package recovery

import (
	"context"
	"errors"
	"fmt"
	"html"
	"io"
	"log/slog"
	"net/http"
	"strconv"
	"strings"
	"time"

	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/audit"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/auth"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/httpx"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/mail"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/users"
	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/web"
)

// genericNotice is the ONLY answer POST /access ever gives, whether or not the
// email matched (SPEC-007 AC-4, A3): the recovery entry must not be an
// enumeration oracle.
const genericNotice = "If a record exists, we have emailed you."

// errHandled marks a decision that already wrote its own HTTP response (a
// client error such as 403/404/409/410). Callers return nil so the door does
// NOT also write a 500 on top the response.
var errHandled = errors.New("recovery: response already written")

// Users is the narrow slice of the user service the recovery flow needs.
type Users interface {
	Get(ctx context.Context, id int64) (users.User, error)
	ByEmail(ctx context.Context, email string) (users.User, error)
	// Manager returns the chain link above id. The approval authority is the
	// CHAIN (R-7): the flow walks this link upward from the requester.
	Manager(ctx context.Context, id int64) (users.Optional[users.UserID], error)
}

// maxChainDepth bounds the manager-line walk (R-7) so a corrupted chain (a
// cycle) can never spin: it fails closed well before the bound.
const maxChainDepth = 64

// Requests is the recovery-request service the flow drives.
type Requests interface {
	// Create opens (or reuses) a pending request. created reports whether a
	// NEW row was inserted — only a new request triggers notification, so a
	// repeated POST cannot spam the chain.
	Create(ctx context.Context, userID int64, clientIP string) (Request, bool, error)
	Get(ctx context.Context, id int64) (Request, error)
	Decide(ctx context.Context, id, decidedBy int64, status Status) error
}

// Deps assembles the recovery flow's dependencies.
type Deps struct {
	// Requests owns the recovery_requests table.
	Requests Requests
	// Users resolves the email-on-record and the chain (the approver).
	Users Users
	// Identities rotates the identity on approval (D-007).
	Identities auth.Identities
	// Mailer delivers the notifications and the enrolment link.
	Mailer mail.Mailer
	// CSRF issues the signed pre-session token for the anonymous /access form
	// (A15).
	CSRF *httpx.PreSessionCSRF
	// Audit receives the request/approve/deny/rotation events (A11).
	Audit audit.Audit
	// BaseURL prefixes the enrolment link in the emailed recovery.
	BaseURL string
	// PendingTTL bounds the approvable window (default DefaultPendingTTL).
	PendingTTL time.Duration
	// Logger receives structured logs. Defaults to slog.Default().
	Logger *slog.Logger
	// Now is the clock (tests). Defaults to time.Now.
	Now func() time.Time
}

// Actions is the SPEC-007 recovery flow.
type Actions struct {
	d Deps
}

// New validates and builds the flow.
func New(d Deps) (*Actions, error) {
	switch {
	case d.Requests == nil:
		return nil, errors.New("recovery: Requests is required")
	case d.Users == nil:
		return nil, errors.New("recovery: Users is required")
	case d.Identities == nil:
		return nil, errors.New("recovery: Identities is required")
	case d.CSRF == nil:
		return nil, errors.New("recovery: CSRF is required")
	}
	if d.Mailer == nil {
		d.Mailer = mail.Nop{}
	}
	if d.Logger == nil {
		d.Logger = slog.Default()
	}
	if d.Now == nil {
		d.Now = time.Now
	}
	if d.PendingTTL <= 0 {
		d.PendingTTL = DefaultPendingTTL
	}
	return &Actions{d: d}, nil
}

// ── the anonymous /access entry ─────────────────────────────────────────────

// Page renders the recovery entry (GET /access). It runs before any session,
// so the form carries the signed pre-session double-submit token (A15).
func (a *Actions) Page(w http.ResponseWriter, r *http.Request) error {
	token, err := a.d.CSRF.Ensure(w, r)
	if err != nil {
		return err
	}
	return renderHTML(w, http.StatusOK, accessPage(token))
}

// Request handles POST /access?do=auth&action=recover. It ALWAYS answers with
// the same generic notice (AC-4): a match opens a pending request and notifies
// the chain; a miss does nothing observable.
func (a *Actions) Request(w http.ResponseWriter, r *http.Request) error {
	ctx := r.Context()
	email := strings.TrimSpace(r.FormValue("email"))

	if email != "" {
		u, err := a.d.Users.ByEmail(ctx, email)
		switch {
		case err == nil:
			a.openRequest(ctx, u)
		case errors.Is(err, users.ErrNotFound):
			// Miss: deliberately indistinguishable from a hit (AC-4).
		default:
			return err // a real fault is a 500, not an enumeration signal
		}
	}
	return renderHTML(w, http.StatusOK, noticePage())
}

