finance-duck/internal/banking/enablebanking.go

package banking

// DTOs and authentication follow https://enablebanking.com/docs/api/reference/.
// In particular entry_reference is stable across sessions, transaction_id is NOT;
// GET /sessions returns UID strings, unlike POST /sessions' account objects.
import (
	"bytes"
	"context"
	"crypto"
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha256"
	"crypto/x509"
	"encoding/base64"
	"encoding/json"
	"encoding/pem"
	"errors"
	"fmt"
	"io"
	"net"
	"net/http"
	"net/url"
	"slices"
	"strconv"
	"strings"
	"time"

	"finance-duck/internal/domain"
	"finance-duck/internal/ratelimit"
)

// ErrReconnect identifies inactive bank consent, not application authentication
// failures or temporary transport/provider errors.
var ErrReconnect = errors.New("bank consent requires reconnection")

type Session struct {
	ID         string           `json:"session_id"`
	ValidUntil string           `json:"valid_until"`
	Accounts   []domain.Account `json:"accounts"`
}

// SessionStatus contains only the current consent expiry and external account
// membership. Full account metadata is captured once by Exchange.
type SessionStatus struct {
	ValidUntil string
	AccountIDs []string
}
type Balance struct {
	Amount        domain.Money `json:"amount"`
	Currency      string       `json:"currency"`
	Type          string       `json:"type"`
	ReferenceDate string       `json:"reference_date,omitempty"`
}
type Provider interface {
	Authorize(context.Context, string, string, string) (string, error)
	Exchange(context.Context, string) (Session, error)
	Status(context.Context, string) (SessionStatus, error)
	Balances(context.Context, string) ([]Balance, error)
	Transactions(ctx context.Context, account domain.Account, from, to string, longest bool) ([]domain.Facts, error)
	Institutions(ctx context.Context, country string) ([]Institution, error)
}
type EnableBanking struct {
	HTTPClient  *http.Client
	BaseURL     string
	appID       string
	key         *rsa.PrivateKey
	redirectURL string
	requests    ratelimit.Controller
	// Accessed only while requests is acquired. Keys exclude pagination/query data.
	backgroundQuotas map[string]time.Time
}

var _ Provider = (*EnableBanking)(nil)

// MaxPrivateKeyPEM bounds uploaded and environment-loaded private keys.
const MaxPrivateKeyPEM = 32 * 1024

