package lafsession

import (
	"crypto/ed25519"
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
)

// Identity is a long-term Ed25519 key pair used for mutual authentication.
type Identity struct {
	Private ed25519.PrivateKey
	Public  ed25519.PublicKey
}

// GenerateIdentity creates a new random identity key pair.
func GenerateIdentity() (*Identity, error) {
	pub, priv, err := ed25519.GenerateKey(rand.Reader)
	if err != nil {
		return nil, err
	}
	return &Identity{Private: priv, Public: pub}, nil
}

// IdentityFromSeed derives an identity from a 32-byte seed (deterministic).
func IdentityFromSeed(seed []byte) (*Identity, error) {
	if len(seed) != ed25519.SeedSize {
		return nil, fmt.Errorf("lafsession: seed must be %d bytes", ed25519.SeedSize)
	}
	priv := ed25519.NewKeyFromSeed(seed)
	return &Identity{Private: priv, Public: priv.Public().(ed25519.PublicKey)}, nil
}

// PublicBytes returns the 32-byte public key.
func (id *Identity) PublicBytes() []byte {
	return append([]byte(nil), id.Public...)
}

// Fingerprint returns a SHA-256 hex fingerprint of the public key (full 32 bytes hex).
func (id *Identity) Fingerprint() string {
	sum := sha256.Sum256(id.Public)
	return hex.EncodeToString(sum[:])
}

// ShortFingerprint returns the first 20 bytes of SHA-256 as spaced hex groups (display).
func (id *Identity) ShortFingerprint() string {
	sum := sha256.Sum256(id.Public)
	h := hex.EncodeToString(sum[:20])
	out := make([]byte, 0, 24+5)
	for i := 0; i < len(h); i += 4 {
		if i > 0 {
			out = append(out, ' ')
		}
		end := i + 4
		if end > len(h) {
			end = len(h)
		}
		out = append(out, h[i:end]...)
	}
	return string(out)
}

// Sign signs a transcript with the identity private key.
func (id *Identity) Sign(msg []byte) []byte {
	return ed25519.Sign(id.Private, msg)
}

// Verify checks a signature under this identity public key.
func (id *Identity) Verify(msg, sig []byte) bool {
	return ed25519.Verify(id.Public, msg, sig)
}

// VerifyPeer checks a signature under an arbitrary peer public key.
func VerifyPeer(peerPub, msg, sig []byte) bool {
	if len(peerPub) != ed25519.PublicKeySize {
		return false
	}
	return ed25519.Verify(ed25519.PublicKey(peerPub), msg, sig)
}

// FingerprintOf returns SHA-256 hex of a raw public key.
func FingerprintOf(pub []byte) string {
	sum := sha256.Sum256(pub)
	return hex.EncodeToString(sum[:])
}