add custom encoding registration function

[wiro34]

I'm new to golang.

This is experimental.

I hope somebody that advise if there is better way.

Fixes #6

Usage Sample:

package foo

import (
	"github.com/axgle/mahonia"
	"golang.org/x/text/encoding"
	"golang.org/x/text/encoding/japanese"
	"golang.org/x/text/transform"
)

var ShiftJIS encoding.Encoding = &sjisEncoding{}

type sjisEncoding struct {
	encoding.Encoding
}

func (e *sjisEncoding) NewDecoder() *encoding.Decoder {
	return japanese.ShiftJIS.NewDecoder()
}

func (e *sjisEncoding) NewEncoder() *encoding.Encoder {
	return &encoding.Encoder{Transformer: &shiftJISEncoder{}}
}

type shiftJISEncoder struct{ transform.NopResetter }

// Transform using mahonia.
func (shiftJISEncoder) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
	encoder := mahonia.NewEncoder("sjis")
	encoded := []byte(encoder.ConvertString(string(src)))
	copy(dst, encoded)
	return nDst + len(encoded), nSrc + len(src), err
}

// override `sjis` encoding
tds.RegisterEncoding("sjis", foo.ShiftJIS)

[thda]

Hi,

many thanks for your input.

I strongly suggest using utf-8 for your use case. That said, if your server does not support it, maybe it would be easier to add a flag to disable charset conversion, named CharConvert, which could be set to off.
This flag would be taken into account in session.go, in the processEnvChange function.
It would probably give better performance.

That said, I'm not opposed to registering a charset encoding as you have done.
Before I merge this, would you mind adding a read lock to getEncoding, and a read/write lock to register encoding?
Would fix eventual race conditions.

A quick word about this in the documentation (doc.go) would also be appreciated.

Thanks,
Thomas

[wiro34]

I strongly suggest using utf-8 for your use case.

Yes, I know I should use utf8. However, our server is still not...

I updated the commit.

Thank you for your advice.
I tried s.charConvert = false, but it didn't work.
How is charConvert used?

[thda]

S.charconvert is not used. I'll add that connection flag and make use of it.
Let me review and merge today. Thanks again!

[thda]

Merging this. I'll add the charConvert connexion parameter and run all my tests before pushing a release.