func NewEnableBanking(appID string, keyPEM []byte, redirectURL string) (*EnableBanking, error) {
	if len(appID) == 0 || len(appID) > 256 {
		return nil, errors.New("invalid Enable Banking application ID")
	}
	for _, c := range appID {
		if c < 33 || c > 126 {
			return nil, errors.New("invalid Enable Banking application ID")
		}
	}
	redirect, err := url.Parse(redirectURL)
	if err != nil || redirect.Hostname() == "" || (redirect.Scheme != "https" && redirect.Scheme != "http") || redirect.User != nil || redirect.Opaque != "" || redirect.Path != "/api/banking/callback" || redirect.RawPath != "" || redirect.RawQuery != "" || redirect.ForceQuery || redirect.Fragment != "" || strings.Contains(redirectURL, "#") {
		return nil, errors.New("invalid Enable Banking redirect URL")
	}
	if len(keyPEM) > MaxPrivateKeyPEM {
		return nil, errors.New("Enable Banking private key exceeds size limit")
	}
	content := bytes.TrimSpace(keyPEM)
	block, rest := pem.Decode(content)
	if block == nil || !bytes.HasPrefix(content, []byte("-----BEGIN "+block.Type+"-----")) || len(bytes.TrimSpace(rest)) != 0 || len(block.Headers) != 0 {
		return nil, errors.New("Enable Banking key must be a single PEM private key")
	}
	var key *rsa.PrivateKey
	switch block.Type {
	case "RSA PRIVATE KEY":
		key, err = x509.ParsePKCS1PrivateKey(block.Bytes)
	case "PRIVATE KEY":
		var parsed any
		parsed, err = x509.ParsePKCS8PrivateKey(block.Bytes)
		if err == nil {
			var ok bool
			key, ok = parsed.(*rsa.PrivateKey)
			if !ok {
				err = fmt.Errorf("not RSA")
			}
		}
	default:
		err = fmt.Errorf("unsupported key type")
	}
	if err != nil || key == nil {
		return nil, fmt.Errorf("invalid Enable Banking RSA private key")
	}
	if key.N.BitLen() < 2048 {
		return nil, fmt.Errorf("Enable Banking RSA key must be at least 2048 bits")
	}
	if err = key.Validate(); err != nil {
		return nil, fmt.Errorf("invalid Enable Banking RSA private key")
	}
	return &EnableBanking{HTTPClient: &http.Client{Timeout: 30 * time.Second}, BaseURL: "https://api.enablebanking.com", appID: appID, key: key, redirectURL: redirectURL, requests: ratelimit.Controller{MinimumInterval: time.Second, InitialBackoff: 30 * time.Second}}, nil
}
func (p *EnableBanking) jwt() (string, error) {
	if p.key == nil || p.appID == "" {
		return "", fmt.Errorf("Enable Banking is not configured")
	}
	now := time.Now().Unix()
	header, _ := json.Marshal(map[string]any{"typ": "JWT", "alg": "RS256", "kid": p.appID})
	claims, _ := json.Marshal(map[string]any{"iss": "enablebanking.com", "aud": "api.enablebanking.com", "iat": now, "exp": now + 3600})
	unsigned := base64.RawURLEncoding.EncodeToString(header) + "." + base64.RawURLEncoding.EncodeToString(claims)
	hash := sha256.Sum256([]byte(unsigned))
	signature, err := rsa.SignPKCS1v15(rand.Reader, p.key, crypto.SHA256, hash[:])
	if err != nil {
		return "", fmt.Errorf("sign Enable Banking token")
	}
	return unsigned + "." + base64.RawURLEncoding.EncodeToString(signature), nil
}

// PSU contains only the documented, nonsecret browser metadata for a person
// actively requesting account data. It must never be stored on a shared client.
type PSU struct {
	IPAddress      string
	UserAgent      string
	Accept         string
	AcceptCharset  string
	AcceptEncoding string
	AcceptLanguage string
}

type psuContextKey struct{}

// WithPSU marks this request context as user initiated. Call only at a manual
// HTTP boundary, never for scheduled retrieval. Unknown metadata stays empty.
func WithPSU(ctx context.Context, psu PSU) context.Context {
	return context.WithValue(ctx, psuContextKey{}, psu)
}

func (psu PSU) setHeaders(header http.Header) {
	if ip := net.ParseIP(psu.IPAddress); ip != nil {
		header.Set("Psu-Ip-Address", ip.String())
	}
	for _, field := range []struct{ name, value string }{
		{"Psu-User-Agent", psu.UserAgent},
		{"Psu-Accept", psu.Accept},
		{"Psu-Accept-Charset", psu.AcceptCharset},
		{"Psu-Accept-Encoding", psu.AcceptEncoding},
		{"Psu-Accept-Language", psu.AcceptLanguage},
	} {
		if field.value != "" {
			header.Set(field.name, field.value)
		}
	}
}

// accountDataPath admits only the internally generated account GET endpoints.
// The escaped account segment is retained; dates and continuation keys are not.
func accountDataPath(method, path string) string {
	if method != http.MethodGet {
		return ""
	}
	path, _, _ = strings.Cut(path, "?")
	parts := strings.Split(path, "/")
	if len(parts) != 4 || parts[0] != "" || parts[1] != "accounts" || (parts[3] != "balances" && parts[3] != "transactions") {
		return ""
	}
	account, err := url.PathUnescape(parts[2])
	if err != nil || account == "" || account == "." || account == ".." || strings.ContainsAny(account, "/\\") || url.PathEscape(account) != parts[2] {
		return ""
	}
	return path
}

