package server

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"
	"sync"
	"time"

	"cc.tiredsysadmin.keeporg/internal/secret"
	"cc.tiredsysadmin.keeporg/internal/store"
)

// OIDCConfig configures single sign-on. The server acts as a confidential
// client: it exchanges the authorization code over TLS directly with the
// provider's token endpoint and then reads the identity from the userinfo
// endpoint, so no local JWT verification is needed.
type OIDCConfig struct {
	Issuer       string `json:"issuer"`
	ClientID     string `json:"client_id"`
	ClientSecret string `json:"client_secret"`
	Scopes       string `json:"scopes"`
	// AutoCreate provisions an account on first successful sign-in.
	AutoCreate bool `json:"auto_create"`
	// DefaultGroupID is the group auto-created users join.
	DefaultGroupID string `json:"default_group_id"`
}

func (c OIDCConfig) Enabled() bool {
	return c.Issuer != "" && c.ClientID != "" && c.ClientSecret != ""
}

type oidcMetadata struct {
	AuthorizationEndpoint string `json:"authorization_endpoint"`
	TokenEndpoint         string `json:"token_endpoint"`
	UserinfoEndpoint      string `json:"userinfo_endpoint"`
}

type oidcProvider struct {
	cfg         OIDCConfig
	redirectURI string
	http        *http.Client

	mu   sync.Mutex
	meta *oidcMetadata
}

func newOIDCProvider(cfg OIDCConfig, redirectURI string) *oidcProvider {
	if cfg.Scopes == "" {
		cfg.Scopes = "openid profile email"
	}
	return &oidcProvider{cfg: cfg, redirectURI: redirectURI,
		http: &http.Client{Timeout: 15 * time.Second}}
}

// metadata fetches and caches the provider's discovery document.
func (p *oidcProvider) metadata() (*oidcMetadata, error) {
	p.mu.Lock()
	defer p.mu.Unlock()
	if p.meta != nil {
		return p.meta, nil
	}
	u := strings.TrimRight(p.cfg.Issuer, "/") + "/.well-known/openid-configuration"
	resp, err := p.http.Get(u)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("oidc: discovery %s: %s", u, resp.Status)
	}
	var m oidcMetadata
	if err := json.NewDecoder(io.LimitReader(resp.Body, 1<<20)).Decode(&m); err != nil {
		return nil, err
	}
	if m.AuthorizationEndpoint == "" || m.TokenEndpoint == "" || m.UserinfoEndpoint == "" {
		return nil, errors.New("oidc: discovery document is missing endpoints")
	}
	p.meta = &m
	return p.meta, nil
}

type oidcIdentity struct {
	Subject  string `json:"sub"`
	Email    string `json:"email"`
	Username string `json:"preferred_username"`
	Name     string `json:"name"`
}

// exchange turns an authorization code into the caller's identity.
func (p *oidcProvider) exchange(code string) (*oidcIdentity, error) {
	meta, err := p.metadata()
	if err != nil {
		return nil, err
	}
	form := url.Values{
		"grant_type":   {"authorization_code"},
		"code":         {code},
		"redirect_uri": {p.redirectURI},
	}
	req, err := http.NewRequest(http.MethodPost, meta.TokenEndpoint, strings.NewReader(form.Encode()))
	if err != nil {
		return nil, err
	}
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
	req.SetBasicAuth(url.QueryEscape(p.cfg.ClientID), url.QueryEscape(p.cfg.ClientSecret))
	resp, err := p.http.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		body, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<12))
		return nil, fmt.Errorf("oidc: token endpoint: %s: %s", resp.Status, strings.TrimSpace(string(body)))
	}
	var tok struct {
		AccessToken string `json:"access_token"`
	}
	if err := json.NewDecoder(io.LimitReader(resp.Body, 1<<20)).Decode(&tok); err != nil {
		return nil, err
	}
	if tok.AccessToken == "" {
		return nil, errors.New("oidc: token response carried no access token")
	}
	return p.userinfo(meta.UserinfoEndpoint, tok.AccessToken)
}

func (p *oidcProvider) userinfo(endpoint, accessToken string) (*oidcIdentity, error) {
	req, err := http.NewRequest(http.MethodGet, endpoint, nil)
	if err != nil {
		return nil, err
	}
	req.Header.Set("Authorization", "Bearer "+accessToken)
	resp, err := p.http.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("oidc: userinfo: %s", resp.Status)
	}
	var id oidcIdentity
	if err := json.NewDecoder(io.LimitReader(resp.Body, 1<<20)).Decode(&id); err != nil {
		return nil, err
	}
	if id.Subject == "" {
		return nil, errors.New("oidc: userinfo carried no subject")
	}
	return &id, nil
}

