package ect

import (
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"encoding/json"
	"errors"
	"time"

	"github.com/go-jose/go-jose/v4"
)

// CreateOptions configures ECT creation.
type CreateOptions struct {
	// KeyID is the kid header value (references public key from WIT).
	KeyID string
	// IATMaxAge caps how far in the past iat may be (recommended 15 min).
	IATMaxAge time.Duration
	// DefaultExpiry is added to time.Now() for exp when zero (recommended 5–15 min).
	DefaultExpiry time.Duration
	// ValidateUUIDs when true requires jti and wid (if set) to be UUID format (RFC 9562).
	ValidateUUIDs bool
	// MaxPredLength is the max number of predecessor references (0 = no limit; recommended 100).
	MaxPredLength int
}

// DefaultCreateOptions returns recommended defaults.
func DefaultCreateOptions() CreateOptions {
	return CreateOptions{
		IATMaxAge:     15 * time.Minute,
		DefaultExpiry: 10 * time.Minute,
	}
}

// Create builds and signs an ECT. Payload must have required claims set;
// Iat/Exp can be zero to use defaults (now, now+DefaultExpiry).
// Create may modify the payload in place (Iat, Exp, Sub, Par) when filling defaults; pass a copy if the original must stay unchanged.
func Create(payload *Payload, privateKey *ecdsa.PrivateKey, opts CreateOptions) (compact string, err error) {
	if payload == nil || privateKey == nil {
		return "", ErrPayloadRequired
	}
	if opts.KeyID == "" {
		return "", ErrKeyIDRequired
	}
	now := time.Now()
	if payload.Iat == 0 {
		payload.Iat = now.Unix()
	}
	if payload.Exp == 0 {
		if opts.DefaultExpiry == 0 {
			opts.DefaultExpiry = 10 * time.Minute
		}
		payload.Exp = now.Add(opts.DefaultExpiry).Unix()
	}
	if payload.Pred == nil {
		payload.Pred = []string{}
	}

	if err := validatePayloadForCreate(payload, opts); err != nil {
		return "", err
	}

	payloadBytes, err := json.Marshal(payload)
	if err != nil {
		return "", err
	}

	sig := jose.SigningKey{Algorithm: jose.ES256, Key: privateKey}
	options := &jose.SignerOptions{
		ExtraHeaders: map[jose.HeaderKey]interface{}{
			jose.HeaderType: ECTType,
			jose.HeaderKey("kid"): opts.KeyID,
		},
	}
	signer, err := jose.NewSigner(sig, options)
	if err != nil {
		return "", err
	}

	jws, err := signer.Sign(payloadBytes)
	if err != nil {
		return "", err
	}

	return jws.CompactSerialize()
}

func validatePayloadForCreate(p *Payload, opts CreateOptions) error {
	if p.Iss == "" {
		return errors.New("ect: iss required")
	}
	if len(p.Aud) == 0 {
		return errors.New("ect: aud required")
	}
	if p.Jti == "" {
		return errors.New("ect: jti required")
	}
	if p.ExecAct == "" {
		return errors.New("ect: exec_act required")
	}
	if opts.ValidateUUIDs {
		if !ValidUUID(p.Jti) {
			return ErrInvalidJTI
		}
		if p.Wid != "" && !ValidUUID(p.Wid) {
			return ErrInvalidWID
		}
	}
	if opts.MaxPredLength > 0 && len(p.Pred) > opts.MaxPredLength {
		return ErrPredLength
	}
	if p.InpHash != "" {
		if err := ValidateHashFormat(p.InpHash); err != nil {
			return err
		}
	}
	if p.OutHash != "" {
		if err := ValidateHashFormat(p.OutHash); err != nil {
			return err
		}
	}
	if err := ValidateExt(p.Ext); err != nil {
		return err
	}
	// compensation_* live in ext per spec
	if p.Ext != nil {
		if _, hasReason := p.Ext["compensation_reason"]; hasReason {
			if v, _ := p.Ext["compensation_required"].(bool); !v {
				return errors.New("ect: ext.compensation_reason requires ext.compensation_required true")
			}
		}
	}
	return nil
}

// GenerateKey creates an ECDSA P-256 key for ES256 (for testing/demo).
func GenerateKey() (*ecdsa.PrivateKey, error) {
	return ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
}