format: Move commitgraph and packfile here

1 files changed, 111 insertions, 0 deletions

diff --git a/format/packfile/ingest/stream.go b/format/packfile/ingest/stream.go
new file mode 100644
index 00000000..a403087a
--- /dev/null
+++ b/format/packfile/ingest/stream.go
@@ -0,0 +1,111 @@
+package ingest
+
+import (
+	"errors"
+	"hash"
+	"io"
+	"os"
+)
+
+const streamScannerBufferSize = 64 << 10
+
+// streamScanner incrementally reads/consumes one pack stream while mirroring
+// consumed bytes into one destination pack file.
+type streamScanner struct {
+	src     io.Reader
+	dstFile *os.File
+
+	// Input buffer window: buf[off:n] is unread.
+	buf []byte
+	off int
+	n   int
+
+	// Absolute consumed stream bytes.
+	consumed uint64
+
+	// Running pack hash over consumed bytes while hashEnabled is true.
+	hash        hash.Hash
+	hashSize    int
+	hashEnabled bool
+
+	// Entry CRC state while one entry is being consumed.
+	entryCRC   uint32
+	inEntryCRC bool
+
+	packTrailer []byte
+}
+
+// newStreamScanner constructs one scanner with fixed input buffering.
+func newStreamScanner(src io.Reader, dstFile *os.File, hash hash.Hash, hashSize int) *streamScanner {
+	return &streamScanner{
+		src:         src,
+		dstFile:     dstFile,
+		buf:         make([]byte, streamScannerBufferSize),
+		hash:        hash,
+		hashSize:    hashSize,
+		hashEnabled: true,
+	}
+}
+
+// Read implements io.Reader.
+func (scanner *streamScanner) Read(dst []byte) (int, error) {
+	if len(dst) == 0 {
+		return 0, nil
+	}
+
+	if scanner.n-scanner.off == 0 {
+		err := scanner.fill(1)
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				return 0, io.EOF
+			}
+
+			return 0, err
+		}
+	}
+
+	unread := scanner.n - scanner.off
+	if unread == 0 {
+		return 0, io.EOF
+	}
+
+	n := min(len(dst), unread)
+
+	copy(dst, scanner.buf[scanner.off:scanner.off+n])
+
+	err := scanner.use(n)
+	if err != nil {
+		return 0, err
+	}
+
+	return n, nil
+}
+
+// ReadByte implements io.ByteReader without allocation.
+func (scanner *streamScanner) ReadByte() (byte, error) {
+	if scanner.n-scanner.off == 0 {
+		err := scanner.fill(1)
+		if err != nil {
+			return 0, err
+		}
+	}
+
+	b := scanner.buf[scanner.off]
+
+	err := scanner.use(1)
+	if err != nil {
+		return 0, err
+	}
+
+	return b, nil
+}
+
+// readFull reads exactly len(dst) bytes through receiver.
+func (scanner *streamScanner) readFull(dst []byte) error {
+	_, err := io.ReadFull(scanner, dst)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}