hash: Key by algorithm, not size

1 files changed, 51 insertions, 10 deletions

diff --git a/hash.go b/hash.go
index 3edb6079..1a0053c2 100644
--- a/hash.go
+++ b/hash.go
@@ -8,44 +8,85 @@ import (
 
 const maxHashSize = 32
 
+// hashAlgorithm identifies the hash algorithm used for Git object IDs.
+type hashAlgorithm uint8
+
+const (
+	hashAlgoUnknown hashAlgorithm = iota
+	hashAlgoSHA1
+	hashAlgoSHA256
+)
+
+// size returns the hash size in bytes.
+func (algo hashAlgorithm) size() int {
+	switch algo {
+	case hashAlgoSHA1:
+		return sha1.Size
+	case hashAlgoSHA256:
+		return sha256.Size
+	default:
+		return 0
+	}
+}
+
+// String returns the canonical name of the hash algorithm.
+func (algo hashAlgorithm) String() string {
+	switch algo {
+	case hashAlgoSHA1:
+		return "sha1"
+	case hashAlgoSHA256:
+		return "sha256"
+	default:
+		return "unknown"
+	}
+}
+
 // Hash represents a Git object ID.
 type Hash struct {
-	size int
+	algo hashAlgorithm
 	data [maxHashSize]byte
 }
 
 // hashFunc is a function that computes a hash from input data.
 type hashFunc func([]byte) Hash
 
-// hashFuncs maps hash size to hash function.
-var hashFuncs = map[int]hashFunc{
-	sha1.Size: func(data []byte) Hash {
+// hashFuncs maps hash algorithm to hash function.
+var hashFuncs = map[hashAlgorithm]hashFunc{
+	hashAlgoSHA1: func(data []byte) Hash {
 		sum := sha1.Sum(data)
 		var h Hash
 		copy(h.data[:], sum[:])
-		h.size = sha1.Size
+		h.algo = hashAlgoSHA1
 		return h
 	},
-	sha256.Size: func(data []byte) Hash {
+	hashAlgoSHA256: func(data []byte) Hash {
 		sum := sha256.Sum256(data)
 		var h Hash
 		copy(h.data[:], sum[:])
-		h.size = sha256.Size
+		h.algo = hashAlgoSHA256
 		return h
 	},
 }
 
 // String returns a hexadecimal string representation of the hash.
 func (hash Hash) String() string {
-	return hex.EncodeToString(hash.data[:hash.size])
+	size := hash.algo.size()
+	if size == 0 {
+		return ""
+	}
+	return hex.EncodeToString(hash.data[:size])
 }
 
 // Bytes returns a copy of the hash's bytes.
 func (hash Hash) Bytes() []byte {
-	return append([]byte(nil), hash.data[:hash.size]...)
+	size := hash.algo.size()
+	if size == 0 {
+		return nil
+	}
+	return append([]byte(nil), hash.data[:size]...)
 }
 
 // Size returns the hash size.
 func (hash Hash) Size() int {
-	return hash.size
+	return hash.algo.size()
 }