package licensing

import (
	"bytes"
	"crypto/ed25519"
	"encoding/base64"
	"encoding/json"
	"errors"
	"io"
	"time"

	"softbox.local/core/internal/canonicaljson"
)

const (
	// MaxRevocationValidity is the maximum signed freshness interval accepted
	// from a Revocation List v1 document.
	MaxRevocationValidity = 31 * 24 * time.Hour
	// RevocationGrace is the bounded offline period after a cached list expires.
	RevocationGrace = 7 * 24 * time.Hour
)

var (
	ErrInvalidRevocation          = errors.New("revocation list is invalid")
	ErrRevocationSignatureMissing = errors.New("revocation list signature is missing")
	ErrRevocationSignatureInvalid = errors.New("revocation list signature is invalid")
)

var revocationFields = map[string]struct{}{
	"schema_version":      {},
	"generated_at":        {},
	"expires_at":          {},
	"revoked_license_ids": {},
	"signature":           {},
}

// RevocationList is a verified Revocation List v1 document. It deliberately
// excludes its source document, signature and signing key.
type RevocationList struct {
	GeneratedAt       time.Time
	ExpiresAt         time.Time
	RevokedLicenseIDs []string
}

// RevocationState is the non-sensitive authorization result of one cached
// revocation list at a caller-supplied instant.
type RevocationState string

const (
	RevocationStateCurrent     RevocationState = "current"
	RevocationStateGrace       RevocationState = "grace"
	RevocationStateUnavailable RevocationState = "unavailable"
	RevocationStateRevoked     RevocationState = "revoked"
)

// RevocationVerifier validates Revocation List v1 documents with one copied
// Ed25519 public key. Callers may inject the same controlled authorization key
// used for License v1, but no fallback key exists.
type RevocationVerifier struct {
	publicKey ed25519.PublicKey
}

// NewRevocationVerifier copies and validates a Revocation List v1 signing key.
func NewRevocationVerifier(publicKey []byte) (RevocationVerifier, error) {
	if len(publicKey) != ed25519.PublicKeySize {
		return RevocationVerifier{}, ErrPublicKeyInvalid
	}
	return RevocationVerifier{publicKey: append(ed25519.PublicKey(nil), publicKey...)}, nil
}

// Verify validates one Revocation List v1 document.
func (verifier RevocationVerifier) Verify(document []byte) (RevocationList, error) {
	if len(verifier.publicKey) != ed25519.PublicKeySize {
		return RevocationList{}, ErrPublicKeyInvalid
	}

	rootValue, err := canonicaljson.Parse(document)
	if err != nil {
		return RevocationList{}, revocationCanonicalError(err)
	}
	root, ok := rootValue.(map[string]any)
	if !ok {
		return RevocationList{}, ErrInvalidRevocation
	}
	signatureValue, exists := root["signature"]
	if !exists {
		return RevocationList{}, ErrRevocationSignatureMissing
	}
	signatureText, ok := signatureValue.(string)
	if !ok {
		return RevocationList{}, ErrRevocationSignatureInvalid
	}
	hasExactFields := hasExactRevocationShape(root)
	delete(root, "signature")
	signedPayload, err := canonicaljson.Marshal(root)
	if err != nil {
		return RevocationList{}, revocationCanonicalError(err)
	}
	signature, err := decodeRevocationSignature(signatureText)
	if err != nil || !ed25519.Verify(verifier.publicKey, signedPayload, signature) {
		return RevocationList{}, ErrRevocationSignatureInvalid
	}
	if !hasExactFields {
		return RevocationList{}, ErrInvalidRevocation
	}

	wire, err := decodeRevocationList(document)
	if err != nil {
		return RevocationList{}, err
	}
	return validateRevocationList(wire)
}

