package io

import (
	"bytes"
	"errors"
)

// ErrDrained is returned on an attempt to use an already drained write buffer.
var ErrDrained = errors.New("buffer already drained")

// BufBinWriter is an additional layer on top of BinWriter that
// automatically creates a buffer to write into that you can get after all
// writes via Bytes().
type BufBinWriter struct {
	*BinWriter
	buf bytes.Buffer
}

// NewBufBinWriter makes a BufBinWriter with an empty byte buffer.
func NewBufBinWriter() *BufBinWriter {
	b := new(BufBinWriter)
	b.BinWriter = NewBinWriterFromIO(&b.buf)
	return b
}

// Len returns the number of bytes of the unread portion of the buffer.
func (bw *BufBinWriter) Len() int {
	return bw.buf.Len()
}

// Bytes returns the resulting buffer and makes future writes return an error.
// Subsequent calls to it will return nil. You can reuse this instance of
// [BufBinWriter] after [BufBinWriter.Reset].
func (bw *BufBinWriter) Bytes() []byte {
	if bw.Err != nil {
		return nil
	}
	bw.Err = ErrDrained
	return bw.buf.Bytes()
}

// Reset resets the state of the buffer, making it usable again. It can
// make buffer usage somewhat more efficient because you don't need to
// create it again. But beware, the buffer is gonna be the same as the one
// returned by Bytes(), so if you need that data after Reset() you have to copy
// it yourself.
func (bw *BufBinWriter) Reset() {
	bw.Err = nil
	bw.buf.Reset()
}