// BackgroundQuotaError identifies the bank's background-only account quota.
// It preserves RateLimitError identity and exposes no provider response text.
type BackgroundQuotaError struct {
	*ratelimit.RateLimitError
}

func (e *BackgroundQuotaError) Error() string {
	return "background bank retrieval quota (normally six hours): " + e.RateLimitError.Error()
}

func (e *BackgroundQuotaError) Unwrap() error {
	return e.RateLimitError
}

// APIError reports a failed Enable Banking call. Its message is built only
// from the HTTP status and, when the response envelope's error code exactly
// matches the documented enumeration, that code with a locally written hint.
// Provider response text is never included.
type APIError struct {
	Status int
	Code   string // documented Enable Banking error code, or empty
}

func (e *APIError) Error() string {
	if hint, known := apiErrorHints[e.Code]; known {
		return fmt.Sprintf("Enable Banking returned HTTP %d (%s: %s)", e.Status, e.Code, hint)
	}
	return fmt.Sprintf("Enable Banking returned HTTP %d", e.Status)
}

// Unwrap exposes inactive-consent codes as ErrReconnect so callers offer
// reconnection instead of a dead-end provider failure.
func (e *APIError) Unwrap() error {
	switch e.Code {
	case "CLOSED_SESSION", "EXPIRED_SESSION", "REVOKED_SESSION", "SESSION_DOES_NOT_EXIST":
		return ErrReconnect
	}
	return nil
}

// apiErrorHints holds locally written descriptions for the documented error
// codes relevant to account information. Only exact matches are ever exposed.
var apiErrorHints = map[string]string{
	"ACCESS_DENIED":                "access to this resource is denied for the application",
	"ACCOUNT_DOES_NOT_EXIST":       "no account matches the stored identifier",
	"ASPSP_ACCOUNT_NOT_ACCESSIBLE": "the bank did not grant access to the requested account",
	"ASPSP_ERROR":                  "the bank reported an error",
	"ASPSP_PSU_ACTION_REQUIRED":    "the bank requires action in your banking app or online banking",
	"ASPSP_TIMEOUT":                "the bank did not respond in time",
	"CLOSED_SESSION":               "the bank session is closed",
	"DATE_FROM_IN_FUTURE":          "the requested start date is in the future",
	"EXPIRED_SESSION":              "the bank session has expired",
	"NO_ACCOUNTS_ADDED":            "no allowed accounts are added to the application",
	"PSU_HEADER_INVALID":           "the forwarded browser metadata was rejected",
	"PSU_HEADER_NOT_PROVIDED":      "this bank requires a user-initiated request",
	"REVOKED_SESSION":              "the bank session was revoked",
	"SESSION_DOES_NOT_EXIST":       "the bank session no longer exists",
	"UNAUTHORIZED_ACCESS":          "the application is not authorized for this request",
	"UNAUTHORIZED_IP":              "this network address is not authorized for the request",
	"WRONG_CONTINUATION_KEY":       "the pagination key was rejected",
	"WRONG_DATE_INTERVAL":          "the start date must not be after the end date",
	"WRONG_REQUEST_PARAMETERS":     "the request parameters were rejected",
	"WRONG_SESSION_STATUS":         "the bank session is in the wrong state for this request",
	"WRONG_TRANSACTIONS_PERIOD":    "the bank does not provide transactions for the requested period; banks commonly limit history to about 90 days after the initial connection",
}

// apiError classifies a non-success response by its documented error code
// without retaining or exposing any other provider response content.
func apiError(response *http.Response) *APIError {
	failure := &APIError{Status: response.StatusCode}
	const envelopeLimit = 16 << 10
	body, err := io.ReadAll(io.LimitReader(response.Body, envelopeLimit+1))
	var envelope struct {
		Error string `json:"error"`
	}
	if err == nil && len(body) <= envelopeLimit && json.Unmarshal(body, &envelope) == nil {
		if _, known := apiErrorHints[envelope.Error]; known {
			failure.Code = envelope.Error
		}
	}
	return failure
}