// StateFor reports whether one license ID is safe to use under this verified
// cache. A future-dated list is unavailable rather than trusted.
func (list RevocationList) StateFor(licenseID string, now time.Time) RevocationState {
	if now.Before(list.GeneratedAt) || !list.ExpiresAt.After(list.GeneratedAt) ||
		list.ExpiresAt.Sub(list.GeneratedAt) > MaxRevocationValidity {
		return RevocationStateUnavailable
	}
	for _, revokedID := range list.RevokedLicenseIDs {
		if revokedID == licenseID {
			return RevocationStateRevoked
		}
	}
	if !now.After(list.ExpiresAt) {
		return RevocationStateCurrent
	}
	if !now.After(list.ExpiresAt.Add(RevocationGrace)) {
		return RevocationStateGrace
	}
	return RevocationStateUnavailable
}

func hasExactRevocationShape(root map[string]any) bool {
	if len(root) != len(revocationFields) {
		return false
	}
	for field := range revocationFields {
		if _, exists := root[field]; !exists {
			return false
		}
	}
	if _, ok := root["schema_version"].(json.Number); !ok {
		return false
	}
	if _, ok := root["revoked_license_ids"].([]any); !ok {
		return false
	}
	for _, field := range []string{"generated_at", "expires_at", "signature"} {
		if _, ok := root[field].(string); !ok {
			return false
		}
	}
	return true
}

type revocationWire struct {
	SchemaVersion     int      `json:"schema_version"`
	GeneratedAt       string   `json:"generated_at"`
	ExpiresAt         string   `json:"expires_at"`
	RevokedLicenseIDs []string `json:"revoked_license_ids"`
	Signature         string   `json:"signature"`
}

func decodeRevocationList(document []byte) (revocationWire, error) {
	decoder := json.NewDecoder(bytes.NewReader(document))
	decoder.DisallowUnknownFields()
	decoder.UseNumber()
	var wire revocationWire
	if err := decoder.Decode(&wire); err != nil {
		return revocationWire{}, ErrInvalidRevocation
	}
	var extra any
	if err := decoder.Decode(&extra); err != io.EOF {
		return revocationWire{}, ErrInvalidRevocation
	}
	return wire, nil
}

func validateRevocationList(wire revocationWire) (RevocationList, error) {
	if wire.SchemaVersion != 1 || wire.Signature == "" {
		return RevocationList{}, ErrInvalidRevocation
	}
	generatedAt, err := time.Parse(timestampLayout, wire.GeneratedAt)
	if err != nil || generatedAt.Format(timestampLayout) != wire.GeneratedAt {
		return RevocationList{}, ErrInvalidRevocation
	}
	expiresAt, err := time.Parse(timestampLayout, wire.ExpiresAt)
	if err != nil || expiresAt.Format(timestampLayout) != wire.ExpiresAt ||
		!expiresAt.After(generatedAt) || expiresAt.Sub(generatedAt) > MaxRevocationValidity {
		return RevocationList{}, ErrInvalidRevocation
	}
	ids := make([]string, len(wire.RevokedLicenseIDs))
	seen := make(map[string]struct{}, len(wire.RevokedLicenseIDs))
	for index, licenseID := range wire.RevokedLicenseIDs {
		if !licenseIDPattern.MatchString(licenseID) {
			return RevocationList{}, ErrInvalidRevocation
		}
		if _, exists := seen[licenseID]; exists {
			return RevocationList{}, ErrInvalidRevocation
		}
		seen[licenseID] = struct{}{}
		ids[index] = licenseID
	}
	return RevocationList{
		GeneratedAt:       generatedAt,
		ExpiresAt:         expiresAt,
		RevokedLicenseIDs: ids,
	}, nil
}

func decodeRevocationSignature(value string) ([]byte, error) {
	signature, err := base64.StdEncoding.Strict().DecodeString(value)
	if err != nil || base64.StdEncoding.EncodeToString(signature) != value ||
		len(signature) != ed25519.SignatureSize {
		return nil, ErrRevocationSignatureInvalid
	}
	return signature, nil
}

func revocationCanonicalError(err error) error {
	switch {
	case errors.Is(err, canonicaljson.ErrDuplicateField):
		return ErrDuplicateField
	case errors.Is(err, canonicaljson.ErrUnsupportedNumber):
		return ErrUnsupportedNumber
	default:
		return ErrInvalidRevocation
	}
}