You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: ecdsa.go
    path of file: ./repos/kubeedge/vendor/github.com/golang-jwt/jwt
    
    the code of the file until where you have to start completion: package jwt

import (
	"crypto"
	"crypto/ecdsa"
	"crypto/rand"
	"errors"
	"math/big"
)

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

// 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
}

// Specific instances for EC256 and company
var (
	SigningMethodES256 *Sign
    
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 {
			return "", ErrInvalidKey
		}

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

		// We serialize the outputs (r and s) into big-endian byte arrays
		// padded with zeros on the left to make sure the sizes work out.
		// Output must be 2*keyBytes long.
		out := make([]byte, 2*keyBytes)
		r.FillBytes(out[0:keyBytes]) // r is assigned to the first half of output.
		s.FillBytes(out[keyBytes:])  // s is assigned to the second half of output.

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