implement reader at

pkging/file.go

@@ -26,6 +26,13 @@ type File interface {
 	// Read reads up to len(b) bytes from the File. It returns the number of bytes read and any error encountered. At end of file, Read returns 0, io.EOF.
 	Read(p []byte) (int, error)
 
+	// ReadAt implements the ReadAt interface for File
+	// (reads len(b) bytes from the File starting at byte offset off.
+	// It returns the number of bytes read and the error, if any.
+	// ReadAt always returns a non-nil error when n < len(b).
+	// At end of file, that error is io.EOF)
+	ReadAt(p []byte, offset int64) (int, error)
+
 	// Readdir reads the contents of the directory associated with file and returns a slice of up to n FileInfo values, as would be returned by Lstat, in directory order. Subsequent calls on the same file will yield further FileInfos.
 	//
 	// If n > 0, Readdir returns at most n FileInfo structures. In this case, if Readdir returns an empty slice, it will return a non-nil error explaining why. At the end of a directory, the error is io.EOF.

pkging/mem/file.go

@@ -81,6 +81,21 @@ func (f *File) Read(p []byte) (int, error) {
 	return 0, fmt.Errorf("unable to read %s", f.Name())
 }
 
+// ReadAt implements the ReadAt interface for File
+// (reads len(b) bytes from the File starting at byte offset off.
+// It returns the number of bytes read and the error, if any.
+// ReadAt always returns a non-nil error when n < len(b).
+// At end of file, that error is io.EOF)
+func (f *File) ReadAt(p []byte, offset int64) (int, error) {
+	if len(f.data) == 0 || offset >= int64(len(f.data)) {
+		return 0, io.EOF
+	}
+
+	f.reader = bytes.NewReader(f.data[offset:])
+
+	return f.reader.Read(p)
+}
+
 // Write writes len(b) bytes to the File. It returns the number of bytes written and an error, if any. Write returns a non-nil error when n != len(b).
 func (f *File) Write(b []byte) (int, error) {
 	if f.writer == nil {

pkging/mem/file_test.go

@@ -60,3 +60,29 @@ func Test_File_Seek(t *testing.T) {
 	r.NotEqual(data, b)
 	r.Equal([]byte("the arm"), b)
 }
+
+func TestFileReadAt(t *testing.T) {
+	r := require.New(t)
+
+	info, err := here.Current()
+	r.NoError(err)
+
+	pkg, err := New(info)
+	r.NoError(err)
+
+	f, err := pkg.Create(":/tolstoy")
+	r.NoError(err)
+
+	data := []byte("pierre always loved natasha")
+	f.Write(data)
+	r.NoError(f.Close())
+
+	f, err = pkg.Open(":/tolstoy")
+	r.NoError(err)
+
+	b := make([]byte, len(data))
+	read, err := f.ReadAt(b, 7)
+	r.NoError(err)
+	r.Equal("always loved natasha", string(b[:read]))
+	r.Equal(read, len(data[7:]))
+}