// backgroundRetryAt follows the bank's six-hour guidance, never shortening a
// longer provider deadline. Zero retains the limiter's unbounded-delay meaning.
func backgroundRetryAt(header string, now time.Time) time.Time {
	next := now.Add(6 * time.Hour)
	header = strings.TrimSpace(header)
	if header == "" {
		return next
	}
	if strings.Trim(header, "0123456789") == "" {
		seconds, err := strconv.ParseUint(header, 10, 64)
		if err != nil || seconds > uint64((1<<63-1)/int64(time.Second)) {
			return time.Time{}
		}
		if delay := time.Duration(seconds) * time.Second; delay > 6*time.Hour {
			return now.Add(delay)
		}
	} else if date, err := http.ParseTime(header); err == nil && date.After(next) {
		return date
	}
	return next
}
func (p *EnableBanking) request(ctx context.Context, method, path string, input, output any) error {
	if err := p.requests.Acquire(ctx); err != nil {
		return fmt.Errorf("Enable Banking: %w", err)
	}
	defer p.requests.Release()
	scope := accountDataPath(method, path)
	psu, foreground := ctx.Value(psuContextKey{}).(PSU)
	now := time.Now()
	for key, next := range p.backgroundQuotas {
		if !next.IsZero() && !next.After(now) {
			delete(p.backgroundQuotas, key)
		}
	}
	if scope != "" && !foreground {
		if next, limited := p.backgroundQuotas[scope]; limited {
			return fmt.Errorf("Enable Banking: %w", &BackgroundQuotaError{ratelimit.NewError(next)})
		}
	}
	token, err := p.jwt()
	if err != nil {
		return err
	}
	var body io.Reader
	if input != nil {
		b, e := json.Marshal(input)
		if e != nil {
			return fmt.Errorf("encode Enable Banking request")
		}
		body = bytes.NewReader(b)
	}
	base, err := url.Parse(p.BaseURL)
	if err != nil || base.Host == "" || base.User != nil || (base.Scheme != "http" && base.Scheme != "https") {
		return fmt.Errorf("invalid Enable Banking base URL")
	}
	req, err := http.NewRequestWithContext(ctx, method, strings.TrimRight(p.BaseURL, "/")+path, body)
	if err != nil {
		return fmt.Errorf("create Enable Banking request")
	}
	req.Header.Set("Authorization", "Bearer "+token)
	req.Header.Set("Accept", "application/json")
	if scope != "" && foreground {
		psu.setHeaders(req.Header)
	}
	if input != nil {
		req.Header.Set("Content-Type", "application/json")
	}
	client := http.Client{Timeout: 30 * time.Second}
	if p.HTTPClient != nil {
		client = *p.HTTPClient
	}
	// Cap each attempt, including response reads, without timing out retry waits.
	if client.Timeout <= 0 || client.Timeout > 30*time.Second {
		client.Timeout = 30 * time.Second
	}
	// Never forward signed credentials or financial requests through redirects.
	client.CheckRedirect = func(*http.Request, []*http.Request) error { return http.ErrUseLastResponse }
	response, err := p.requests.Do(ctx, func(ctx context.Context) (*http.Response, error) {
		response, err := client.Do(req.Clone(ctx))
		if err != nil {
			if errors.Is(err, context.Canceled) {
				return nil, context.Canceled
			}
			if errors.Is(err, context.DeadlineExceeded) {
				return nil, fmt.Errorf("request timed out: %w", context.DeadlineExceeded)
			}
			return nil, errors.New("connection failed")
		}
		if response.StatusCode == http.StatusTooManyRequests && scope != "" && !foreground {
			// Inspect only a small structured error envelope, never exposing its
			// detail/message or retaining response bytes. Other 429s remain the
			// common controller's responsibility (including body closure).
			const errorLimit = 16 << 10
			body, readErr := io.ReadAll(io.LimitReader(response.Body, errorLimit+1))
			var envelope struct {
				Error string `json:"error"`
			}
			if readErr == nil && len(body) <= errorLimit && json.Unmarshal(body, &envelope) == nil && envelope.Error == "ASPSP_RATE_LIMIT_EXCEEDED" {
				next := backgroundRetryAt(response.Header.Get("Retry-After"), time.Now())
				if p.backgroundQuotas == nil {
					p.backgroundQuotas = make(map[string]time.Time)
				}
				p.backgroundQuotas[scope] = next
				response.Body.Close()
				return nil, &BackgroundQuotaError{ratelimit.NewError(next)}
			}
		}
		return response, nil
	}, method == http.MethodGet)
	if err != nil {
		return fmt.Errorf("Enable Banking: %w", err)
	}
	defer response.Body.Close()
	if response.StatusCode < 200 || response.StatusCode >= 300 {
		return apiError(response)
	}
	const limit = 16 << 20
	b, err := io.ReadAll(io.LimitReader(response.Body, limit+1))
	if err != nil {
		if errors.Is(err, context.Canceled) {
			return context.Canceled
		}
		if errors.Is(err, context.DeadlineExceeded) {
			return fmt.Errorf("Enable Banking response timed out: %w", context.DeadlineExceeded)
		}
		return fmt.Errorf("read Enable Banking response")
	}
	if len(b) > limit {
		return fmt.Errorf("Enable Banking response exceeded size limit")
	}
	if err = json.Unmarshal(b, output); err != nil {
		return fmt.Errorf("invalid Enable Banking response")
	}
	return nil
}

