	for _, h := range candidates {
+		if strings.HasPrefix(h.String(), hashStr) {
+			hashes = append(hashes, h)
+		}
+	}
+	return hashes
+}
+
 type RepackConfig struct {
 	// UseRefDeltas configures whether packfile encoder will use reference deltas.
 	// By default OFSDeltaObject is used.
@@ -1659,3 +1699,31 @@ func (r *Repository) createNewObjectPack(cfg *RepackConfig) (h plumbing.Hash, er
 
 	return h, err
 }
+
+func expandPartialHash(st storer.EncodedObjectStorer, prefix []byte) (hashes []plumbing.Hash) {
+	// The fast version is implemented by storage/filesystem.ObjectStorage.
+	type fastIter interface {
+		HashesWithPrefix(prefix []byte) ([]plumbing.Hash, error)
+	}
+	if fi, ok := st.(fastIter); ok {
+		h, err := fi.HashesWithPrefix(prefix)
+		if err != nil {
+			return nil
+		}
+		return h
+	}
+
+	// Slow path.
+	iter, err := st.IterEncodedObjects(plumbing.AnyObject)
+	if err != nil {
+		return nil
+	}
+	iter.ForEach(func(obj plumbing.EncodedObject) error {
+		h := obj.Hash()
+		if bytes.HasPrefix(h[:], prefix) {
+			hashes = append(hashes, h)
+		}
+		return nil
+	})
+	return
+}