1 files changed, 79 insertions, 0 deletions
diff --git a/internal/format/packrev/write.go b/internal/format/packrev/write.go
new file mode 100644
index 00000000..399c9157
--- /dev/null
+++ b/internal/format/packrev/write.go
@@ -0,0 +1,79 @@
+package packrev
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"math"
+
+	"lindenii.org/go/furgit/internal/stickyio"
+	"lindenii.org/go/furgit/object/id"
+)
+
+// ErrInvalidPositions reports that
+// positions supplied for a reverse index write
+// are out of range or too numerous.
+var ErrInvalidPositions = errors.New("internal/format/packrev: invalid positions")
+
+// Write writes one pack reverse index to w.
+//
+// positions holds, for each object in pack offset order,
+// the object's pack index position.
+// packHash must be the pack's trailer hash;
+// Write panics when its length does not match the object format.
+func Write(w io.Writer, objectFormat id.ObjectFormat, positions []uint32, packHash []byte) error {
+	hashID, err := hashFunctionID(objectFormat)
+	if err != nil {
+		return err
+	}
+
+	if len(packHash) != objectFormat.Size() {
+		panic("internal/format/packrev: invalid pack hash length")
+	}
+
+	if len(positions) > math.MaxUint32 {
+		return fmt.Errorf("%w: too many positions", ErrInvalidPositions)
+	}
+
+	for _, position := range positions {
+		if uint64(position) >= uint64(len(positions)) {
+			return fmt.Errorf("%w: index position out of range", ErrInvalidPositions)
+		}
+	}
+
+	hashImpl, err := objectFormat.New()
+	if err != nil {
+		return fmt.Errorf("internal/format/packrev: %w", err)
+	}
+
+	bw := bufio.NewWriter(io.MultiWriter(w, hashImpl))
+	sw := stickyio.New(bw)
+
+	sw.PutUint32(signature)
+	sw.PutUint32(version)
+	sw.PutUint32(hashID)
+
+	for _, position := range positions {
+		sw.PutUint32(position)
+	}
+
+	sw.Put(packHash)
+
+	err = sw.Err()
+	if err != nil {
+		return fmt.Errorf("internal/format/packrev: %w", err)
+	}
+
+	err = bw.Flush()
+	if err != nil {
+		return fmt.Errorf("internal/format/packrev: %w", err)
+	}
+
+	_, err = w.Write(hashImpl.Sum(nil))
+	if err != nil {
+		return fmt.Errorf("internal/format/packrev: %w", err)
+	}
+
+	return nil
+}