package main

import (
	"bytes"
	"cmp"
	"context"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"sync"
	"time"
)

const (
	// authIssuer is the OpenAI login service.
	authIssuer = "https://auth.openai.com"

	// codexClientID is the OAuth client ID of the Codex CLI. The ChatGPT
	// backend takes the tokens of this client only.
	codexClientID = "app_EMoamEEZ73f0CkXaXp7hrann"

	// authClaim is the JWT claim that holds the ChatGPT account ID.
	authClaim = "https://api.openai.com/auth"

	// refreshMargin is how long before its expiry an access token is
	// refreshed.
	refreshMargin = 60 * time.Second

	// loginTimeout is how long a device code login waits for the user.
	loginTimeout = 15 * time.Minute

	// tokenFileName is the name of the token file in the data directory.
	tokenFileName = "auth.json"

	// defaultPollInterval is the poll interval of a device code login when
	// the login service gives none.
	defaultPollInterval = 5 * time.Second
)

// errLoginNeeded tells that no valid token is available, so the user must run
// "onechat login".
var errLoginNeeded = errors.New(`login needed: run "onechat login"`)

// chatgptAuth authorizes model calls to the ChatGPT backend with the tokens in
// the token file. The tokens belong to OneChat only. The Codex CLI rotates the
// refresh token on each refresh, so two programs with one token file log each
// other out.
//
// Before each call it reads the expiry from the access token. Within
// refreshMargin of the expiry it gets new tokens with the refresh token and
// replaces the file atomically. A mutex lets one refresh run at a time, and a
// call that waits for it uses the new tokens.
type chatgptAuth struct {
	mutex  sync.Mutex
	path   string
	issuer string
	http   *http.Client
}

// tokenFile is the content of the token file.
type tokenFile struct {
	AccessToken  string `json:"access_token"`
	RefreshToken string `json:"refresh_token"`
	IDToken      string `json:"id_token"`
	AccountID    string `json:"account_id"`
}

// tokenResponse is the answer of the token endpoint.
type tokenResponse struct {
	AccessToken  string `json:"access_token"`
	RefreshToken string `json:"refresh_token"`
	IDToken      string `json:"id_token"`
}

// deviceCode is the answer of the user code endpoint. The interval is a number
// or a string of seconds.
type deviceCode struct {
	DeviceAuthID string          `json:"device_auth_id"`
	UserCode     string          `json:"user_code"`
	UserCodeAlt  string          `json:"usercode"`
	Interval     json.RawMessage `json:"interval"`
}

// deviceGrant is the answer of the device token endpoint after the user
// approves the code.
type deviceGrant struct {
	AuthorizationCode string `json:"authorization_code"`
	CodeVerifier      string `json:"code_verifier"`
}

// login runs "onechat login": the device code login into the token file of the
// data directory.
func login(ctx context.Context, out io.Writer) error {
	dir, err := dataDir()
	if err != nil {
		return err
	}

	return newChatGPTAuth(filepath.Join(dir, tokenFileName), authIssuer).Login(ctx, out)
}

// newChatGPTAuth returns the authorizer for the token file at path, with the
// login service at issuer.
func newChatGPTAuth(path, issuer string) *chatgptAuth {
	return &chatgptAuth{
		path:   path,
		issuer: issuer,
		http:   &http.Client{},
	}
}

// Login runs the device code login. It writes the link and the code for the
// user to out, waits until the user approves the code, and writes the tokens
// to the token file.
func (a *chatgptAuth) Login(ctx context.Context, out io.Writer) error {
	a.mutex.Lock()
	defer a.mutex.Unlock()

	var code deviceCode
	err := a.postJSON(ctx, "/api/accounts/deviceauth/usercode", map[string]string{
		"client_id": codexClientID,
	}, &code)
	if err != nil {
		return fmt.Errorf("request device code (device code login must be allowed in the ChatGPT security settings): %w", err)
	}

	userCode := cmp.Or(code.UserCode, code.UserCodeAlt)
	if code.DeviceAuthID == "" || userCode == "" {
		return errors.New("device code response has no device ID or user code")
	}

	_, err = fmt.Fprintf(out, "Open %s/codex/device on any device and enter this code:\n\n    %s\n\nWaiting for the approval...\n", a.issuer, userCode)
	if err != nil {
		return err
	}

	grant, err := a.poll(ctx, code.DeviceAuthID, userCode, pollInterval(code.Interval))
	if err != nil {
		return err
	}

	tokens, err := a.exchange(ctx, grant)
	if err != nil {
		return err
	}

	err = a.write(tokens)
	if err != nil {
		return err
	}

	_, err = fmt.Fprintln(out, "Logged in.")

