1 files changed, 65 insertions, 0 deletions
diff --git a/format/pack/ingest/trailer.go b/format/pack/ingest/trailer.go
new file mode 100644
index 00000000..be8156d3
--- /dev/null
+++ b/format/pack/ingest/trailer.go
@@ -0,0 +1,65 @@
+package ingest
+
+import (
+	"bytes"
+	"fmt"
+	"hash"
+)
+
+// trailerVerifier incrementally verifies trailing hash bytes in a stream.
+type trailerVerifier struct {
+	hash     hash.Hash
+	hashSize int
+	tail     []byte
+	seen     int64
+}
+
+// newTrailerVerifier creates a trailing hash verifier.
+func newTrailerVerifier(hash hash.Hash, hashSize int) *trailerVerifier {
+	return &trailerVerifier{
+		hash:     hash,
+		hashSize: hashSize,
+		tail:     make([]byte, 0, hashSize),
+	}
+}
+
+// write feeds one chunk of stream bytes into the verifier.
+func (verifier *trailerVerifier) write(src []byte) {
+	if len(src) == 0 {
+		return
+	}
+
+	verifier.seen += int64(len(src))
+	if len(verifier.tail) == 0 && len(src) <= verifier.hashSize {
+		verifier.tail = append(verifier.tail, src...)
+
+		return
+	}
+
+	tmp := make([]byte, 0, len(verifier.tail)+len(src))
+	tmp = append(tmp, verifier.tail...)
+	tmp = append(tmp, src...)
+	if len(tmp) <= verifier.hashSize {
+		verifier.tail = tmp
+
+		return
+	}
+
+	flushN := len(tmp) - verifier.hashSize
+	_, _ = verifier.hash.Write(tmp[:flushN])
+	verifier.tail = append(verifier.tail[:0], tmp[flushN:]...)
+}
+
+// verify finalizes verification against the stream trailer.
+func (verifier *trailerVerifier) verify() error {
+	if len(verifier.tail) != verifier.hashSize {
+		return fmt.Errorf("format/pack/ingest: stream too short for trailer hash")
+	}
+
+	computed := verifier.hash.Sum(nil)
+	if !bytes.Equal(computed, verifier.tail) {
+		return &ErrPackTrailerMismatch{}
+	}
+
+	return nil
+}