// Package acme wraps github.com/go-acme/lego with EdgeGuard's
// HTTP-01-via-shared-webroot setup.
//
// The webroot is /var/lib/edgeguard/acme — HAProxy already proxies
// /.well-known/acme-challenge/* to edgeguard-api which serves files
// from that directory (see internal/handlers/acme.go). Lego writes
// challenge tokens there; the existing handler answers from disk.
//
// Account state (the ACME-account private key + URL) lives in
// /var/lib/edgeguard/acme-account/account.key + account.json so a
// renewal doesn't need to re-register.
package acme

import (
	"crypto"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/x509"
	"encoding/json"
	"encoding/pem"
	"errors"
	"fmt"
	"os"
	"path/filepath"

	"github.com/go-acme/lego/v4/certificate"
	"github.com/go-acme/lego/v4/lego"
	"github.com/go-acme/lego/v4/providers/http/webroot"
	"github.com/go-acme/lego/v4/registration"
)

const (
	// DefaultWebroot matches what postinst created and HAProxy
	// proxies to.
	DefaultWebroot = "/var/lib/edgeguard/acme"

	// DefaultAccountDir holds account.key + account.json. Mode 0700
	// — the account key is a pseudo-credential.
	DefaultAccountDir = "/var/lib/edgeguard/acme-account"

	// caDirURL is the production Let's Encrypt directory. Tests
	// override via NewService's dirURL parameter.
	caDirURL = "https://acme-v02.api.letsencrypt.org/directory"
)

// Service issues + renews certs against Let's Encrypt. One Service
// per running edgeguard-api; lazily registers the account on first
// Issue() call.
type Service struct {
	WebrootPath string
	AccountDir  string
	DirURL      string
	Email       string

	// loaded lazily on first call
	user *acmeUser
}

// New returns a Service with sensible defaults. Email comes from
// setup.json's acme_email; the caller plumbs it through main.go.
func New(email string) *Service {
	return &Service{
		WebrootPath: DefaultWebroot,
		AccountDir:  DefaultAccountDir,
		DirURL:      caDirURL,
		Email:       email,
	}
}

// Issue runs an HTTP-01 ACME challenge for `domain` via the shared
// webroot, returning the leaf cert PEM, chain (issuer cert) PEM, and
// account-independent private key PEM.
func (s *Service) Issue(domain string) (certPEM, chainPEM, keyPEM string, err error) {
	if domain == "" {
		return "", "", "", errors.New("domain required")
	}
	if s.Email == "" {
		return "", "", "", errors.New("acme: email not configured (set via setup wizard)")
	}

	user, err := s.loadOrRegisterAccount()
	if err != nil {
		return "", "", "", fmt.Errorf("acme: account: %w", err)
	}

	cfg := lego.NewConfig(user)
	cfg.CADirURL = s.DirURL
	client, err := lego.NewClient(cfg)
	if err != nil {
		return "", "", "", fmt.Errorf("acme: client: %w", err)
	}

	provider, err := webroot.NewHTTPProvider(s.WebrootPath)
	if err != nil {
		return "", "", "", fmt.Errorf("acme: webroot provider: %w", err)
	}
	if err := client.Challenge.SetHTTP01Provider(provider); err != nil {
		return "", "", "", fmt.Errorf("acme: set http-01: %w", err)
	}

	// First-time account registration — idempotent (lego skips when
	// the account is already registered against the directory).
	if user.Registration == nil {
		reg, err := client.Registration.Register(registration.RegisterOptions{TermsOfServiceAgreed: true})
		if err != nil {
			return "", "", "", fmt.Errorf("acme: register: %w", err)
		}
		user.Registration = reg
		if err := s.saveAccount(user); err != nil {
			return "", "", "", fmt.Errorf("acme: save account: %w", err)
		}
	}

	req := certificate.ObtainRequest{
		Domains: []string{domain},
		Bundle:  true,
	}
	res, err := client.Certificate.Obtain(req)
	if err != nil {
		return "", "", "", fmt.Errorf("acme: obtain: %w", err)
	}

	// res.Certificate is the leaf+chain bundle (because Bundle=true).
	// The frontend wants leaf separated from chain so it can store
	// fields cleanly — split on the second BEGIN CERTIFICATE marker.
	leaf, chain := splitBundle(string(res.Certificate))
	return leaf, chain, string(res.PrivateKey), nil
}

func splitBundle(bundle string) (leaf, chain string) {
	// Find the second "-----BEGIN CERTIFICATE-----" marker. Everything
	// before it is the leaf, after (incl. marker) is the chain.
	const marker = "-----BEGIN CERTIFICATE-----"
	first := indexOfNth(bundle, marker, 1)
	second := indexOfNth(bundle, marker, 2)
	if first < 0 {
		return bundle, ""
	}
	if second < 0 {
		return bundle, ""
	}
	return bundle[:second], bundle[second:]
}

func indexOfNth(s, sub string, n int) int {
	idx := -1
	pos := 0
	for i := 0; i < n; i++ {
		j := indexAfter(s, sub, pos)
		if j < 0 {
			return -1
		}
		idx = j
		pos = j + len(sub)
	}
	return idx
}

func indexAfter(s, sub string, from int) int {
	if from >= len(s) {
		return -1
	}
	rel := -1
	for i := from; i+len(sub) <= len(s); i++ {
		if s[i:i+len(sub)] == sub {
			rel = i
			break
		}
	}
	return rel
}

// acmeUser implements lego's registration.User interface against
// our on-disk account state.
type acmeUser struct {
	Email        string
	Registration *registration.Resource
	key          crypto.PrivateKey
}

func (u *acmeUser) GetEmail() string                        { return u.Email }
func (u *acmeUser) GetRegistration() *registration.Resource { return u.Registration }
func (u *acmeUser) GetPrivateKey() crypto.PrivateKey        { return u.key }

func (s *Service) loadOrRegisterAccount() (*acmeUser, error) {
	if err := os.MkdirAll(s.AccountDir, 0o700); err != nil {
		return nil, err
	}
	keyPath := filepath.Join(s.AccountDir, "account.key")
	regPath := filepath.Join(s.AccountDir, "account.json")

	user := &acmeUser{Email: s.Email}

	if b, err := os.ReadFile(keyPath); err == nil {
		block, _ := pem.Decode(b)
		if block == nil {
			return nil, errors.New("acme: account.key has no PEM block")
		}
		k, err := x509.ParseECPrivateKey(block.Bytes)
		if err != nil {
			return nil, fmt.Errorf("acme: parse account.key: %w", err)
		}
		user.key = k
	} else {
		k, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
		if err != nil {
			return nil, err
		}
		der, err := x509.MarshalECPrivateKey(k)
		if err != nil {
			return nil, err
		}
		buf := pem.EncodeToMemory(&pem.Block{Type: "EC PRIVATE KEY", Bytes: der})
		if err := os.WriteFile(keyPath, buf, 0o600); err != nil {
			return nil, err
		}
		user.key = k
	}

	if b, err := os.ReadFile(regPath); err == nil {
		var reg registration.Resource
		if err := json.Unmarshal(b, &reg); err == nil {
			user.Registration = &reg
		}
	}
	return user, nil
}

func (s *Service) saveAccount(u *acmeUser) error {
	if u.Registration == nil {
		return nil
	}
	b, err := json.MarshalIndent(u.Registration, "", "  ")
	if err != nil {
		return err
	}
	return os.WriteFile(filepath.Join(s.AccountDir, "account.json"), b, 0o600)
}