type accessDTO struct {
	ValidUntil string `json:"valid_until"`
}
type institutionDTO struct {
	Name    string `json:"name"`
	Country string `json:"country"`
}
type accountIdentificationDTO struct {
	IBAN string `json:"iban"`
}
type accountDTO struct {
	UID                string                   `json:"uid"`
	IdentificationHash string                   `json:"identification_hash"`
	AccountID          accountIdentificationDTO `json:"account_id"`
	Name               string                   `json:"name"`
	Details            string                   `json:"details"`
	Currency           string                   `json:"currency"`
}

func (a accountDTO) account(institution string) (domain.Account, error) {
	if !validCurrency(a.Currency) {
		return domain.Account{}, fmt.Errorf("Enable Banking account has invalid currency")
	}
	stable := a.IdentificationHash
	if stable == "" {
		stable = normalizeIBAN(a.AccountID.IBAN)
	}
	if stable == "" {
		return domain.Account{}, fmt.Errorf("Enable Banking account lacks stable identification")
	}
	name := a.Details
	if name == "" {
		name = a.Name
	}
	if name == "" {
		name = institution
	}
	return domain.Account{ID: "acct_" + digest("enablebanking", stable), DisplayName: name, Institution: institution, Currency: a.Currency, ExternalAccountID: a.UID, IBAN: normalizeIBAN(a.AccountID.IBAN), Active: a.UID != ""}, nil
}

// Institution describes a bank available for personal account information.
// Logo is retained only when it is an https Enable Banking URL, matching the
// Content-Security-Policy image allowlist under which the UI displays it.
type Institution struct {
	Name    string `json:"name"`
	Country string `json:"country"`
	Logo    string `json:"logo,omitempty"`
}

// safeLogoURL admits only https Enable Banking brand URLs. Any other
// provider-supplied location renders no logo rather than a third-party fetch.
func safeLogoURL(logo string) string {
	u, err := url.Parse(logo)
	if err != nil || u.Scheme != "https" || u.User != nil {
		return ""
	}
	host := strings.ToLower(u.Hostname())
	if host != "enablebanking.com" && !strings.HasSuffix(host, ".enablebanking.com") {
		return ""
	}
	return logo
}

type aspspDTO struct {
	institutionDTO
	Logo                   string `json:"logo"`
	MaximumConsentValidity int64  `json:"maximum_consent_validity"`
}

func (p *EnableBanking) aspsps(ctx context.Context, country string) ([]aspspDTO, error) {
	var list struct {
		ASPSPs []aspspDTO `json:"aspsps"`
	}
	query := url.Values{"country": {country}, "psu_type": {"personal"}, "service": {"AIS"}}
	if err := p.request(ctx, http.MethodGet, "/aspsps?"+query.Encode(), nil, &list); err != nil {
		return nil, err
	}
	return list.ASPSPs, nil
}

