package clusterauth

import (
	"crypto/ed25519"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"strings"
	"time"
)

const (
	AuthoritySchemaVersion = "rap.cluster_authority.v1"
	SignatureSchemaVersion = "rap.cluster_authority.signature.v1"
	AlgorithmEd25519       = "ed25519"
)

var (
	ErrInvalidKey       = errors.New("invalid cluster authority key")
	ErrInvalidSignature = errors.New("invalid cluster authority signature")
	ErrInvalidPayload   = errors.New("invalid cluster authority payload")
)

type KeyPair struct {
	PublicKeyB64  string
	PrivateKeyB64 string
	Fingerprint   string
}

type Signature struct {
	SchemaVersion  string    `json:"schema_version"`
	Algorithm      string    `json:"algorithm"`
	KeyFingerprint string    `json:"key_fingerprint"`
	Signature      string    `json:"signature"`
	SignedAt       time.Time `json:"signed_at"`
}

func GenerateKeyPair() (KeyPair, error) {
	publicKey, privateKey, err := ed25519.GenerateKey(rand.Reader)
	if err != nil {
		return KeyPair{}, err
	}
	fingerprint := Fingerprint(publicKey)
	return KeyPair{
		PublicKeyB64:  base64.StdEncoding.EncodeToString(publicKey),
		PrivateKeyB64: base64.StdEncoding.EncodeToString(privateKey),
		Fingerprint:   fingerprint,
	}, nil
}

func Fingerprint(publicKey ed25519.PublicKey) string {
	sum := sha256.Sum256(publicKey)
	return "rap-ca-ed25519-" + hex.EncodeToString(sum[:16])
}

func FingerprintFromBase64(publicKeyB64 string) (string, error) {
	publicKey, err := DecodePublicKey(publicKeyB64)
	if err != nil {
		return "", err
	}
	return Fingerprint(publicKey), nil
}

func SignRaw(privateKeyB64 string, payload json.RawMessage, signedAt time.Time) (Signature, error) {
	privateKey, err := DecodePrivateKey(privateKeyB64)
	if err != nil {
		return Signature{}, err
	}
	canonical, err := CanonicalJSON(payload)
	if err != nil {
		return Signature{}, err
	}
	publicKey, ok := privateKey.Public().(ed25519.PublicKey)
	if !ok {
		return Signature{}, ErrInvalidKey
	}
	signature := ed25519.Sign(privateKey, canonical)
	return Signature{
		SchemaVersion:  SignatureSchemaVersion,
		Algorithm:      AlgorithmEd25519,
		KeyFingerprint: Fingerprint(publicKey),
		Signature:      base64.StdEncoding.EncodeToString(signature),
		SignedAt:       signedAt.UTC(),
	}, nil
}

func SignPayload(privateKeyB64 string, payload any, signedAt time.Time) (json.RawMessage, Signature, error) {
	raw, err := json.Marshal(payload)
	if err != nil {
		return nil, Signature{}, fmt.Errorf("%w: marshal: %v", ErrInvalidPayload, err)
	}
	signature, err := SignRaw(privateKeyB64, raw, signedAt)
	if err != nil {
		return nil, Signature{}, err
	}
	return json.RawMessage(raw), signature, nil
}

func VerifyRaw(publicKeyB64 string, payload json.RawMessage, signature Signature) error {
	if signature.SchemaVersion != SignatureSchemaVersion {
		return fmt.Errorf("%w: schema_version must be %s", ErrInvalidSignature, SignatureSchemaVersion)
	}
	if signature.Algorithm != AlgorithmEd25519 {
		return fmt.Errorf("%w: algorithm must be %s", ErrInvalidSignature, AlgorithmEd25519)
	}
	publicKey, err := DecodePublicKey(publicKeyB64)
	if err != nil {
		return err
	}
	if signature.KeyFingerprint != Fingerprint(publicKey) {
		return fmt.Errorf("%w: key fingerprint mismatch", ErrInvalidSignature)
	}
	canonical, err := CanonicalJSON(payload)
	if err != nil {
		return err
	}
	decodedSignature, err := decodeBase64(strings.TrimSpace(signature.Signature))
	if err != nil || len(decodedSignature) != ed25519.SignatureSize {
		return fmt.Errorf("%w: signature must be base64 ed25519 signature", ErrInvalidSignature)
	}
	if !ed25519.Verify(publicKey, canonical, decodedSignature) {
		return ErrInvalidSignature
	}
	return nil
}

func CanonicalJSON(raw json.RawMessage) ([]byte, error) {
	if len(raw) == 0 {
		return nil, fmt.Errorf("%w: empty payload", ErrInvalidPayload)
	}
	var value any
	if err := json.Unmarshal(raw, &value); err != nil {
		return nil, fmt.Errorf("%w: invalid json: %v", ErrInvalidPayload, err)
	}
	canonical, err := json.Marshal(value)
	if err != nil {
		return nil, fmt.Errorf("%w: canonical json: %v", ErrInvalidPayload, err)
	}
	return canonical, nil
}

func HashRaw(raw json.RawMessage) (string, error) {
	canonical, err := CanonicalJSON(raw)
	if err != nil {
		return "", err
	}
	sum := sha256.Sum256(canonical)
	return hex.EncodeToString(sum[:]), nil
}

func DecodePublicKey(value string) (ed25519.PublicKey, error) {
	decoded, err := decodeBase64(strings.TrimSpace(value))
	if err != nil {
		return nil, fmt.Errorf("%w: public key must be base64 encoded", ErrInvalidKey)
	}
	if len(decoded) != ed25519.PublicKeySize {
		return nil, fmt.Errorf("%w: public key must decode to %d bytes", ErrInvalidKey, ed25519.PublicKeySize)
	}
	return ed25519.PublicKey(decoded), nil
}

func DecodePrivateKey(value string) (ed25519.PrivateKey, error) {
	decoded, err := decodeBase64(strings.TrimSpace(value))
	if err != nil {
		return nil, fmt.Errorf("%w: private key must be base64 encoded", ErrInvalidKey)
	}
	if len(decoded) != ed25519.PrivateKeySize {
		return nil, fmt.Errorf("%w: private key must decode to %d bytes", ErrInvalidKey, ed25519.PrivateKeySize)
	}
	return ed25519.PrivateKey(decoded), nil
}

func decodeBase64(value string) ([]byte, error) {
	if value == "" {
		return nil, errors.New("empty base64 value")
	}
	decoded, err := base64.StdEncoding.DecodeString(value)
	if err == nil {
		return decoded, nil
	}
	return base64.RawStdEncoding.DecodeString(value)
}