1 files changed, 112 insertions, 0 deletions
diff --git a/internal/format/packidx/write_test.go b/internal/format/packidx/write_test.go
new file mode 100644
index 00000000..68df3ece
--- /dev/null
+++ b/internal/format/packidx/write_test.go
@@ -0,0 +1,112 @@
+package packidx_test
+
+import (
+	"bytes"
+	"errors"
+	"testing"
+
+	"lindenii.org/go/furgit/internal/format/packidx"
+	"lindenii.org/go/furgit/object/id"
+)
+
+// syntheticEntries builds n distinct entries sorted by object ID,
+// spread over the fanout table.
+func syntheticEntries(n int) []packidx.Entry {
+	entries := make([]packidx.Entry, n)
+	for i := range entries {
+		entries[i].OID[0] = byte(i * 7)
+		entries[i].OID[1] = byte(i + 1)
+		entries[i].Offset = uint64(i+1) * 100
+		entries[i].CRC32 = uint32(i+1) * 0x01010101
+	}
+
+	return entries
+}
+
+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, idx := parseGitIdxFile(t, prefix, objectFormat)
+
+			entries := make([]packidx.Entry, idx.NumObjects())
+			for pos := range entries {
+				copy(entries[pos].OID[:], idx.OIDAt(pos))
+				entries[pos].CRC32 = idx.CRCAt(pos)
+
+				offset, err := idx.OffsetAt(pos)
+				if err != nil {
+					t.Fatalf("OffsetAt(%d): %v", pos, err)
+				}
+
+				entries[pos].Offset = offset
+			}
+
+			var buf bytes.Buffer
+
+			err := packidx.Write(&buf, objectFormat, entries, idx.PackHash())
+			if err != nil {
+				t.Fatalf("Write: %v", err)
+			}
+
+			if !bytes.Equal(buf.Bytes(), gitData) {
+				t.Fatalf("Write output differs from git's index (%d vs %d bytes)", buf.Len(), len(gitData))
+			}
+		})
+	}
+}
+
+func TestWriteInvalidEntries(t *testing.T) {
+	t.Parallel()
+
+	unsorted := syntheticEntries(3)
+	unsorted[0], unsorted[2] = unsorted[2], unsorted[0]
+
+	duplicated := syntheticEntries(3)
+	duplicated[1] = duplicated[0]
+
+	cases := []struct {
+		name    string
+		entries []packidx.Entry
+	}{
+		{name: "unsorted", entries: unsorted},
+		{name: "duplicated", entries: duplicated},
+	}
+
+	for _, objectFormat := range id.SupportedObjectFormats() {
+		t.Run(objectFormat.String(), func(t *testing.T) {
+			t.Parallel()
+
+			packHash := bytes.Repeat([]byte{0x5a}, objectFormat.Size())
+
+			for _, tc := range cases {
+				t.Run(tc.name, func(t *testing.T) {
+					t.Parallel()
+
+					err := packidx.Write(&bytes.Buffer{}, objectFormat, tc.entries, packHash)
+					if !errors.Is(err, packidx.ErrInvalidEntries) {
+						t.Fatalf("Write error = %v, want ErrInvalidEntries", err)
+					}
+				})
+			}
+		})
+	}
+}
+
+func TestWriteBadPackHashPanics(t *testing.T) {
+	t.Parallel()
+
+	objectFormat := id.ObjectFormatSHA256
+
+	defer func() {
+		if recover() == nil {
+			t.Fatalf("Write with short pack hash: expected panic")
+		}
+	}()
+
+	_ = packidx.Write(&bytes.Buffer{}, objectFormat, nil, []byte{0x01})
+}