// Institutions lists the banks connectable for personal account information
// in a country, excluding entries the Authorize flow would reject anyway.
func (p *EnableBanking) Institutions(ctx context.Context, country string) ([]Institution, error) {
	country = strings.ToUpper(strings.TrimSpace(country))
	if len(country) != 2 {
		return nil, fmt.Errorf("country must be a two-letter code")
	}
	aspsps, err := p.aspsps(ctx, country)
	if err != nil {
		return nil, err
	}
	result := make([]Institution, 0, len(aspsps))
	for _, a := range aspsps {
		if a.Name == "" || a.Country != country || a.MaximumConsentValidity <= 0 {
			continue
		}
		result = append(result, Institution{Name: a.Name, Country: a.Country, Logo: safeLogoURL(a.Logo)})
	}
	slices.SortFunc(result, func(a, b Institution) int { return strings.Compare(a.Name, b.Name) })
	return result, nil
}

func (p *EnableBanking) Authorize(ctx context.Context, institution, country, state string) (string, error) {
	country = strings.ToUpper(strings.TrimSpace(country))
	institution = strings.TrimSpace(institution)
	if institution == "" || len(country) != 2 || state == "" {
		return "", fmt.Errorf("institution, country and authorization state are required")
	}
	aspsps, err := p.aspsps(ctx, country)
	if err != nil {
		return "", err
	}
	var validity int64
	for _, a := range aspsps {
		if a.Name == institution && a.Country == country {
			validity = a.MaximumConsentValidity
			break
		}
	}
	if validity <= 0 {
		return "", fmt.Errorf("institution is unavailable for personal account information or has no valid consent duration")
	}
	// Avoid overflow or unexpectedly long access while honoring each bank's limit.
	if validity > 180*24*3600 {
		validity = 180 * 24 * 3600
	}
	request := struct {
		Access struct {
			ValidUntil   string `json:"valid_until"`
			Balances     bool   `json:"balances"`
			Transactions bool   `json:"transactions"`
		} `json:"access"`
		ASPSP       institutionDTO `json:"aspsp"`
		State       string         `json:"state"`
		RedirectURL string         `json:"redirect_url"`
		PSUType     string         `json:"psu_type"`
	}{ASPSP: institutionDTO{institution, country}, State: state, RedirectURL: p.redirectURL, PSUType: "personal"}
	request.Access.ValidUntil = time.Now().UTC().Add(time.Duration(validity) * time.Second).Format(time.RFC3339)
	request.Access.Balances = true
	request.Access.Transactions = true
	var response struct {
		URL string `json:"url"`
	}
	if err := p.request(ctx, http.MethodPost, "/auth", request, &response); err != nil {
		return "", err
	}
	parsed, err := url.Parse(response.URL)
	if err != nil || parsed.Host == "" || parsed.Scheme != "https" || parsed.User != nil {
		return "", fmt.Errorf("Enable Banking returned invalid authorization URL")
	}
	return response.URL, nil
}
func (p *EnableBanking) Exchange(ctx context.Context, code string) (Session, error) {
	if code == "" {
		return Session{}, fmt.Errorf("authorization code is required")
	}
	var response struct {
		ID       string         `json:"session_id"`
		Accounts []accountDTO   `json:"accounts"`
		Access   accessDTO      `json:"access"`
		ASPSP    institutionDTO `json:"aspsp"`
	}
	if err := p.request(ctx, http.MethodPost, "/sessions", map[string]string{"code": code}, &response); err != nil {
		return Session{}, err
	}
	if response.ID == "" {
		return Session{}, fmt.Errorf("Enable Banking returned no session ID")
	}
	if _, err := time.Parse(time.RFC3339, response.Access.ValidUntil); err != nil {
		return Session{}, fmt.Errorf("Enable Banking returned invalid session expiry")
	}
	result := Session{ID: response.ID, ValidUntil: response.Access.ValidUntil, Accounts: []domain.Account{}}
	for _, a := range response.Accounts {
		account, err := a.account(response.ASPSP.Name)
		if err != nil {
			return Session{}, err
		}
		result.Accounts = append(result.Accounts, account)
	}
	return result, nil
}
func (p *EnableBanking) Status(ctx context.Context, sessionID string) (SessionStatus, error) {
	if sessionID == "" {
		return SessionStatus{}, fmt.Errorf("session ID is required")
	}
	var response struct {
		Status   string    `json:"status"`
		Accounts []string  `json:"accounts"`
		Access   accessDTO `json:"access"`
	}
	if err := p.request(ctx, http.MethodGet, "/sessions/"+url.PathEscape(sessionID), nil, &response); err != nil {
		return SessionStatus{}, err
	}
	if response.Status != "AUTHORIZED" {
		return SessionStatus{}, fmt.Errorf("Enable Banking session is not authorized: %w", ErrReconnect)
	}
	expires, err := time.Parse(time.RFC3339, response.Access.ValidUntil)
	if err != nil {
		return SessionStatus{}, fmt.Errorf("Enable Banking returned invalid session expiry")
	}
	if !expires.After(time.Now()) {
		return SessionStatus{}, fmt.Errorf("Enable Banking session expired: %w", ErrReconnect)
	}
	for _, id := range response.Accounts {
		if id == "" {
			return SessionStatus{}, fmt.Errorf("Enable Banking returned empty account identifier")
		}
	}
	return SessionStatus{ValidUntil: response.Access.ValidUntil, AccountIDs: response.Accounts}, nil
}

