package public

import (
	"crypto"
	"crypto/ecdsa"
	"crypto/rand"
	"encoding/base64"
	"errors"
	"fmt"
	"math/big"
	"strings"
)

var (
	ErrInvalidKey      = errors.New("key is invalid")
	ErrInvalidKeyType  = errors.New("key is of invalid type")
	ErrHashUnavailable = errors.New("the requested hash function is unavailable")
)

var (
	// Sadly this is missing from crypto/ecdsa compared to crypto/rsa
	ErrECDSAVerification = errors.New("crypto/ecdsa: verification error")
)

func GetSignVerifyMgr() *SigningMethodECDSA {
	return &SigningMethodECDSA{"ES512", crypto.SHA512, 66, 521}
}

// Implements the ECDSA family of signing methods signing methods
// Expects *ecdsa.PrivateKey for signing and *ecdsa.PublicKey for verification
type SigningMethodECDSA struct {
	Name      string
	Hash      crypto.Hash
	KeySize   int
	CurveBits int
}

func (m *SigningMethodECDSA) Alg() string {
	return m.Name
}

// Implements the Verify method from SigningMethod
// For this verify method, key must be an ecdsa.PublicKey struct
func (m *SigningMethodECDSA) Verify(signingString, signature string, key interface{}) error {
	var err error

	// Decode the signature
	var sig []byte
	if sig, err = DecodeSegment(signature); err != nil {
		return err
	}

	// Get the key
	var ecdsaKey *ecdsa.PublicKey
	switch k := key.(type) {
	case *ecdsa.PublicKey:
		ecdsaKey = k
	default:
		return ErrInvalidKeyType
	}

	if len(sig) != 2*m.KeySize {
		return ErrECDSAVerification
	}

	r := big.NewInt(0).SetBytes(sig[:m.KeySize])
	s := big.NewInt(0).SetBytes(sig[m.KeySize:])

	// Create hasher
	if !m.Hash.Available() {
		return ErrHashUnavailable
	}
	hasher := m.Hash.New()
	hasher.Write([]byte(signingString))

	// Verify the signature
	if verifystatus := ecdsa.Verify(ecdsaKey, hasher.Sum(nil), r, s); verifystatus == true {
		return nil
	} else {
		return ErrECDSAVerification
	}
}

// Implements the Sign method from SigningMethod
// For this signing method, key must be an ecdsa.PrivateKey struct
func (m *SigningMethodECDSA) Sign(signingString string, key interface{}) (string, error) {
	// Get the key
	var ecdsaKey *ecdsa.PrivateKey
	switch k := key.(type) {
	case *ecdsa.PrivateKey:
		ecdsaKey = k
	default:
		return "", ErrInvalidKeyType
	}

	// Create the hasher
	if !m.Hash.Available() {
		return "", ErrHashUnavailable
	}

	hasher := m.Hash.New()
	hasher.Write([]byte(signingString))

	// Sign the string and return r, s
	if r, s, err := ecdsa.Sign(rand.Reader, ecdsaKey, hasher.Sum(nil)); err == nil {
		curveBits := ecdsaKey.Curve.Params().BitSize

		if m.CurveBits != curveBits {
			fmt.Println(m.CurveBits, curveBits)
			return "", ErrInvalidKey
		}

		keyBytes := curveBits / 8
		if curveBits%8 > 0 {
			keyBytes += 1
		}

		// We serialize the outpus (r and s) into big-endian byte arrays and pad
		// them with zeros on the left to make sure the sizes work out. Both arrays
		// must be keyBytes long, and the output must be 2*keyBytes long.
		rBytes := r.Bytes()
		rBytesPadded := make([]byte, keyBytes)
		copy(rBytesPadded[keyBytes-len(rBytes):], rBytes)

		sBytes := s.Bytes()
		sBytesPadded := make([]byte, keyBytes)
		copy(sBytesPadded[keyBytes-len(sBytes):], sBytes)

		out := append(rBytesPadded, sBytesPadded...)

		return EncodeSegment(out), nil
	} else {
		return "", err
	}
}

// Encode JWT specific base64url encoding with padding stripped
func EncodeSegment(seg []byte) string {
	return strings.TrimRight(base64.URLEncoding.EncodeToString(seg), "=")
}

// Decode JWT specific base64url encoding with padding stripped
func DecodeSegment(seg string) ([]byte, error) {
	if l := len(seg) % 4; l > 0 {
		seg += strings.Repeat("=", 4-l)
	}

	return base64.URLEncoding.DecodeString(seg)
}