// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package packet

import (
	"crypto/openpgp/error"
	"io"
	"os"
)

// Reader reads packets from an io.Reader and allows packets to be 'unread' so
// that they result from the next call to Next.
type Reader struct {
	q       []Packet
	readers []io.Reader
}

// Next returns the most recently unread Packet, or reads another packet from
// the top-most io.Reader. Unknown packet types are skipped.
func (r *Reader) Next() (p Packet, err os.Error) {
	if len(r.q) > 0 {
		p = r.q[len(r.q)-1]
		r.q = r.q[:len(r.q)-1]
		return
	}

	for len(r.readers) > 0 {
		p, err = Read(r.readers[len(r.readers)-1])
		if err == nil {
			return
		}
		if err == os.EOF {
			r.readers = r.readers[:len(r.readers)-1]
			continue
		}
		if _, ok := err.(error.UnknownPacketTypeError); !ok {
			return nil, err
		}
	}

	return nil, os.EOF
}

// Push causes the Reader to start reading from a new io.Reader. When an EOF
// error is seen from the new io.Reader, it is popped and the Reader continues
// to read from the next most recent io.Reader.
func (r *Reader) Push(reader io.Reader) {
	r.readers = append(r.readers, reader)
}

// Unread causes the given Packet to be returned from the next call to Next.
func (r *Reader) Unread(p Packet) {
	r.q = append(r.q, p)
}

func NewReader(r io.Reader) *Reader {
	return &Reader{
		q:       nil,
		readers: []io.Reader{r},
	}
}