type amountDTO struct {
	Amount   string `json:"amount"`
	Currency string `json:"currency"`
}

func (p *EnableBanking) Balances(ctx context.Context, externalAccountID string) ([]Balance, error) {
	if externalAccountID == "" {
		return nil, fmt.Errorf("account is not connected to Enable Banking")
	}
	var response struct {
		Balances []struct {
			Amount        amountDTO `json:"balance_amount"`
			Type          string    `json:"balance_type"`
			ReferenceDate string    `json:"reference_date"`
		} `json:"balances"`
	}
	if err := p.request(ctx, http.MethodGet, "/accounts/"+url.PathEscape(externalAccountID)+"/balances", nil, &response); err != nil {
		return nil, err
	}
	result := make([]Balance, 0, len(response.Balances))
	for _, b := range response.Balances {
		amount, err := domain.ParseMoney(b.Amount.Amount)
		if err != nil || !validCurrency(b.Amount.Currency) {
			return nil, fmt.Errorf("Enable Banking returned invalid balance amount")
		}
		result = append(result, Balance{Amount: amount, Currency: b.Amount.Currency, Type: b.Type, ReferenceDate: b.ReferenceDate})
	}
	return result, nil
}

type transactionDTO struct {
	EntryReference  string    `json:"entry_reference"`
	Amount          amountDTO `json:"transaction_amount"`
	Indicator       string    `json:"credit_debit_indicator"`
	Status          string    `json:"status"`
	BookingDate     string    `json:"booking_date"`
	ValueDate       string    `json:"value_date"`
	TransactionDate string    `json:"transaction_date"`
	Remittance      []string  `json:"remittance_information"`
	ReferenceNumber string    `json:"reference_number"`
	Creditor        struct {
		Name string `json:"name"`
	} `json:"creditor"`
	Debtor struct {
		Name string `json:"name"`
	} `json:"debtor"`
	CreditorAccount accountIdentificationDTO `json:"creditor_account"`
	DebtorAccount   accountIdentificationDTO `json:"debtor_account"`
}

