intial release

asar/storage.go

@@ -0,0 +1,99 @@
+package asar
+
+import (
+	"bytes"
+	"errors"
+	"io"
+)
+
+type SizedReaderAt interface {
+	io.ReaderAt
+	Size() int64
+}
+
+type WriterReaderAtWriterTo interface {
+	io.Writer
+	io.ReaderAt
+	io.WriterTo
+}
+
+type BytesReadAtWriter struct {
+	s []byte
+	i int64
+}
+
+func NewBytesReadAtWriter(s []byte) *BytesReadAtWriter {
+	return &BytesReadAtWriter{s: s, i: 0}
+}
+
+func (b *BytesReadAtWriter) Write(p []byte) (n int, err error) {
+	b.s = append(b.s, p...)
+	return len(p), nil
+}
+
+func (b *BytesReadAtWriter) ReadAt(p []byte, off int64) (n int, err error) {
+	// cannot modify state - see io.ReaderAt
+	if off < 0 {
+		return 0, errors.New("negative offset")
+	}
+	if off >= int64(len(b.s)) {
+		return 0, io.EOF
+	}
+	n = copy(p, b.s[off:])
+	if n < len(p) {
+		err = io.EOF
+	}
+	return
+}
+
+func (b *BytesReadAtWriter) WriteTo(w io.Writer) (n int64, err error) {
+	return io.Copy(w, bytes.NewReader(b.s))
+}
+
+type FilesystemStorage struct {
+	existing  SizedReaderAt
+	new       WriterReaderAtWriterTo
+	newOffset int64
+}
+
+func NewFilesystemStorage(existing SizedReaderAt, new WriterReaderAtWriterTo) *FilesystemStorage {
+	return &FilesystemStorage{
+		existing:  existing,
+		new:       new,
+		newOffset: 0,
+	}
+}
+
+func (fs *FilesystemStorage) Size() int64 {
+	return fs.existing.Size() + fs.newOffset
+}
+
+func (fs *FilesystemStorage) ReadAt(p []byte, off int64) (n int, err error) {
+	existingSize := fs.existing.Size()
+	if off < existingSize {
+		// error when cross boundary
+		if off+int64(len(p)) > existingSize {
+			return 0, io.ErrUnexpectedEOF
+		}
+		return fs.existing.ReadAt(p, off)
+	}
+
+	return fs.new.ReadAt(p, off-existingSize)
+}
+
+func (fs *FilesystemStorage) Write(p []byte) (n int, err error) {
+	n, err = fs.new.Write(p)
+	fs.newOffset += int64(n)
+	return
+}
+
+func (fs *FilesystemStorage) WriteTo(w io.Writer) (n int64, err error) {
+	// write existing
+	n, err = io.Copy(w, io.NewSectionReader(fs.existing, 0, fs.existing.Size()))
+	if err != nil {
+		return n, err
+	}
+
+	n2, err := fs.new.WriteTo(w)
+	return n + n2, err
+}