// Package fingerprint provides JA4/JA3 fingerprint generation for TLS ClientHello
package fingerprint

import (
	"fmt"

	"ja4sentinel/api"

	tlsfingerprint "github.com/psanford/tlsfingerprint"
)

// EngineImpl implements the api.Engine interface for fingerprint generation
type EngineImpl struct{}

// NewEngine creates a new fingerprint engine
func NewEngine() *EngineImpl {
	return &EngineImpl{}
}

// FromClientHello generates JA4 (and optionally JA3) fingerprints from a TLS ClientHello
// Note: JA4Hash is populated for internal use but should NOT be serialized to LogRecord
// as the JA4 format already includes its own hash portions (per architecture.yml)
func (e *EngineImpl) FromClientHello(ch api.TLSClientHello) (*api.Fingerprints, error) {
	if len(ch.Payload) == 0 {
		return nil, fmt.Errorf("empty ClientHello payload")
	}

	// Parse the ClientHello using tlsfingerprint
	fp, err := tlsfingerprint.ParseClientHello(ch.Payload)
	if err != nil {
		return nil, fmt.Errorf("failed to parse ClientHello: %w", err)
	}

	// Generate JA4 fingerprint
	// Note: JA4 string format already includes the hash portion
	// e.g., "t13d1516h2_8daaf6152771_02cb136f2775" where the last part is the SHA256 hash
	ja4 := fp.JA4String()

	// Generate JA3 fingerprint and its MD5 hash
	ja3 := fp.JA3String()
	ja3Hash := fp.JA3Hash()

	// Extract JA4 hash portion (last segment after underscore)
	// JA4 format: <tls_ver><ciphers><extensions>_<sni_hash>_<cipher_extension_hash>
	// This is kept for internal use but NOT serialized to LogRecord
	ja4Hash := extractJA4Hash(ja4)

	return &api.Fingerprints{
		JA4:     ja4,
		JA4Hash: ja4Hash, // Internal use only - not serialized to LogRecord
		JA3:     ja3,
		JA3Hash: ja3Hash,
	}, nil
}

// extractJA4Hash extracts the hash portion from a JA4 string
// JA4 format: <base>_<sni_hash>_<cipher_hash> -> returns "<sni_hash>_<cipher_hash>"
func extractJA4Hash(ja4 string) string {
	// JA4 string format: t13d1516h2_8daaf6152771_02cb136f2775
	// We extract everything after the first underscore as the "hash" portion
	for i, c := range ja4 {
		if c == '_' {
			return ja4[i+1:]
		}
	}
	return ""
}