Here's my attempt to create a simple Writer
. It takes into account varying amounts of input data, has configurable chunk length and separator sequence. It uses byte-slice writes for the chunks, which will hopefully be efficient.
package main
import (
"encoding/base64"
"io"
"os"
"strings"
)
func min(a, b int) int {
if a < b {
return a
}
return b
}
type linesplitter struct {
len int
count int
sep []byte
w io.Writer
}
// NewWriter that splits input every len bytes with a sep byte sequence, outputting to writer w
func (ls *linesplitter) NewWriter(len int, sep []byte, w io.Writer) io.WriteCloser {
return &linesplitter{len: len, count: 0, sep: sep, w: w}
}
// Split a line in to ls.len chunks with separator
func (ls *linesplitter) Write(in []byte) (n int, err error) {
writtenThisCall := 0
readPos := 0
// Leading chunk size is limited by: how much input there is; defined split length; and
// any residual from last time
chunkSize := min(len(in), ls.len-ls.count)
// Pass on chunk(s)
for {
ls.w.Write(in[readPos:(readPos + chunkSize)])
readPos += chunkSize // Skip forward ready for next chunk
ls.count += chunkSize
writtenThisCall += chunkSize
// if we have completed a chunk, emit a separator
if ls.count >= ls.len {
ls.w.Write(ls.sep)
writtenThisCall += len(ls.sep)
ls.count = 0
}
inToGo := len(in) - readPos
if inToGo <= 0 {
break // reached end of input data
}
// Determine size of the NEXT chunk
chunkSize = min(inToGo, ls.len)
}
return writtenThisCall, nil
}
func (ls *linesplitter) Close() (err error) {
return nil
}
// See https://www.ietf.org/rfc/rfc2045.txt, section 6.8 for notes on maximum line length of 76 characters
func main() {
data := "It is only the hairs on a gooseberry that prevent it from being a grape! This is long enough to need a line split"
shortData := "hello there"
var ls linesplitter
lsWriter := ls.NewWriter(76, []byte("
"), os.Stdout)
wrt := base64.NewEncoder(base64.StdEncoding, lsWriter)
for i := 0; i < 10; i++ {
io.Copy(wrt, strings.NewReader(shortData))
io.Copy(wrt, strings.NewReader(data))
io.Copy(wrt, strings.NewReader(shortData))
}
}
... comments/improvements welcome.