// Package pqcrypto implements SEC-013 (Homomorphic Encryption research)
// and SEC-014 (Post-Quantum Signatures).
//
// SEC-013: Provides an interface for future lattice-based HE integration
// (CKKS/BFV schemes) to enable correlation on encrypted events.
//
// SEC-014: Implements CRYSTALS-Dilithium-like post-quantum signatures
// using a hybrid classical+PQ approach for Decision Logger chain.
//
// Current state: Research stubs with interface definitions.
// Production: requires official NIST PQC library bindings.
package pqcrypto

import (
	"crypto/ed25519"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"log/slog"
	"sync"
	"time"
)

// --- SEC-014: Post-Quantum Signatures ---

// SignatureScheme defines the signature algorithm.
type SignatureScheme string

const (
	SchemeClassical SignatureScheme = "ed25519"
	SchemeHybrid    SignatureScheme = "hybrid-ed25519-dilithium"
	SchemeDilithium SignatureScheme = "dilithium3" // CRYSTALS-Dilithium Level 3
)

// HybridSignature combines classical Ed25519 + post-quantum signature.
type HybridSignature struct {
	ClassicalSig string          `json:"classical_sig"` // Ed25519
	PQSig        string          `json:"pq_sig"`        // Dilithium (simulated)
	Scheme       SignatureScheme `json:"scheme"`
	Hash         string          `json:"hash"`
	Timestamp    time.Time       `json:"timestamp"`
}

// HybridSigner provides quantum-resistant signing with classical fallback.
type HybridSigner struct {
	mu          sync.RWMutex
	scheme      SignatureScheme
	classicalPub  ed25519.PublicKey
	classicalPriv ed25519.PrivateKey
	logger      *slog.Logger
	stats       SignerStats
}

// SignerStats tracks signing metrics.
type SignerStats struct {
	mu           sync.Mutex
	TotalSigns   int64          `json:"total_signs"`
	TotalVerifies int64         `json:"total_verifies"`
	Scheme       SignatureScheme `json:"scheme"`
	StartedAt    time.Time      `json:"started_at"`
}

// NewHybridSigner creates a new post-quantum hybrid signer.
func NewHybridSigner(scheme SignatureScheme) (*HybridSigner, error) {
	pub, priv, err := ed25519.GenerateKey(nil)
	if err != nil {
		return nil, fmt.Errorf("pqcrypto: generate ed25519 key: %w", err)
	}

	signer := &HybridSigner{
		scheme:        scheme,
		classicalPub:  pub,
		classicalPriv: priv,
		logger:        slog.Default().With("component", "sec-014-pqcrypto"),
		stats: SignerStats{
			Scheme:    scheme,
			StartedAt: time.Now(),
		},
	}

	signer.logger.Info("hybrid signer initialized",
		"scheme", scheme,
		"classical", "ed25519",
	)

	return signer, nil
}

// Sign creates a hybrid (classical + PQ) signature.
func (hs *HybridSigner) Sign(data []byte) (*HybridSignature, error) {
	hs.stats.mu.Lock()
	hs.stats.TotalSigns++
	hs.stats.mu.Unlock()

	hash := sha256.Sum256(data)
	hashHex := hex.EncodeToString(hash[:])

	// Classical Ed25519 signature.
	classicalSig := ed25519.Sign(hs.classicalPriv, hash[:])

	// Post-quantum signature (simulated — real impl needs CRYSTALS-Dilithium).
	pqSig := simulateDilithiumSign(hash[:])

	return &HybridSignature{
		ClassicalSig: hex.EncodeToString(classicalSig),
		PQSig:        pqSig,
		Scheme:       hs.scheme,
		Hash:         hashHex,
		Timestamp:    time.Now(),
	}, nil
}

// Verify checks both classical and PQ signatures.
func (hs *HybridSigner) Verify(data []byte, sig *HybridSignature) bool {
	hs.stats.mu.Lock()
	hs.stats.TotalVerifies++
	hs.stats.mu.Unlock()

	hash := sha256.Sum256(data)

	// Verify classical signature.
	classicalSigBytes, err := hex.DecodeString(sig.ClassicalSig)
	if err != nil {
		return false
	}
	if !ed25519.Verify(hs.classicalPub, hash[:], classicalSigBytes) {
		return false
	}

	// Verify PQ signature (simulated).
	if !simulateDilithiumVerify(hash[:], sig.PQSig) {
		return false
	}

	return true
}

// PublicKeyHex returns the classical public key.
func (hs *HybridSigner) PublicKeyHex() string {
	return hex.EncodeToString(hs.classicalPub)
}

// Stats returns signer metrics.
func (hs *HybridSigner) Stats() SignerStats {
	hs.stats.mu.Lock()
	defer hs.stats.mu.Unlock()
	return SignerStats{
		TotalSigns:    hs.stats.TotalSigns,
		TotalVerifies: hs.stats.TotalVerifies,
		Scheme:        hs.stats.Scheme,
		StartedAt:     hs.stats.StartedAt,
	}
}

// --- SEC-013: Homomorphic Encryption (Research Interface) ---

// HEScheme defines the homomorphic encryption scheme.
type HEScheme string

const (
	HE_CKKS HEScheme = "CKKS" // Approximate arithmetic (ML-friendly)
	HE_BFV  HEScheme = "BFV"  // Exact integer arithmetic
)

// EncryptedEvent represents a homomorphically encrypted SOC event.
type EncryptedEvent struct {
	CiphertextID string   `json:"ciphertext_id"`
	Scheme       HEScheme `json:"scheme"`
	FieldCount   int      `json:"field_count"`
	Created      time.Time `json:"created"`
}

// HEEngine defines the interface for homomorphic encryption operations.
// This is a research interface — real implementation requires a lattice-based
// HE library (e.g., Microsoft SEAL, OpenFHE, or Lattigo for Go).
type HEEngine interface {
	// Encrypt encrypts event fields for correlation without decryption.
	Encrypt(fields map[string]float64) (*EncryptedEvent, error)

	// CorrelateEncrypted runs correlation rules on encrypted events.
	CorrelateEncrypted(events []*EncryptedEvent) (float64, error)

	// Decrypt recovers plaintext (requires private key).
	Decrypt(event *EncryptedEvent) (map[string]float64, error)
}

// --- Simulated PQ functions ---

func simulateDilithiumSign(hash []byte) string {
	// Simulated Dilithium signature: SHA-256 of hash with prefix.
	// In production: use circl or pqcrypto-go for real Dilithium.
	prefixed := append([]byte("DILITHIUM3-SIM:"), hash...)
	sig := sha256.Sum256(prefixed)
	return hex.EncodeToString(sig[:])
}

func simulateDilithiumVerify(hash []byte, sigHex string) bool {
	expected := simulateDilithiumSign(hash)
	return expected == sigHex
}