	return err
}

// postJSON posts body as JSON to the path of the login service and decodes the
// answer into result.
func (a *chatgptAuth) postJSON(ctx context.Context, path string, body, result any) error {
	data, err := json.Marshal(body)
	if err != nil {
		return fmt.Errorf("encode request: %w", err)
	}

	_, err = a.post(ctx, path, "application/json", data, result)

	return err
}

// post posts body to the path of the login service and decodes a 200 answer
// into result. It returns the HTTP status, or 0 when no answer came. The error
// holds the status, but not the body, which can hold a token.
func (a *chatgptAuth) post(ctx context.Context, path, contentType string, body []byte, result any) (status int, err error) {
	request, err := http.NewRequestWithContext(ctx, http.MethodPost, a.issuer+path, bytes.NewReader(body))
	if err != nil {
		return 0, fmt.Errorf("create request: %w", err)
	}

	request.Header.Set("Content-Type", contentType)
	response, err := a.http.Do(request)
	if err != nil {
		return 0, fmt.Errorf("send request: %w", err)
	}

	defer func() {
		err = errors.Join(err, response.Body.Close())
	}()

	if response.StatusCode != http.StatusOK {
		return response.StatusCode, fmt.Errorf("HTTP %d", response.StatusCode)
	}

	err = json.NewDecoder(response.Body).Decode(result)
	if err != nil {
		return response.StatusCode, fmt.Errorf("decode answer: %w", err)
	}

	return response.StatusCode, nil
}

// poll asks for the grant of the device code until the user approves it or the
// login times out.
func (a *chatgptAuth) poll(ctx context.Context, deviceID, userCode string, interval time.Duration) (deviceGrant, error) {
	ctx, cancel := context.WithTimeout(ctx, loginTimeout)
	defer cancel()

	body, err := json.Marshal(map[string]string{
		"device_auth_id": deviceID,
		"user_code":      userCode,
	})
	if err != nil {
		return deviceGrant{}, fmt.Errorf("encode poll request: %w", err)
	}

	for {
		var grant deviceGrant
		status, err := a.post(ctx, "/api/accounts/deviceauth/token", "application/json", body, &grant)
		if err == nil && (grant.AuthorizationCode == "" || grant.CodeVerifier == "") {
			err = errors.New("device grant has no authorization code or code verifier")
		}

		if err == nil {
			return grant, nil
		}

		if status != http.StatusForbidden && status != http.StatusNotFound {
			return deviceGrant{}, fmt.Errorf("poll device code: %w", err)
		}

		timer := time.NewTimer(interval)
		select {
		case <-ctx.Done():
			timer.Stop()
			return deviceGrant{}, fmt.Errorf("wait for approval: %w", ctx.Err())
		case <-timer.C:
		}
	}
}

// exchange trades the grant of the device code for tokens.
func (a *chatgptAuth) exchange(ctx context.Context, grant deviceGrant) (tokenFile, error) {
	form := url.Values{
		"grant_type":    {"authorization_code"},
		"code":          {grant.AuthorizationCode},
		"redirect_uri":  {a.issuer + "/deviceauth/callback"},
		"client_id":     {codexClientID},
		"code_verifier": {grant.CodeVerifier},
	}

	var response tokenResponse
	_, err := a.post(ctx, "/oauth/token", "application/x-www-form-urlencoded", []byte(form.Encode()), &response)
	if err != nil {
		return tokenFile{}, fmt.Errorf("exchange code for tokens: %w", err)
	}

	if response.AccessToken == "" || response.RefreshToken == "" || response.IDToken == "" {
		return tokenFile{}, errors.New("token response has no access, refresh, or ID token")
	}

	tokens := tokenFile{
		AccessToken:  response.AccessToken,
		RefreshToken: response.RefreshToken,
		IDToken:      response.IDToken,
	}

	tokens.AccountID = cmp.Or(accountID(tokens.AccessToken), accountID(tokens.IDToken))
	if tokens.AccountID == "" {
		return tokenFile{}, errors.New("tokens have no ChatGPT account ID")
	}

	return tokens, nil
}

// write replaces the token file atomically. The file has mode 0600.
func (a *chatgptAuth) write(tokens tokenFile) error {
	data, err := json.Marshal(tokens)
	if err != nil {
		return fmt.Errorf("encode tokens: %w", err)
	}

