1 files changed, 140 insertions, 0 deletions
diff --git a/object/id/algorithms.go b/object/id/algorithms.go
new file mode 100644
index 00000000..404d3bbf
--- /dev/null
+++ b/object/id/algorithms.go
@@ -0,0 +1,140 @@
+package objectid
+
+import (
+	"crypto/sha1" //#nosec gosec
+	"crypto/sha256"
+	"hash"
+)
+
+// maxObjectIDSize MUST be >= the largest supported algorithm size.
+const maxObjectIDSize = sha256.Size
+
+// Algorithm identifies the hash algorithm used for Git object IDs.
+type Algorithm uint8
+
+const (
+	AlgorithmUnknown Algorithm = iota
+	AlgorithmSHA1
+	AlgorithmSHA256
+)
+
+type algorithmDetails struct {
+	name       string
+	size       int
+	packHashID uint32
+	sum        func([]byte) ObjectID
+	new        func() hash.Hash
+}
+
+//nolint:gochecknoglobals
+var algorithmTable = [...]algorithmDetails{
+	AlgorithmUnknown: {},
+	AlgorithmSHA1: {
+		name:       "sha1",
+		size:       sha1.Size,
+		packHashID: 1,
+		sum: func(data []byte) ObjectID {
+			sum := sha1.Sum(data) //#nosec G401
+
+			var id ObjectID
+			copy(id.data[:], sum[:])
+			id.algo = AlgorithmSHA1
+
+			return id
+		},
+		new: sha1.New,
+	},
+	AlgorithmSHA256: {
+		name:       "sha256",
+		size:       sha256.Size,
+		packHashID: 2,
+		sum: func(data []byte) ObjectID {
+			sum := sha256.Sum256(data)
+
+			var id ObjectID
+			copy(id.data[:], sum[:])
+			id.algo = AlgorithmSHA256
+
+			return id
+		},
+		new: sha256.New,
+	},
+}
+
+var (
+	//nolint:gochecknoglobals
+	algorithmByName = map[string]Algorithm{}
+	//nolint:gochecknoglobals
+	supportedAlgorithms []Algorithm
+)
+
+func init() { //nolint:gochecknoinits
+	for algo := Algorithm(0); int(algo) < len(algorithmTable); algo++ {
+		info := algorithmTable[algo]
+		if info.name == "" {
+			continue
+		}
+
+		algorithmByName[info.name] = algo
+		supportedAlgorithms = append(supportedAlgorithms, algo)
+	}
+}
+
+// SupportedAlgorithms returns all object ID algorithms supported by furgit.
+// Do not mutate.
+func SupportedAlgorithms() []Algorithm {
+	return supportedAlgorithms
+}
+
+// ParseAlgorithm parses a canonical algorithm name (e.g. "sha1", "sha256").
+func ParseAlgorithm(s string) (Algorithm, bool) {
+	algo, ok := algorithmByName[s]
+
+	return algo, ok
+}
+
+// Size returns the hash size in bytes.
+func (algo Algorithm) Size() int {
+	return algo.info().size
+}
+
+// String returns the canonical algorithm name.
+func (algo Algorithm) String() string {
+	inf := algo.info()
+	if inf.name == "" {
+		return "unknown"
+	}
+
+	return inf.name
+}
+
+// HexLen returns the encoded hexadecimal length.
+func (algo Algorithm) HexLen() int {
+	return algo.Size() * 2
+}
+
+// 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
+}
+
+// Sum computes an object ID from raw data using the selected algorithm.
+func (algo Algorithm) Sum(data []byte) ObjectID {
+	return algo.info().sum(data)
+}
+
+// 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
+}
+
+func (algo Algorithm) info() algorithmDetails {
+	return algorithmTable[algo]
+}