oid: Add ObjectID

The tests are not rreeeaallly necessary but I guess if I somehow mess
things up here, that'd pick it up earlier than debugging other things.

3 files changed, 339 insertions, 0 deletions

diff --git a/go.mod b/go.mod
new file mode 100644
index 00000000..390ca978
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module codeberg.org/lindenii/furgit
+
+go 1.26.0
diff --git a/oid/objectid.go b/oid/objectid.go
new file mode 100644
index 00000000..92031d92
--- /dev/null
+++ b/oid/objectid.go
@@ -0,0 +1,192 @@
+// Package oid provides object ID and algorithm primitives for Git objects.
+package oid
+
+import (
+	"crypto/sha1"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"hash"
+)
+
+var (
+	// ErrInvalidAlgorithm indicates an unsupported object ID algorithm.
+	ErrInvalidAlgorithm = errors.New("oid: invalid algorithm")
+	// ErrInvalidObjectID indicates malformed object ID data.
+	ErrInvalidObjectID = errors.New("oid: invalid object id")
+)
+
+// 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
+	sum  func([]byte) ObjectID
+	new  func() hash.Hash
+}
+
+var algorithmTable = [...]algorithmDetails{
+	AlgorithmUnknown: {},
+	AlgorithmSHA1: {
+		name: "sha1",
+		size: sha1.Size,
+		sum: func(data []byte) ObjectID {
+			sum := sha1.Sum(data)
+			var id ObjectID
+			copy(id.data[:], sum[:])
+			id.algo = AlgorithmSHA1
+			return id
+		},
+		new: func() hash.Hash {
+			return sha1.New()
+		},
+	},
+	AlgorithmSHA256: {
+		name: "sha256",
+		size: sha256.Size,
+		sum: func(data []byte) ObjectID {
+			sum := sha256.Sum256(data)
+			var id ObjectID
+			copy(id.data[:], sum[:])
+			id.algo = AlgorithmSHA256
+			return id
+		},
+		new: func() hash.Hash {
+			return sha256.New()
+		},
+	},
+}
+
+var algorithmByName = map[string]Algorithm{}
+
+func init() {
+	for algo, info := range algorithmTable {
+		if info.name == "" {
+			continue
+		}
+		algorithmByName[info.name] = Algorithm(algo)
+	}
+}
+
+func (algo Algorithm) info() algorithmDetails {
+	return algorithmTable[algo]
+}
+
+// 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
+}
+
+// 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
+}
+
+// ObjectID represents a Git object ID.
+type ObjectID struct {
+	algo Algorithm
+	data [maxObjectIDSize]byte
+}
+
+// Algorithm returns the object ID's hash algorithm.
+func (id ObjectID) Algorithm() Algorithm {
+	return id.algo
+}
+
+// Size returns the object ID size in bytes.
+func (id ObjectID) Size() int {
+	return id.algo.Size()
+}
+
+// String returns the canonical hex representation.
+func (id ObjectID) String() string {
+	size := id.Size()
+	if size == 0 {
+		return ""
+	}
+	return hex.EncodeToString(id.data[:size])
+}
+
+// Bytes returns a copy of the object ID bytes.
+func (id ObjectID) Bytes() []byte {
+	size := id.Size()
+	if size == 0 {
+		return nil
+	}
+	return append([]byte(nil), id.data[:size]...)
+}
+
+// ParseHex parses an object ID from hex for the specified algorithm.
+func ParseHex(algo Algorithm, s string) (ObjectID, error) {
+	var id ObjectID
+	if algo.Size() == 0 {
+		return id, ErrInvalidAlgorithm
+	}
+	if len(s)%2 != 0 {
+		return id, fmt.Errorf("%w: odd hex length %d", ErrInvalidObjectID, len(s))
+	}
+	if len(s) != algo.HexLen() {
+		return id, fmt.Errorf("%w: got %d chars, expected %d", ErrInvalidObjectID, len(s), algo.HexLen())
+	}
+	decoded, err := hex.DecodeString(s)
+	if err != nil {
+		return id, fmt.Errorf("%w: decode: %v", ErrInvalidObjectID, err)
+	}
+	copy(id.data[:], decoded)
+	id.algo = algo
+	return id, nil
+}
+
+// FromBytes builds an object ID from raw bytes for the specified algorithm.
+func FromBytes(algo Algorithm, b []byte) (ObjectID, error) {