	err = writeFileAtomic(a.path, append(data, '\n'))
	if err != nil {
		return fmt.Errorf("write token file: %w", err)
	}

	return nil
}

// Authorize sets the access token and the account ID, after a refresh when the
// token expires within refreshMargin.
func (a *chatgptAuth) Authorize(ctx context.Context, header http.Header) error {
	a.mutex.Lock()
	defer a.mutex.Unlock()

	tokens, err := a.read()
	if err != nil {
		return err
	}

	expiry, found := tokenExpiry(tokens.AccessToken)
	if !found || !time.Now().Before(expiry.Add(-refreshMargin)) {
		tokens, err = a.refresh(ctx, tokens)
		if err != nil {
			return err
		}
	}

	header.Set("Authorization", "Bearer "+tokens.AccessToken)
	header.Set("ChatGPT-Account-Id", tokens.AccountID)

	return nil
}

// read reads the token file.
func (a *chatgptAuth) read() (tokenFile, error) {
	data, err := os.ReadFile(a.path)
	if errors.Is(err, os.ErrNotExist) {
		return tokenFile{}, errLoginNeeded
	}

	if err != nil {
		return tokenFile{}, fmt.Errorf("read token file: %w", err)
	}

	var tokens tokenFile
	err = json.Unmarshal(data, &tokens)
	if err == nil && (tokens.AccessToken == "" || tokens.RefreshToken == "" || tokens.AccountID == "") {
		err = errors.New("a token or the account ID is missing")
	}

	if err != nil {
		return tokenFile{}, fmt.Errorf("token file: %w: %w", err, errLoginNeeded)
	}

	return tokens, nil
}

// refresh gets new tokens with the refresh token and replaces the token file.
// It keeps the old refresh token and ID token when the answer has none.
func (a *chatgptAuth) refresh(ctx context.Context, tokens tokenFile) (tokenFile, error) {
	var response tokenResponse
	err := a.postJSON(ctx, "/oauth/token", map[string]string{
		"client_id":     codexClientID,
		"grant_type":    "refresh_token",
		"refresh_token": tokens.RefreshToken,
		"scope":         "openid profile email",
	}, &response)
	if err == nil && response.AccessToken == "" {
		err = errors.New("refresh response has no access token")
	}

	if err != nil {
		return tokenFile{}, fmt.Errorf("refresh tokens: %w: %w", err, errLoginNeeded)
	}

	tokens.AccessToken = response.AccessToken
	tokens.RefreshToken = cmp.Or(response.RefreshToken, tokens.RefreshToken)
	tokens.IDToken = cmp.Or(response.IDToken, tokens.IDToken)
	tokens.AccountID = cmp.Or(accountID(tokens.AccessToken), accountID(tokens.IDToken), tokens.AccountID)

	err = a.write(tokens)
	if err != nil {
		return tokenFile{}, err
	}

	return tokens, nil
}

// pollInterval returns the poll interval of a device code, given in seconds as
// a number or a string.
func pollInterval(raw json.RawMessage) time.Duration {
	seconds, err := strconv.ParseFloat(strings.Trim(string(raw), `"`), 64)
	if err != nil || seconds < 0 {
		return defaultPollInterval
	}

	return time.Duration(seconds * float64(time.Second))
}

// accountID returns the ChatGPT account ID of a JWT, or "" when it has none.
func accountID(token string) string {
	var claims map[string]json.RawMessage
	if !decodeClaims(token, &claims) {
		return ""
	}

	var auth struct {
		AccountID string `json:"chatgpt_account_id"`
	}

	err := json.Unmarshal(claims[authClaim], &auth)
	if err != nil {
		return ""
	}

	return auth.AccountID
}

// decodeClaims decodes the payload of a JWT into claims, without a check of
// the signature. The token comes from the login service over HTTPS, and the
// client reads only the expiry and the account ID from it.
func decodeClaims(token string, claims any) bool {
	parts := strings.Split(token, ".")
	if len(parts) != 3 {
		return false
	}

	payload, err := base64.RawURLEncoding.DecodeString(strings.TrimRight(parts[1], "="))
	if err != nil {
		return false
	}

	return json.Unmarshal(payload, claims) == nil
}

// tokenExpiry returns the expiry of a JWT from its exp claim.
func tokenExpiry(token string) (time.Time, bool) {
	var claims struct {
		Exp *float64 `json:"exp"`
	}

	if !decodeClaims(token, &claims) || claims.Exp == nil {
		return time.Time{}, false
	}

	return time.Unix(int64(*claims.Exp), 0), true
}