Generalize hash in backend

backend/generic.go

@@ -19,6 +19,13 @@ var (
 	ErrMultipleIDMatches = errors.New("multiple IDs with prefix found")
 )
 
+var (
+	newHash  = sha256.New
+	hashData = sha256.Sum256
+)
+
+const hashSize = sha256.Size
+
 // Each lists all entries of type t in the backend and calls function f() with
 // the id and data.
 func Each(be interface {
@@ -92,7 +99,7 @@ func Uncompress(data []byte) []byte {
 
 // Hash returns the ID for data.
 func Hash(data []byte) ID {
-	h := sha256.Sum256(data)
+	h := hashData(data)
 	id := idPool.Get().(ID)
 	copy(id, h[:])
 	return id

backend/id.go

@@ -2,7 +2,6 @@ package backend
 
 import (
 	"bytes"
-	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
 	"errors"
@@ -10,7 +9,7 @@ import (
 )
 
 // IDSize contains the size of an ID, in bytes.
-const IDSize = sha256.Size
+const IDSize = hashSize
 
 // References content within a repository.
 type ID []byte
@@ -26,7 +25,7 @@ func ParseID(s string) (ID, error) {
 	}
 
 	if len(b) != IDSize {
-		return nil, errors.New("invalid length for sha256 hash")
+		return nil, errors.New("invalid length for hash")
 	}
 
 	return ID(b), nil
@@ -83,7 +82,7 @@ func (id *ID) UnmarshalJSON(b []byte) error {
 }
 
 func IDFromData(d []byte) ID {
-	hash := sha256.Sum256(d)
+	hash := hashData(d)
 	id := idPool.Get().(ID)
 	copy(id, hash[:])
 	return id