1 files changed, 60 insertions, 0 deletions
diff --git a/plumbing/hash/hash.go b/plumbing/hash/hash.go
new file mode 100644
index 0000000..82d1856
--- /dev/null
+++ b/plumbing/hash/hash.go
@@ -0,0 +1,60 @@
+// package hash provides a way for managing the
+// underlying hash implementations used across go-git.
+package hash
+
+import (
+	"crypto"
+	"fmt"
+	"hash"
+
+	"github.com/pjbgf/sha1cd"
+)
+
+// algos is a map of hash algorithms.
+var algos = map[crypto.Hash]func() hash.Hash{}
+
+func init() {
+	reset()
+}
+
+// reset resets the default algos value. Can be used after running tests
+// that registers new algorithms to avoid side effects.
+func reset() {
+	algos[crypto.SHA1] = sha1cd.New
+	algos[crypto.SHA256] = crypto.SHA256.New
+}
+
+// RegisterHash allows for the hash algorithm used to be overriden.
+// This ensures the hash selection for go-git must be explicit, when
+// overriding the default value.
+func RegisterHash(h crypto.Hash, f func() hash.Hash) error {
+	if f == nil {
+		return fmt.Errorf("cannot register hash: f is nil")
+	}
+
+	switch h {
+	case crypto.SHA1:
+		algos[h] = f
+	case crypto.SHA256:
+		algos[h] = f
+	default:
+		return fmt.Errorf("unsupported hash function: %v", h)
+	}
+	return nil
+}
+
+// Hash is the same as hash.Hash. This allows consumers
+// to not having to import this package alongside "hash".
+type Hash interface {
+	hash.Hash
+}
+
+// New returns a new Hash for the given hash function.
+// It panics if the hash function is not registered.
+func New(h crypto.Hash) Hash {
+	hh, ok := algos[h]
+	if !ok {
+		panic(fmt.Sprintf("hash algorithm not registered: %v", h))
+	}
+	return hh()
+}