// Transactions retrieves booked transactions in the requested window. With
// longest, the documented "longest" fetching strategy asks the provider for
// the maximum period the bank permits instead of rejecting an out-of-range
// start date; rows outside the requested window are still filtered out here.
func (p *EnableBanking) Transactions(ctx context.Context, account domain.Account, from, to string, longest bool) ([]domain.Facts, error) {
	if account.ID == "" || account.ExternalAccountID == "" {
		return nil, fmt.Errorf("account is not connected to Enable Banking")
	}
	for _, date := range []string{from, to} {
		if date != "" {
			if _, err := time.Parse("2006-01-02", date); err != nil {
				return nil, fmt.Errorf("invalid transaction date range")
			}
		}
	}
	if from != "" && to != "" && from > to {
		return nil, fmt.Errorf("invalid transaction date range")
	}
	query := url.Values{"transaction_status": {"BOOK"}}
	if from != "" {
		query.Set("date_from", from)
	}
	if to != "" {
		query.Set("date_to", to)
	}
	if longest {
		query.Set("strategy", "longest")
	}
	result := make([]domain.Facts, 0)
	seen := map[string]bool{}
	for range 1000 {
		var response struct {
			Transactions    []transactionDTO `json:"transactions"`
			ContinuationKey string           `json:"continuation_key"`
		}
		if err := p.request(ctx, http.MethodGet, "/accounts/"+url.PathEscape(account.ExternalAccountID)+"/transactions?"+query.Encode(), nil, &response); err != nil {
			return nil, err
		}
		for _, t := range response.Transactions {
			if t.Status != "BOOK" {
				continue
			}
			amount, err := domain.ParseMoney(t.Amount.Amount)
			if err != nil || strings.HasPrefix(amount.String(), "-") || !validCurrency(t.Amount.Currency) {
				return nil, fmt.Errorf("Enable Banking returned invalid transaction amount")
			}
			party, iban := t.Debtor.Name, t.DebtorAccount.IBAN
			switch t.Indicator {
			case "DBIT":
				amount, err = domain.ParseMoney("-" + amount.String())
				if err != nil {
					return nil, fmt.Errorf("invalid debit amount")
				}
				party, iban = t.Creditor.Name, t.CreditorAccount.IBAN
			case "CRDT":
			default:
				return nil, fmt.Errorf("Enable Banking returned invalid credit/debit indicator")
			}
			// Booked records without a booking date cannot be placed truthfully in the journal.
			if _, err := time.Parse("2006-01-02", t.BookingDate); err != nil {
				return nil, fmt.Errorf("Enable Banking booked transaction has no valid booking date")
			}
			if (from != "" && t.BookingDate < from) || (to != "" && t.BookingDate > to) {
				continue
			}
			if t.ValueDate != "" {
				if _, err := time.Parse("2006-01-02", t.ValueDate); err != nil {
					return nil, fmt.Errorf("Enable Banking returned invalid value date")
				}
			}
			description := strings.Join(t.Remittance, "\n")
			if description == "" {
				description = t.ReferenceNumber
			}
			result = append(result, domain.Facts{Source: "enablebanking", AccountID: account.ID, BookingDate: t.BookingDate, ValueDate: t.ValueDate, Amount: amount, Currency: t.Amount.Currency, RawDescription: description, ExternalID: t.EntryReference, Counterparty: party, CounterpartyIBAN: normalizeIBAN(iban)})
		}
		if response.ContinuationKey == "" {
			return result, nil
		}
		if seen[response.ContinuationKey] {
			return nil, fmt.Errorf("Enable Banking repeated a pagination key")
		}
		seen[response.ContinuationKey] = true
		query.Set("continuation_key", response.ContinuationKey)
	}
	return nil, fmt.Errorf("Enable Banking transaction pagination exceeded limit")
}