// handleOIDCLogin starts the authorization code flow for a pending device.
func (s *Server) handleOIDCLogin(w http.ResponseWriter, r *http.Request) {
	if s.oidc == nil {
		s.renderDone(w, "Single sign-on unavailable", "This server has no OIDC provider configured.")
		return
	}
	meta, err := s.oidc.metadata()
	if err != nil {
		s.log.Printf("oidc discovery: %v", err)
		s.renderDone(w, "Single sign-on unavailable", "The identity provider could not be reached.")
		return
	}
	// With a user code this approves a waiting client; without one it signs
	// a browser in to the web interface.
	userCode := strings.ToUpper(strings.TrimSpace(r.URL.Query().Get("code")))
	state := secret.Token()
	if err := s.st.Update(func(st *store.State) error {
		if userCode == "" {
			st.Devices[state] = &store.DeviceAuth{
				DeviceCode: state, Web: true, Label: "web", OIDCState: state,
				CreatedAt: time.Now(), ExpiresAt: time.Now().Add(deviceTTL),
			}
			return nil
		}
		d := st.DeviceByUserCode(userCode)
		if d == nil || time.Now().After(d.ExpiresAt) {
			return errors.New("unknown or expired device code")
		}
		d.OIDCState = state
		return nil
	}); err != nil {
		s.renderDevice(w, userCode, err.Error())
		return
	}
	q := url.Values{
		"response_type": {"code"},
		"client_id":     {s.oidc.cfg.ClientID},
		"redirect_uri":  {s.oidc.redirectURI},
		"scope":         {s.oidc.cfg.Scopes},
		"state":         {state},
	}
	http.Redirect(w, r, meta.AuthorizationEndpoint+"?"+q.Encode(), http.StatusFound)
}

// handleOIDCCallback finishes the flow and approves the waiting device.
func (s *Server) handleOIDCCallback(w http.ResponseWriter, r *http.Request) {
	if s.oidc == nil {
		s.renderDone(w, "Single sign-on unavailable", "This server has no OIDC provider configured.")
		return
	}
	q := r.URL.Query()
	if e := q.Get("error"); e != "" {
		s.renderDone(w, "Sign-in failed", e)
		return
	}
	state, code := q.Get("state"), q.Get("code")
	if state == "" || code == "" {
		s.renderDone(w, "Sign-in failed", "The provider did not return a code.")
		return
	}
	// Bind the state to a pending device before spending the code.
	var found bool
	s.st.Read(func(st *store.State) {
		for _, d := range st.Devices {
			if d.OIDCState != "" && secret.Equal(d.OIDCState, state) && time.Now().Before(d.ExpiresAt) {
				found = true
			}
		}
	})
	if !found {
		s.renderDone(w, "Sign-in failed", "This sign-in request has expired. Start again from the client.")
		return
	}
	id, err := s.oidc.exchange(code)
	if err != nil {
		s.log.Printf("oidc exchange: %v", err)
		s.renderDone(w, "Sign-in failed", "The identity provider rejected the sign-in.")
		return
	}
	var webToken, returnURL string
	if err := s.st.Update(func(st *store.State) error {
		var dev *store.DeviceAuth
		for _, d := range st.Devices {
			if d.OIDCState != "" && secret.Equal(d.OIDCState, state) {
				dev = d
			}
		}
		if dev == nil || time.Now().After(dev.ExpiresAt) {
			return errors.New("this sign-in request has expired")
		}
		u, err := s.linkOIDCUser(st, id)
		if err != nil {
			return err
		}
		dev.OIDCState = ""
		token := s.newSession(st, u.ID, dev.Label).Token
		if dev.Web {
			// Nothing is waiting to poll for this one; the browser in front
			// of us is the client.
			webToken = token
			delete(st.Devices, dev.DeviceCode)
			return nil
		}
		dev.SessionToken = token
		returnURL = dev.ReturnURL
		return nil
	}); err != nil {
		s.renderDone(w, "Sign-in failed", err.Error())
		return
	}
	if webToken != "" {
		s.setWebCookie(w, webToken, s.cfg.SessionTTL)
		http.Redirect(w, r, "/web/vaults", http.StatusFound)
		return
	}
	s.renderApproved(w, returnURL)
}

// linkOIDCUser finds the account behind an identity, provisioning one when
// the deployment allows it.
func (s *Server) linkOIDCUser(st *store.State, id *oidcIdentity) (*store.User, error) {
	issuer := strings.TrimRight(s.oidc.cfg.Issuer, "/")
	if u := st.UserByOIDC(issuer, id.Subject); u != nil {
		if u.Disabled {
			return nil, errors.New("this account is disabled")
		}
		return u, nil
	}
	// A local account may claim the identity by matching username or email,
	// which is how an invited user upgrades to single sign-on.
	for _, u := range st.Users {
		if u.OIDCSubject != "" {
			continue
		}
		if (id.Username != "" && u.Username == id.Username) ||
			(id.Email != "" && strings.EqualFold(u.Email, id.Email)) {
			u.OIDCIssuer, u.OIDCSubject = issuer, id.Subject
			return u, nil
		}
	}
	if !s.oidc.cfg.AutoCreate {
		return nil, errors.New("no account is linked to this identity; redeem an invite link first")
	}
	name := id.Username
	if name == "" {
		name = id.Email
	}
	if name == "" {
		name = "sso-" + secret.ID()
	}
	if validUsername(name) != nil || st.UserByName(name) != nil {
		name = "sso-" + secret.ID()
	}
	u := &store.User{ID: secret.ID(), Username: name, Email: id.Email,
		OIDCIssuer: issuer, OIDCSubject: id.Subject, CreatedAt: time.Now()}
	st.Users[u.ID] = u
	if g := s.oidc.cfg.DefaultGroupID; g != "" && st.Groups[g] != nil {
		st.Memberships = append(st.Memberships, store.Membership{UserID: u.ID, GroupID: g, Role: store.RoleMember})
	}
	return u, nil
}