package bencode

import (
	"bufio"
	"bytes"
	"errors"
	"io"
	"strconv"
)

// A Decoder reads bencoded objects from an input stream.
type Decoder struct {
	r *bufio.Reader
}

// NewDecoder returns a new decoder that reads from r.
func NewDecoder(r io.Reader) *Decoder {
	return &Decoder{r: bufio.NewReader(r)}
}

// Decode unmarshals the next bencoded value in the stream.
func (dec *Decoder) Decode() (interface{}, error) {
	return unmarshal(dec.r)
}

// Unmarshal deserializes and returns the bencoded value in buf.
func Unmarshal(buf []byte) (interface{}, error) {
	r := bufio.NewReader(bytes.NewBuffer(buf))
	return unmarshal(r)
}

// unmarshal reads bencoded values from a bufio.Reader
func unmarshal(r *bufio.Reader) (interface{}, error) {
	tok, err := r.ReadByte()
	if err != nil {
		return nil, err
	}

	switch tok {
	case 'i':
		return readTerminatedInt(r, 'e')

	case 'l':
		return readList(r)

	case 'd':
		return readDict(r)

	default:
		err = r.UnreadByte()
		if err != nil {
			return nil, err
		}

		length, err := readTerminatedInt(r, ':')
		if err != nil {
			return nil, errors.New("bencode: unknown input sequence")
		}

		buf := make([]byte, length)
		n, err := r.Read(buf)

		if err != nil {
			return nil, err
		} else if int64(n) != length {
			return nil, errors.New("bencode: short read")
		}

		return string(buf), nil
	}
}

func readTerminator(r io.ByteScanner, term byte) (bool, error) {
	tok, err := r.ReadByte()
	if err != nil {
		return false, err
	} else if tok == term {
		return true, nil
	}
	return false, r.UnreadByte()
}

func readTerminatedInt(r *bufio.Reader, term byte) (int64, error) {
	buf, err := r.ReadSlice(term)
	if err != nil {
		return 0, err
	} else if len(buf) <= 1 {
		return 0, errors.New("bencode: empty integer field")
	}

	return strconv.ParseInt(string(buf[:len(buf)-1]), 10, 64)
}

func readList(r *bufio.Reader) (List, error) {
	list := NewList()
	for {
		ok, err := readTerminator(r, 'e')
		if err != nil {
			return nil, err
		} else if ok {
			break
		}

		v, err := unmarshal(r)
		if err != nil {
			return nil, err
		}
		list = append(list, v)
	}

	return list, nil
}

func readDict(r *bufio.Reader) (Dict, error) {
	dict := NewDict()
	for {
		ok, err := readTerminator(r, 'e')
		if err != nil {
			return nil, err
		} else if ok {
			break
		}

		v, err := unmarshal(r)
		if err != nil {
			return nil, err
		}

		key, ok := v.(string)
		if !ok {
			return nil, errors.New("bencode: non-string map key")
		}

		dict[key], err = unmarshal(r)
		if err != nil {
			return nil, err
		}
	}

	return dict, nil
}