add sectionwriter

This commit is contained in:
siddontang 2014-04-26 11:58:30 +08:00
parent d6a86f7340
commit 3042bf0c03
2 changed files with 125 additions and 0 deletions

69
ioext/sectionwriter.go Normal file
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@ -0,0 +1,69 @@
package ioext
import (
"errors"
"io"
)
var ErrExceedLimit = errors.New("write exceed limit")
func NewSectionWriter(w io.WriterAt, off int64, n int64) *SectionWriter {
return &SectionWriter{w, off, off, off + n}
}
type SectionWriter struct {
w io.WriterAt
base int64
off int64
limit int64
}
func (s *SectionWriter) Write(p []byte) (n int, err error) {
if s.off >= s.limit {
return 0, ErrExceedLimit
}
if max := s.limit - s.off; int64(len(p)) > max {
return 0, ErrExceedLimit
}
n, err = s.w.WriteAt(p, s.off)
s.off += int64(n)
return
}
var errWhence = errors.New("Seek: invalid whence")
var errOffset = errors.New("Seek: invalid offset")
func (s *SectionWriter) Seek(offset int64, whence int) (int64, error) {
switch whence {
default:
return 0, errWhence
case 0:
offset += s.base
case 1:
offset += s.off
case 2:
offset += s.limit
}
if offset < s.base {
return 0, errOffset
}
s.off = offset
return offset - s.base, nil
}
func (s *SectionWriter) WriteAt(p []byte, off int64) (n int, err error) {
if off < 0 || off >= s.limit-s.base {
return 0, errOffset
}
off += s.base
if max := s.limit - off; int64(len(p)) > max {
return 0, ErrExceedLimit
}
return s.w.WriteAt(p, off)
}
// Size returns the size of the section in bytes.
func (s *SectionWriter) Size() int64 { return s.limit - s.base }

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package ioext
import (
"io/ioutil"
"os"
"testing"
)
func TestSectionWriter(t *testing.T) {
f, err := ioutil.TempFile(".", "test_")
if err != nil {
t.Fatal(err)
}
defer func() {
n := f.Name()
f.Close()
os.Remove(n)
}()
f.Truncate(3)
rw := NewSectionWriter(f, 0, 1)
_, err = rw.Write([]byte{'1'})
if err != nil {
t.Fatal(err)
}
_, err = rw.Write([]byte{'1'})
if err == nil {
t.Fatal("must err")
}
rw = NewSectionWriter(f, 1, 2)
_, err = rw.Write([]byte{'2', '3', '4'})
if err == nil {
t.Fatal("must err")
}
_, err = rw.Write([]byte{'2', '3'})
if err != nil {
t.Fatal(err)
}
buf := make([]byte, 3)
_, err = f.ReadAt(buf, 0)
if err != nil {
t.Fatal(err)
}
if string(buf) != "123" {
t.Fatal(string(buf))
}
}