put docbuf into an external pkg

Signed-off-by: Ava Affine <ava@sunnypup.io>

pkg/docbuf/read_write_seek_string.go

@@ -0,0 +1,100 @@
+package docbuf
+
+import (
+	"errors"
+	"io"
+	"fmt"
+)
+
+/* WARNING:
+ * This code is meant to assist with testing and mock ups
+ * It is not only not designed to any rigorous standards,
+ * but additionally does not offer any benefit over a static
+ * in memory single layer cache.
+ */
+
+type ReadWriteSeekString struct {
+	inner  string
+	cursor int
+}
+
+func NewReadWriteSeekString() ReadWriteSeekString{
+	return ReadWriteSeekString{
+		inner: "",
+		cursor: 0,
+	}
+}
+
+func (s *ReadWriteSeekString) Read(
+	buf []byte,
+) (int, error) {
+	i := 0
+	for ; i < len(buf); i++ {
+		if len(s.inner) <= s.cursor {
+			return i, nil
+		}
+		buf[i] = s.inner[s.cursor]
+		s.cursor += 1
+	}
+	return i, nil
+}
+
+func (s *ReadWriteSeekString) Write(
+	buf []byte,
+) (int, error) {
+	backfillDelta := s.cursor - (len(s.inner) - 1)
+	if backfillDelta > 0 {
+		for range backfillDelta {
+			s.inner += "\x00"
+		}
+	}
+
+	tmpBuf := ""
+	if s.cursor > 0 {
+		tmpBuf += s.inner[:s.cursor]
+	}
+	tmpBuf += string(buf)
+	if s.cursor + len(buf) < (len(s.inner) - 1) {
+		tmpBuf += s.inner[s.cursor + len(buf):]
+	}
+
+	s.inner = tmpBuf
+	s.cursor += len(buf)
+	return len(buf), nil
+}
+
+func (s *ReadWriteSeekString) Seek(
+	offset int64,
+	whence int,
+) (int64, error) {
+	var tmpCur int64
+	tmpCur = 0
+
+	switch whence {
+	case io.SeekCurrent:
+		tmpCur = int64(s.cursor)
+	case io.SeekEnd:
+		tmpCur = int64(len(s.inner))
+	case io.SeekStart:
+		tmpCur = int64(0)
+	default:
+		return int64(s.cursor),
+			errors.New("invalid whence value")
+	}
+
+	tmpCur += offset
+	if tmpCur < 0 {
+		msg := fmt.Sprintf("seek index (%d) is negative", tmpCur)
+		return int64(s.cursor), errors.New(msg)
+	}
+	s.cursor = int(tmpCur)
+	return tmpCur, nil
+}
+
+func (s *ReadWriteSeekString) Contents() string {
+	return s.inner
+}
+
+func (s *ReadWriteSeekString) Cursor() int {
+	return s.cursor
+}