反向读取文件

测了500W条数据上面还是有点问题（换行的边界没处理好），这下OK了。😅😅😅 ```go package file import ( "bytes" "fmt" "io" "os" ) var ( buffSize = 1 << 20 ) // ReadLineFromEnd -- type ReadLineFromEnd struct { f *os.File fileSize int bwr *bytes.Buffer lineBuff []byte swapBuff []byte isFirst bool } // NewReadLineFromEnd -- func NewReadLineFromEnd(name string) (rd *ReadLineFromEnd, err error) { f, err := os.Open(name) if err != nil { return nil, err } info, err := f.Stat() if info.IsDir() { return nil, fmt.Errorf("not file") } fileSize := int(info.Size()) rd = &ReadLineFromEnd{ f: f, fileSize: fileSize, bwr: bytes.NewBuffer([]byte{}), lineBuff: make([]byte, 0), swapBuff: make([]byte, buffSize), isFirst: true, } return rd, nil } // ReadLine 结尾包含'\n' func (c *ReadLineFromEnd) ReadLine() (line []byte, err error) { var ok bool for { ok, err = c.buff() if err != nil { return nil, err } if ok { break } } line, err = c.bwr.ReadBytes('\n') if err == io.EOF && c.fileSize > 0 { err = nil } return line, err } // Close -- func (c *ReadLineFromEnd) Close() (err error) { return c.f.Close() } func (c *ReadLineFromEnd) buff() (ok bool, err error) { if c.fileSize == 0 { return true, nil } if c.bwr.Len() >= buffSize { return true, nil } offset := 0 if c.fileSize > buffSize { offset = c.fileSize - buffSize } _, err = c.f.Seek(int64(offset), 0) if err != nil { return false, err } n, err := c.f.Read(c.swapBuff) if err != nil && err != io.EOF { return false, err } if c.fileSize < n { n = c.fileSize } if n == 0 { return true, nil } for { m := bytes.LastIndex(c.swapBuff[:n], []byte{'\n'}) if m == -1 { break } if m < n-1 { err = c.writeLine(c.swapBuff[m+1 : n]) if err != nil { return false, err } ok = true } else if m == n-1 && !c.isFirst { err = c.writeLine(nil) if err != nil { return false, err } ok = true } n = m if n == 0 { break } } if n > 0 { reverseBytes(c.swapBuff[:n]) c.lineBuff = append(c.lineBuff, c.swapBuff[:n]...) } if offset == 0 { err = c.writeLine(nil) if err != nil { return false, err } ok = true } c.fileSize = offset if c.isFirst { c.isFirst = false } return ok, nil } func (c *ReadLineFromEnd) writeLine(b []byte) (err error) { if len(b) > 0 { _, err = c.bwr.Write(b) if err != nil { return err } } if len(c.lineBuff) > 0 { reverseBytes(c.lineBuff) _, err = c.bwr.Write(c.lineBuff) if err != nil { return err } c.lineBuff = c.lineBuff[:0] } _, err = c.bwr.Write([]byte{'\n'}) if err != nil { return err } return nil } func reverseBytes(b []byte) { n := len(b) if n <= 1 { return } for i := 0; i < n; i++ { k := n - 1 if k != i { b[i], b[k] = b[k], b[i] } n-- } } ```

请问这样做的需求是什么？可以试试：先读取文件，然后转为string后，反转一下？

临时写了一个，测试了一下可以，如用于生产环境的话还需要严格测试。 ```go package file import ( "bytes" "fmt" "io" "os" ) var ( buffSize = 1 << 20 ) // ReadLineFromEnd -- type ReadLineFromEnd struct { f *os.File fileSize int bwr *bytes.Buffer swapBuff []byte } // NewReadLineFromEnd -- func NewReadLineFromEnd(name string) (rd *ReadLineFromEnd, err error) { f, err := os.Open(name) if err != nil { return nil, err } info, err := f.Stat() if info.IsDir() { return nil, fmt.Errorf("not file") } fileSize := int(info.Size()) rd = &ReadLineFromEnd{ f: f, fileSize: fileSize, bwr: bytes.NewBuffer([]byte{}), swapBuff: make([]byte, buffSize), } return rd, nil } // Read -- func (c *ReadLineFromEnd) Read(p []byte) (n int, err error) { err = c.buff() if err != nil { return n, err } return c.bwr.Read(p) } // ReadLine 结尾包含'\n' func (c *ReadLineFromEnd) ReadLine() (line []byte, err error) { err = c.buff() if err != nil { return nil, err } return c.bwr.ReadBytes('\n') } // Close -- func (c *ReadLineFromEnd) Close() (err error) { return c.f.Close() } func (c *ReadLineFromEnd) buff() (err error) { if c.fileSize == 0 { return nil } if c.bwr.Len() >= buffSize { return nil } offset := 0 if c.fileSize > buffSize { offset = c.fileSize - buffSize } _, err = c.f.Seek(int64(offset), 0) if err != nil { return err } n, err := c.f.Read(c.swapBuff) if err != nil && err != io.EOF { return err } if n == 0 { return nil } for { m := bytes.LastIndex(c.swapBuff[:n], []byte{'\n'}) if m == -1 { break } if m < n-1 { _, err = c.bwr.Write(c.swapBuff[m+1 : n]) if err != nil { return err } _, err = c.bwr.Write([]byte{'\n'}) if err != nil { return err } } n = m if n == 0 { break } } if n > 0 { _, err := c.bwr.Write(c.swapBuff[:n]) if err != nil { return err } } c.fileSize = offset return nil } ``` 测试代码 ```go func TestNewReadLineFromEnd(t *testing.T) { rd, err := NewReadLineFromEnd("./123.log") if err != nil { t.Fatal(err) } defer rd.Close() for { data, err := rd.ReadLine() if err != nil { if err != io.EOF { t.Fatal(err) } break } fmt.Print(string(data)) } } ```