124 lines
2.7 KiB
Go
124 lines
2.7 KiB
Go
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package prefixer
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import (
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"bufio"
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"io"
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)
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// Prefixer implements io.Reader and io.WriterTo. It reads
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// data from the underlying reader and prepends every line
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// with a given string.
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type Prefixer struct {
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reader *bufio.Reader
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prefix []byte
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unread []byte
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eof bool
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}
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// New creates a new instance of Prefixer.
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func New(r io.Reader, prefix string) *Prefixer {
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return &Prefixer{
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reader: bufio.NewReader(r),
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prefix: []byte(prefix),
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}
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}
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// Read implements io.Reader. It reads data into p from the
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// underlying reader and prepends every line with a prefix.
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// It does not block if no data is available yet.
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// It returns the number of bytes read into p.
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func (r *Prefixer) Read(p []byte) (n int, err error) {
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for {
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// Write unread data from previous read.
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if len(r.unread) > 0 {
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m := copy(p[n:], r.unread)
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n += m
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r.unread = r.unread[m:]
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if len(r.unread) > 0 {
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return n, nil
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}
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}
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// The underlying Reader already returned EOF, do not read again.
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if r.eof {
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return n, io.EOF
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}
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// Read new line, including delim.
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r.unread, err = r.reader.ReadBytes('\n')
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if err == io.EOF {
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r.eof = true
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}
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// No new data, do not block.
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if len(r.unread) == 0 {
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return n, err
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}
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// Some new data, prepend prefix.
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// TODO: We could write the prefix to r.unread buffer just once
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// and re-use it instead of prepending every time.
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r.unread = append(r.prefix, r.unread...)
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if err != nil {
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if err == io.EOF && len(r.unread) > 0 {
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// The underlying Reader already returned EOF, but we still
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// have some unread data to send, thus clear the error.
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return n, nil
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}
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return n, err
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}
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}
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panic("unreachable")
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}
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func (r *Prefixer) WriteTo(w io.Writer) (n int64, err error) {
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for {
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// Write unread data from previous read.
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if len(r.unread) > 0 {
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m, err := w.Write(r.unread)
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n += int64(m)
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if err != nil {
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return n, err
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}
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r.unread = r.unread[m:]
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if len(r.unread) > 0 {
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return n, nil
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}
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}
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// The underlying Reader already returned EOF, do not read again.
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if r.eof {
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return n, io.EOF
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}
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// Read new line, including delim.
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r.unread, err = r.reader.ReadBytes('\n')
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if err == io.EOF {
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r.eof = true
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}
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// No new data, do not block.
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if len(r.unread) == 0 {
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return n, err
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}
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// Some new data, prepend prefix.
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// TODO: We could write the prefix to r.unread buffer just once
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// and re-use it instead of prepending every time.
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r.unread = append(r.prefix, r.unread...)
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if err != nil {
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if err == io.EOF && len(r.unread) > 0 {
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// The underlying Reader already returned EOF, but we still
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// have some unread data to send, thus clear the error.
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return n, nil
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}
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return n, err
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}
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}
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panic("unreachable")
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}
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