1 files changed, 135 insertions, 0 deletions
diff --git a/internal/format/packrev/write_test.go b/internal/format/packrev/write_test.go
new file mode 100644
index 00000000..b5c1fcb9
--- /dev/null
+++ b/internal/format/packrev/write_test.go
@@ -0,0 +1,135 @@
+package packrev_test
+
+import (
+	"bytes"
+	"errors"
+	"os"
+	"testing"
+
+	"lindenii.org/go/furgit/internal/format/packidx"
+	"lindenii.org/go/furgit/internal/format/packrev"
+	"lindenii.org/go/furgit/object/id"
+)
+
+// writeSyntheticRev writes one reverse index over positions
+// with a fixed fake pack hash.
+func writeSyntheticRev(t *testing.T, objectFormat id.ObjectFormat, positions []uint32) []byte {
+	t.Helper()
+
+	packHash := bytes.Repeat([]byte{0x5a}, objectFormat.Size())
+
+	var buf bytes.Buffer
+
+	err := packrev.Write(&buf, objectFormat, positions, packHash)
+	if err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+
+	return buf.Bytes()
+}
+
+func TestWriteRoundTrip(t *testing.T) {
+	t.Parallel()
+
+	for _, objectFormat := range id.SupportedObjectFormats() {
+		t.Run(objectFormat.String(), func(t *testing.T) {
+			t.Parallel()
+
+			positions := []uint32{8, 6, 7, 5, 3, 0, 4, 1, 2}
+			data := writeSyntheticRev(t, objectFormat, positions)
+
+			rev, err := packrev.Parse(data, objectFormat)
+			if err != nil {
+				t.Fatalf("Parse: %v", err)
+			}
+
+			if rev.NumObjects() != len(positions) {
+				t.Fatalf("NumObjects = %d, want %d", rev.NumObjects(), len(positions))
+			}
+
+			if !bytes.Equal(rev.PackHash(), bytes.Repeat([]byte{0x5a}, objectFormat.Size())) {
+				t.Fatalf("PackHash mismatch")
+			}
+
+			for packOrder, want := range positions {
+				position, err := rev.PositionAt(packOrder)
+				if err != nil {
+					t.Fatalf("PositionAt(%d): %v", packOrder, err)
+				}
+
+				if position != int(want) {
+					t.Fatalf("PositionAt(%d) = %d, want %d", packOrder, position, want)
+				}
+			}
+		})
+	}
+}
+
+func TestWriteMatchesGit(t *testing.T) {
+	t.Parallel()
+
+	for _, objectFormat := range id.SupportedObjectFormats() {
+		t.Run(objectFormat.String(), func(t *testing.T) {
+			t.Parallel()
+
+			prefix := makeGitPack(t, objectFormat)
+
+			gitData, err := os.ReadFile(prefix + ".rev") //nolint:gosec
+			if err != nil {
+				t.Fatalf("ReadFile: %v", err)
+			}
+
+			idxData, err := os.ReadFile(prefix + ".idx") //nolint:gosec
+			if err != nil {
+				t.Fatalf("ReadFile: %v", err)
+			}
+
+			idx, err := packidx.Parse(idxData, objectFormat.Size())
+			if err != nil {
+				t.Fatalf("packidx.Parse: %v", err)
+			}
+
+			positions := packOrderPositions(t, &idx)
+
+			var buf bytes.Buffer
+
+			err = packrev.Write(&buf, objectFormat, positions, idx.PackHash())
+			if err != nil {
+				t.Fatalf("Write: %v", err)
+			}
+
+			if !bytes.Equal(buf.Bytes(), gitData) {
+				t.Fatalf("Write output differs from git's reverse index (%d vs %d bytes)", buf.Len(), len(gitData))
+			}
+		})
+	}
+}
+
+func TestWriteInvalidPositions(t *testing.T) {
+	t.Parallel()
+
+	for _, objectFormat := range id.SupportedObjectFormats() {
+		t.Run(objectFormat.String(), func(t *testing.T) {
+			t.Parallel()
+
+			packHash := bytes.Repeat([]byte{0x5a}, objectFormat.Size())
+
+			err := packrev.Write(&bytes.Buffer{}, objectFormat, []uint32{0, 5}, packHash)
+			if !errors.Is(err, packrev.ErrInvalidPositions) {
+				t.Fatalf("Write error = %v, want ErrInvalidPositions", err)
+			}
+		})
+	}
+}
+
+func TestWriteBadPackHashPanics(t *testing.T) {
+	t.Parallel()
+
+	defer func() {
+		if recover() == nil {
+			t.Fatalf("Write with short pack hash: expected panic")
+		}
+	}()
+
+	_ = packrev.Write(&bytes.Buffer{}, id.ObjectFormatSHA256, nil, []byte{0x01})
+}