package session

import (
	"bytes"
	"crypto/cipher"
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha1"
	"crypto/subtle"
	"encoding/base64"
	"encoding/gob"
	"errors"
	"fmt"
	"io"
	"strconv"
	"time"
)

func init() {
	gob.Register([]interface{}{})
	gob.Register(map[int]interface{}{})
	gob.Register(map[string]interface{}{})
	gob.Register(map[interface{}]interface{}{})
	gob.Register(map[string]string{})
	gob.Register(map[int]string{})
	gob.Register(map[int]int{})
	gob.Register(map[int]int64{})
}

func encodeGob(obj map[interface{}]interface{}) ([]byte, error) {
	buf := bytes.NewBuffer(nil)
	enc := gob.NewEncoder(buf)
	err := enc.Encode(obj)
	if err != nil {
		return []byte(""), err
	}
	return buf.Bytes(), nil
}

func decodeGob(encoded []byte) (map[interface{}]interface{}, error) {
	buf := bytes.NewBuffer(encoded)
	dec := gob.NewDecoder(buf)
	var out map[interface{}]interface{}
	err := dec.Decode(&out)
	if err != nil {
		return nil, err
	}
	return out, nil
}

// generateRandomKey creates a random key with the given strength.
func generateRandomKey(strength int) []byte {
	k := make([]byte, strength)
	if _, err := io.ReadFull(rand.Reader, k); err != nil {
		return nil
	}
	return k
}

// Encryption -----------------------------------------------------------------

// encrypt encrypts a value using the given block in counter mode.
//
// A random initialization vector (http://goo.gl/zF67k) with the length of the
// block size is prepended to the resulting ciphertext.
func encrypt(block cipher.Block, value []byte) ([]byte, error) {
	iv := generateRandomKey(block.BlockSize())
	if iv == nil {
		return nil, errors.New("encrypt: failed to generate random iv")
	}
	// Encrypt it.
	stream := cipher.NewCTR(block, iv)
	stream.XORKeyStream(value, value)
	// Return iv + ciphertext.
	return append(iv, value...), nil
}

// decrypt decrypts a value using the given block in counter mode.
//
// The value to be decrypted must be prepended by a initialization vector
// (http://goo.gl/zF67k) with the length of the block size.
func decrypt(block cipher.Block, value []byte) ([]byte, error) {
	size := block.BlockSize()
	if len(value) > size {
		// Extract iv.
		iv := value[:size]
		// Extract ciphertext.
		value = value[size:]
		// Decrypt it.
		stream := cipher.NewCTR(block, iv)
		stream.XORKeyStream(value, value)
		return value, nil
	}
	return nil, errors.New("decrypt: the value could not be decrypted")
}

func encodeCookie(block cipher.Block, hashKey, name string, value map[interface{}]interface{}) (string, error) {
	var err error
	var b []byte
	// 1. encodeGob.
	if b, err = encodeGob(value); err != nil {
		return "", err
	}
	// 2. Encrypt (optional).
	if b, err = encrypt(block, b); err != nil {
		return "", err
	}
	b = encode(b)
	// 3. Create MAC for "name|date|value". Extra pipe to be used later.
	b = []byte(fmt.Sprintf("%s|%d|%s|", name, time.Now().UTC().Unix(), b))
	h := hmac.New(sha1.New, []byte(hashKey))
	h.Write(b)
	sig := h.Sum(nil)
	// Append mac, remove name.
	b = append(b, sig...)[len(name)+1:]
	// 4. Encode to base64.
	b = encode(b)
	// Done.
	return string(b), nil
}

func decodeCookie(block cipher.Block, hashKey, name, value string, gcmaxlifetime int64) (map[interface{}]interface{}, error) {
	// 1. Decode from base64.
	b, err := decode([]byte(value))
	if err != nil {
		return nil, err
	}
	// 2. Verify MAC. Value is "date|value|mac".
	parts := bytes.SplitN(b, []byte("|"), 3)
	if len(parts) != 3 {
		return nil, errors.New("Decode: invalid value %v")
	}

	b = append([]byte(name+"|"), b[:len(b)-len(parts[2])]...)
	h := hmac.New(sha1.New, []byte(hashKey))
	h.Write(b)
	sig := h.Sum(nil)
	if len(sig) != len(parts[2]) || subtle.ConstantTimeCompare(sig, parts[2]) != 1 {
		return nil, errors.New("Decode: the value is not valid")
	}
	// 3. Verify date ranges.
	var t1 int64
	if t1, err = strconv.ParseInt(string(parts[0]), 10, 64); err != nil {
		return nil, errors.New("Decode: invalid timestamp")
	}
	t2 := time.Now().UTC().Unix()
	if t1 > t2 {
		return nil, errors.New("Decode: timestamp is too new")
	}
	if t1 < t2-gcmaxlifetime {
		return nil, errors.New("Decode: expired timestamp")
	}
	// 4. Decrypt (optional).
	b, err = decode(parts[1])
	if err != nil {
		return nil, err
	}
	if b, err = decrypt(block, b); err != nil {
		return nil, err
	}
	// 5. decodeGob.
	if dst, err := decodeGob(b); err != nil {
		return nil, err
	} else {
		return dst, nil
	}
	// Done.
	return nil, nil
}

// Encoding -------------------------------------------------------------------

// encode encodes a value using base64.
func encode(value []byte) []byte {
	encoded := make([]byte, base64.URLEncoding.EncodedLen(len(value)))
	base64.URLEncoding.Encode(encoded, value)
	return encoded
}

// decode decodes a cookie using base64.
func decode(value []byte) ([]byte, error) {
	decoded := make([]byte, base64.URLEncoding.DecodedLen(len(value)))
	b, err := base64.URLEncoding.Decode(decoded, value)
	if err != nil {
		return nil, err
	}
	return decoded[:b], nil
}