1 files changed, 100 insertions, 0 deletions
diff --git a/internal/format/packfile/ofs_test.go b/internal/format/packfile/ofs_test.go
new file mode 100644
index 00000000..736f1ac9
--- /dev/null
+++ b/internal/format/packfile/ofs_test.go
@@ -0,0 +1,100 @@
+package packfile_test
+
+import (
+	"bytes"
+	"errors"
+	"math"
+	"testing"
+
+	"lindenii.org/go/furgit/internal/format/packfile"
+)
+
+func TestParseOfsDeltaDistance(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name     string
+		data     []byte
+		dist     uint64
+		consumed int
+	}{
+		{name: "zero", data: []byte{0x00}, dist: 0, consumed: 1},
+		{name: "one byte max", data: []byte{0x7f}, dist: 127, consumed: 1},
+		{name: "two byte min", data: []byte{0x80, 0x00}, dist: 128, consumed: 2},
+		{name: "two byte max", data: []byte{0xff, 0x7f}, dist: 16511, consumed: 2},
+		{name: "trailing bytes ignored", data: []byte{0x7f, 0xde, 0xad}, dist: 127, consumed: 1},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			dist, consumed, err := packfile.ParseOfsDeltaDistance(tc.data)
+			if err != nil {
+				t.Fatalf("ParseOfsDeltaDistance: %v", err)
+			}
+
+			if dist != tc.dist {
+				t.Fatalf("ParseOfsDeltaDistance = %d, want %d", dist, tc.dist)
+			}
+
+			if consumed != tc.consumed {
+				t.Fatalf("ParseOfsDeltaDistance consumed = %d, want %d", consumed, tc.consumed)
+			}
+		})
+	}
+}
+
+func TestParseOfsDeltaDistanceMalformed(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name string
+		data []byte
+	}{
+		{name: "empty", data: []byte{}},
+		{name: "truncated", data: []byte{0x80}},
+		{name: "overflow", data: bytes.Repeat([]byte{0xff}, 10)},
+		{name: "overlong", data: append(bytes.Repeat([]byte{0x80}, 10), 0x00)},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			_, _, err := packfile.ParseOfsDeltaDistance(tc.data)
+			if !errors.Is(err, packfile.ErrMalformedOfsDeltaDistance) {
+				t.Fatalf("ParseOfsDeltaDistance error = %v, want ErrMalformedOfsDeltaDistance", err)
+			}
+		})
+	}
+}
+
+func TestOfsDeltaDistanceRoundTrip(t *testing.T) {
+	t.Parallel()
+
+	distances := []uint64{
+		0, 1, 127, 128, 16511, 16512, 1 << 31, 1 << 57, math.MaxUint64,
+	}
+
+	for _, dist := range distances {
+		data := packfile.AppendOfsDeltaDistance(nil, dist)
+		if len(data) > packfile.MaxOfsDeltaDistanceLen {
+			t.Fatalf("AppendOfsDeltaDistance(%d) length = %d, want <= %d",
+				dist, len(data), packfile.MaxOfsDeltaDistanceLen)
+		}
+
+		got, consumed, err := packfile.ParseOfsDeltaDistance(data)
+		if err != nil {
+			t.Fatalf("ParseOfsDeltaDistance: %v", err)
+		}
+
+		if got != dist {
+			t.Fatalf("round trip = %d, want %d", got, dist)
+		}
+
+		if consumed != len(data) {
+			t.Fatalf("round trip consumed = %d, want %d", consumed, len(data))
+		}
+	}
+}