6 files changed, 164 insertions, 1 deletions
diff --git a/object/id/algorithm.go b/object/id/algorithm.go
index 47a1b02a..1b02cbc4 100644
--- a/object/id/algorithm.go
+++ b/object/id/algorithm.go
@@ -5,7 +5,7 @@ type Algorithm uint8
 
 const (
 	// AlgorithmUnknown identifies an unknown object ID hash algorithm.
-	AlgorithmUknown Algorithm = iota
+	AlgorithmUnknown Algorithm = iota
 
 	// AlgorithmSHA1 identifies the SHA-1 object ID hash algorithm.
 	// This is the default for all versions of Git until Git 3.0.
@@ -15,3 +15,9 @@ const (
 	// This is the default for Git 3.0 and beyond.
 	AlgorithmSHA256
 )
+
+// SupportedAlgorithms returns all object ID algorithms supported by furgit.
+// Labels: Mut-No.
+func SupportedAlgorithms() []Algorithm {
+	return supportedAlgorithms
+}
diff --git a/object/id/algorithm_details.go b/object/id/algorithm_details.go
new file mode 100644
index 00000000..d3fd665d
--- /dev/null
+++ b/object/id/algorithm_details.go
@@ -0,0 +1,68 @@
+package id
+
+import (
+	"crypto/sha1" //#nosec:G505
+	"crypto/sha256"
+	"hash"
+)
+
+type algorithmDetails struct {
+	name                string
+	size                int
+	packHashID          uint32
+	signatureHeaderName string
+	sum                 func([]byte) ObjectID
+	new                 func() hash.Hash
+	emptyTree           ObjectID
+}
+
+func (algo Algorithm) info() algorithmDetails {
+	return algorithmTable[algo]
+}
+
+//nolint:gochecknoglobals
+var algorithmTable = [...]algorithmDetails{
+	AlgorithmUnknown: {},
+	AlgorithmSHA1: {
+		name:                "sha1",
+		size:                sha1.Size,
+		packHashID:          1,
+		signatureHeaderName: "gpgsig",
+		new:                 sha1.New,
+	},
+	AlgorithmSHA256: {
+		name:                "sha256",
+		size:                sha256.Size,
+		packHashID:          2,
+		signatureHeaderName: "gpgsig-sha256",
+		new:                 sha256.New,
+	},
+}
+
+// maxObjectIDSize MUST be >= the largest supported algorithm size.
+const maxObjectIDSize = sha256.Size
+
+var (
+	//nolint:gochecknoglobals
+	algorithmByName = map[string]Algorithm{}
+	//nolint:gochecknoglobals
+	algorithmBySignatureHeaderName = map[string]Algorithm{}
+	//nolint:gochecknoglobals
+	supportedAlgorithms []Algorithm
+)
+
+func init() { //nolint:gochecknoinits
+	// Skip over AlgorithmUnknown.
+	for algo := Algorithm(1); int(algo) < len(algorithmTable); algo++ {
+		info := &algorithmTable[algo]
+
+		info.emptyTree = info.sum([]byte("tree 0\x00"))
+
+		algorithmByName[info.name] = algo
+		if info.signatureHeaderName != "" {
+			algorithmBySignatureHeaderName[info.signatureHeaderName] = algo
+		}
+
+		supportedAlgorithms = append(supportedAlgorithms, algo)
+	}
+}
diff --git a/object/id/algorithm_ops.go b/object/id/algorithm_ops.go
new file mode 100644
index 00000000..b746dc5f
--- /dev/null
+++ b/object/id/algorithm_ops.go
@@ -0,0 +1,60 @@
+package id
+
+import "hash"
+
+// EmptyTree returns the object ID of
+// an empty tree ("tree 0\x00")
+// for this algorithm.
+func (algo Algorithm) EmptyTree() ObjectID {
+	return algo.info().emptyTree
+}
+
+// HexLen returns the encoded hexadecimal length.
+func (algo Algorithm) HexLen() int {
+	return algo.Size() * 2
+}
+
+// Size returns the hash size in bytes.
+func (algo Algorithm) Size() int {
+	return algo.info().size
+}
+
+// New returns a new hash.Hash for this algorithm.
+func (algo Algorithm) New() (hash.Hash, error) {
+	newFn := algo.info().new
+	if newFn == nil {
+		return nil, ErrInvalidAlgorithm
+	}
+
+	return newFn(), nil
+}
+
+// PackHashID returns the Git pack/rev hash-id encoding for this algorithm.
+//
+// Unknown algorithms return 0.
+func (algo Algorithm) PackHashID() uint32 {
+	return algo.info().packHashID
+}
+
+// SignatureHeaderName returns the signature header name for this algorithm.
+func (algo Algorithm) SignatureHeaderName() string {
+	return algo.info().signatureHeaderName
+}
+
+// String returns the canonical algorithm name.
+func (algo Algorithm) String() string {
+	return algo.info().name
+}
+
+// Sum computes an object ID from raw data using the selected algorithm.
+func (algo Algorithm) Sum(data []byte) ObjectID {
+	return algo.info().sum(data)
+}
+
+// Zero returns the all-zero object ID for this algorithm.
+func (algo Algorithm) Zero() ObjectID {
+	return ObjectID{
+		algo: algo,
+		data: [maxObjectIDSize]byte{},
+	}
+}
diff --git a/object/id/algorithm_parse.go b/object/id/algorithm_parse.go
new file mode 100644
index 00000000..7349288f
--- /dev/null
+++ b/object/id/algorithm_parse.go
@@ -0,0 +1,8 @@
+package id
+
+// ParseAlgorithm parses a canonical algorithm name (e.g. "sha1", "sha256").
+func ParseAlgorithm(s string) (Algorithm, bool) {
+	algo, ok := algorithmByName[s]
+
+	return algo, ok
+}
diff --git a/object/id/errors.go b/object/id/errors.go
new file mode 100644
index 00000000..11c471ed
--- /dev/null
+++ b/object/id/errors.go
@@ -0,0 +1,12 @@
+package id
+
+import "errors"
+
+var (
+	// ErrInvalidAlgorithm indicates an unsupported object ID algorithm.
+	ErrInvalidAlgorithm = errors.New("objectid: invalid algorithm")
+
+	// ErrInvalidObjectID indicates malformed object ID data.
+	// ErrInvalidObjectID = errors.New("objectid: invalid object id")
+	// TODO
+)
diff --git a/object/id/object_id.go b/object/id/object_id.go
new file mode 100644
index 00000000..4cf562fa
--- /dev/null
+++ b/object/id/object_id.go
@@ -0,0 +1,9 @@
+package id
+
+// ObjectID represents a Git object ID.
+//
+//nolint:recvcheck
+type ObjectID struct {
+	algo Algorithm
+	data [maxObjectIDSize]byte
+}