add writeseeker wrapper for fixed buffers

io/bytewriter.go

@@ -0,0 +1,54 @@
+package io
+
+import (
+	"errors"
+	"io"
+)
+
+type Writer interface {
+	io.WriteSeeker
+}
+
+type writer struct {
+	data   []byte
+	offset int
+}
+
+func NewWriter(data []byte) Writer {
+	return &writer{data, 0}
+}
+
+func (w *writer) Write(data []byte) (int, error) {
+	length := len(data)
+	if w.offset+length > len(w.data) {
+		length = len(w.data) - w.offset
+	}
+
+	if length == 0 {
+		return 0, errors.New("cannot write past end of buffer")
+	}
+
+	w.offset += length
+	return length, nil
+}
+
+func (w *writer) Seek(offset int64, whence int) (int64, error) {
+	result := w.offset
+	switch whence {
+	case io.SeekStart:
+		result = int(offset)
+	case io.SeekCurrent:
+		result = w.offset + int(offset)
+	case io.SeekEnd:
+		result = len(w.data) - int(offset)
+	}
+
+	if result < 0 {
+		return int64(w.offset), errors.New("cannot seek before beginning of buffer")
+	}
+	if result >= len(w.data) {
+		return int64(w.offset), errors.New("cannot seek past end of buffer")
+	}
+
+	return int64(w.offset), nil
+}