package mcrypto

import (
	"bytes"
	"crypto/rand"
	"encoding/binary"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"time"

	"github.com/mediocregopher/mediocre-go-lib/mlog"
)

const (
	uuidV0Prefix = "0u"
	uuidV0Len    = 34 // prefix:2 + hexEncodedLen(time:8 + random:8)
)

// UUID is a universally unique identifier which embeds within it a timestamp.
//
// Only Unmarshal methods should be called on the zero UUID value.
//
// Comparing the equality of two UUID's should always be done using the Equal
// method, or by comparing their string forms.
//
// The string form of UUIDs (returned by String or MarshalText) are
// lexigraphically order-able by their embedded timestamp.
type UUID struct {
	// the UUID type is actually just an opaque wrapper. For the most part
	// UUID's don't ever need the information in them (like their timestamp)
	// unpacked, so it's more efficient to just keep the string and unpack
	// on-the-fly
	str string
}

// NewUUID populates and returns a new UUID instance which embeds the given time
func NewUUID(t time.Time) UUID {
	b := make([]byte, 16)
	binary.BigEndian.PutUint64(b[:8], uint64(t.UnixNano()))
	if _, err := rand.Read(b[8:]); err != nil {
		panic(err)
	}
	return UUID{
		str: uuidV0Prefix + hex.EncodeToString(b),
	}
}

func (u UUID) String() string {
	return u.str
}

// Equal returns whether or not the two UUID's are the same value
func (u UUID) Equal(u2 UUID) bool {
	return u.str == u2.str
}

// Time unpacks and returns the timestamp embedded in the UUID
func (u UUID) Time() time.Time {
	b, err := hex.DecodeString(u.str[2:])
	if err != nil {
		// once a UUID has been created it should always be valid
		panic(fmt.Sprintf("malformed UUID: %q", u.str))
	}
	unixNano := int64(binary.BigEndian.Uint64(b[:8]))
	return time.Unix(0, unixNano).Local()
}

// KV implements the method for the mlog.KVer interface
func (u UUID) KV() mlog.KV {
	return mlog.KV{"uuid": u.String()}
}

// MarshalText implements the method for the encoding.TextMarshaler interface
func (u UUID) MarshalText() ([]byte, error) {
	return []byte(u.String()), nil
}

// UnmarshalText implements the method for the encoding.TextUnmarshaler
// interface
func (u *UUID) UnmarshalText(b []byte) error {
	if !bytes.HasPrefix(b, []byte(uuidV0Prefix)) || len(b) != uuidV0Len {
		err := errors.New("malformed uuid string")
		return mlog.ErrWithKV(err, mlog.KV{"uuidStr": string(b)})
	}
	u.str = string(b)
	return nil
}

// MarshalJSON implements the method for the json.Marshaler interface
func (u UUID) MarshalJSON() ([]byte, error) {
	return json.Marshal(u.String())
}

// UnmarshalJSON implements the method for the json.Unmarshaler interface
func (u *UUID) UnmarshalJSON(b []byte) error {
	var s string
	if err := json.Unmarshal(b, &s); err != nil {
		return err
	}
	return u.UnmarshalText([]byte(s))
}