+	var id ObjectID
+	if algo.Size() == 0 {
+		return id, ErrInvalidAlgorithm
+	}
+	if len(b) != algo.Size() {
+		return id, fmt.Errorf("%w: got %d bytes, expected %d", ErrInvalidObjectID, len(b), algo.Size())
+	}
+	copy(id.data[:], b)
+	id.algo = algo
+	return id, nil
+}
diff --git a/oid/objectid_test.go b/oid/objectid_test.go
new file mode 100644
index 00000000..e650da1e
--- /dev/null
+++ b/oid/objectid_test.go
@@ -0,0 +1,144 @@
+package oid
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestParseAlgorithm(t *testing.T) {
+	t.Parallel()
+
+	algo, ok := ParseAlgorithm("sha1")
+	if !ok || algo != AlgorithmSHA1 {
+		t.Fatalf("ParseAlgorithm(sha1) = (%v,%v)", algo, ok)
+	}
+
+	algo, ok = ParseAlgorithm("sha256")
+	if !ok || algo != AlgorithmSHA256 {
+		t.Fatalf("ParseAlgorithm(sha256) = (%v,%v)", algo, ok)
+	}
+
+	if _, ok := ParseAlgorithm("md5"); ok {
+		t.Fatalf("ParseAlgorithm(md5) should fail")
+	}
+}
+
+func TestParseHexRoundtrip(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		algo Algorithm
+		hex  string
+	}{
+		{
+			name: "sha1",
+			algo: AlgorithmSHA1,
+			hex:  "0123456789abcdef0123456789abcdef01234567",
+		},
+		{
+			name: "sha256",
+			algo: AlgorithmSHA256,
+			hex:  "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			id, err := ParseHex(tt.algo, tt.hex)
+			if err != nil {
+				t.Fatalf("ParseHex failed: %v", err)
+			}
+			if got := id.String(); got != tt.hex {
+				t.Fatalf("String() = %q, want %q", got, tt.hex)
+			}
+			if got := id.Size(); got != tt.algo.Size() {
+				t.Fatalf("Size() = %d, want %d", got, tt.algo.Size())
+			}
+
+			raw := id.Bytes()
+			if len(raw) != tt.algo.Size() {
+				t.Fatalf("Bytes len = %d, want %d", len(raw), tt.algo.Size())
+			}
+
+			id2, err := FromBytes(tt.algo, raw)
+			if err != nil {
+				t.Fatalf("FromBytes failed: %v", err)
+			}
+			if id2.String() != tt.hex {
+				t.Fatalf("FromBytes roundtrip = %q, want %q", id2.String(), tt.hex)
+			}
+		})
+	}
+}
+
+func TestParseHexErrors(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		algo Algorithm
+		hex  string
+	}{
+		{"unknown algo", AlgorithmUnknown, "00"},
+		{"odd len", AlgorithmSHA1, "0"},
+		{"wrong len", AlgorithmSHA1, "0123"},
+		{"invalid hex", AlgorithmSHA1, "zz23456789abcdef0123456789abcdef01234567"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if _, err := ParseHex(tt.algo, tt.hex); err == nil {
+				t.Fatalf("expected ParseHex error")
+			}
+		})
+	}
+}
+
+func TestFromBytesErrors(t *testing.T) {
+	t.Parallel()
+
+	if _, err := FromBytes(AlgorithmUnknown, []byte{1, 2}); err == nil {
+		t.Fatalf("expected FromBytes unknown algo error")
+	}
+	if _, err := FromBytes(AlgorithmSHA1, []byte{1, 2}); err == nil {
+		t.Fatalf("expected FromBytes wrong size error")
+	}
+}
+
+func TestBytesReturnsCopy(t *testing.T) {
+	t.Parallel()
+
+	id, err := ParseHex(AlgorithmSHA1, "0123456789abcdef0123456789abcdef01234567")
+	if err != nil {
+		t.Fatalf("ParseHex failed: %v", err)
+	}
+
+	b1 := id.Bytes()
+	b2 := id.Bytes()
+	if !bytes.Equal(b1, b2) {
+		t.Fatalf("Bytes mismatch")
+	}
+	b1[0] ^= 0xff
+	if bytes.Equal(b1, b2) {
+		t.Fatalf("Bytes should return independent copies")
+	}
+}
+
+func TestAlgorithmSum(t *testing.T) {
+	t.Parallel()
+
+	id1 := AlgorithmSHA1.Sum([]byte("hello"))
+	if id1.Algorithm() != AlgorithmSHA1 || id1.Size() != AlgorithmSHA1.Size() {
+		t.Fatalf("sha1 sum produced invalid object id")
+	}
+
+	id2 := AlgorithmSHA256.Sum([]byte("hello"))
+	if id2.Algorithm() != AlgorithmSHA256 || id2.Size() != AlgorithmSHA256.Size() {
+		t.Fatalf("sha256 sum produced invalid object id")
+	}
+
+	if id1.String() == id2.String() {
+		t.Fatalf("sha1 and sha256 should differ")
+	}
+}