feat: add writer

writer.go

@@ -0,0 +1,62 @@
+package spb
+
+import (
+	"bytes"
+	"io"
+)
+
+type Constructor struct {
+	structlen, reallen uint32
+	body               bytes.Buffer
+}
+
+// NewWriter 新的写入构造器
+func NewWriter(buf []byte) (c Constructor) {
+	c.body = *bytes.NewBuffer(buf)
+	return
+}
+
+// WriteString 按字符串写入一项
+func (c *Constructor) WriteString(s string, cap uint32) error {
+	if int(cap) < len(s) {
+		return ErrInvalidDataLen
+	}
+	cnt, err := WriteNum(&c.body, cap)
+	if err != nil {
+		return err
+	}
+	c.structlen += cap
+	c.reallen += cnt
+	cnt, err = WriteNum(&c.body, uint32(len(s)))
+	if err != nil {
+		return err
+	}
+	c.reallen += cnt
+	_, err = c.body.WriteString(s)
+	if err != nil {
+		return err
+	}
+	c.reallen += uint32(len(s))
+	return nil
+}
+
+// Cap structlen 目前写入的总长, 带 padding
+func (c *Constructor) Cap() uint32 {
+	return c.structlen
+}
+
+// Len reallen 目前写入的实值总长, 不带 padding
+func (c *Constructor) Len() uint32 {
+	return c.reallen
+}
+
+// WriteTo 将当前结果写出, 返回写入的实际长度
+func (c *Constructor) WriteTo(w io.Writer) (int64, error) {
+	cnt, err := WriteNum(w, c.structlen)
+	if err != nil {
+		return int64(cnt), err
+	}
+	n, err := w.Write(c.body.Bytes())
+	cnt += uint32(n)
+	return int64(cnt), err
+}