// openRequest opens the pending request and notifies the chain and the
// employee. A reused request is NOT re-notified (no email spam). Failures are
// logged, never surfaced — the response is generic.
func (a *Actions) openRequest(ctx context.Context, requester users.User) {
	req, created, err := a.d.Requests.Create(ctx, requester.ID, clientIP(ctx))
	if err != nil {
		if !errors.Is(err, ErrTooManyPending) {
			a.d.Logger.Error("recovery: open request failed", "user_id", requester.ID, "error", err)
		}
		return
	}
	if created {
		a.notify(ctx, requester, req)
	}
}

// notify emails the approver (the requester's manager, the chain link above
// them) and the employee (SPEC-007 §4: "notify the approver chain + employee").
func (a *Actions) notify(ctx context.Context, requester users.User, req Request) {
	if requester.Manager.Set {
		if mgr, err := a.d.Users.Get(ctx, requester.Manager.Value); err == nil && mgr.Email != "" {
			a.send(ctx, mail.Message{
				To:      mgr.Email,
				Subject: "MyDay: a recovery request awaits your decision",
				Body:    a.approverBody(requester, req),
			})
		}
	}
	if requester.Email != "" {
		a.send(ctx, mail.Message{
			To:      requester.Email,
			Subject: "MyDay: your access recovery request",
			Body:    a.employeeBody(requester, req),
		})
	}
}

// ── the approver actions (session-gated by the door) ────────────────────────

// Approve decides a pending request: an approver on the requester's chain
// (an ancestor, or the admin) and never the requester (AC-2/R-7). On success it
// revokes the OLD identity and every session, mints a NEW GUID and emails the
// enrolment link (AC-3, D-007).
func (a *Actions) Approve(w http.ResponseWriter, r *http.Request, actorID int64) error {
	req, requester, approver, err := a.resolveDecision(w, r, actorID)
	switch {
	case errors.Is(err, errHandled):
		return nil
	case err != nil:
		return err
	}
	ctx := r.Context()

	// Claim the request ATOMICALLY first (a status-conditional UPDATE): when
	// two approvers race, exactly one wins and only the winner rotates. The
	// decision is audited by the store.
	if err := a.d.Requests.Decide(ctx, req.ID, approver.ID, StatusApproved); err != nil {
		writeError(w, http.StatusConflict, "the request is no longer pending")
		return nil
	}

	// Revoke the OLD identity + every session and mint a NEW GUID (D-007). If
	// this fails the request stays approved and audited; the operator can
	// intervene — we never rotate twice for one request.
	guid, err := a.d.Identities.Rotate(ctx, requester.ID)
	if err != nil {
		a.d.Logger.Error("recovery: approved but rotation failed",
			"request", req.ID, "user_id", requester.ID, "error", err)
		return err
	}

	a.send(ctx, mail.Message{
		To:      requester.Email,
		Subject: "MyDay: your access recovery was approved",
		Body:    a.enrolmentBody(requester, guid),
	})
	writeJSON(w, http.StatusOK, `{"status":"approved"}`)
	return nil
}

// Deny rejects a pending request. No rotation happens.
func (a *Actions) Deny(w http.ResponseWriter, r *http.Request, actorID int64) error {
	req, requester, approver, err := a.resolveDecision(w, r, actorID)
	switch {
	case errors.Is(err, errHandled):
		return nil
	case err != nil:
		return err
	}
	ctx := r.Context()

	// The store's conditional UPDATE both decides the request and audits it
	// (A11); a request already decided by someone else is refused.
	if err := a.d.Requests.Decide(ctx, req.ID, approver.ID, StatusDenied); err != nil {
		writeError(w, http.StatusConflict, "the request is no longer pending")
		return nil
	}

	if requester.Email != "" {
		a.send(ctx, mail.Message{
			To:      requester.Email,
			Subject: "MyDay: your access recovery request was denied",
			Body:    "Your access recovery request was denied. Contact your manager if this was unexpected.\n",
		})
	}
	writeJSON(w, http.StatusOK, `{"status":"denied"}`)
	return nil
}

// resolveDecision loads and validates a decision: the request (pending, in
// window), the requester, and the approver's authority (an ancestor in the
// chain, or the admin — never the requester, R-7). A client-side rejection
// returns errHandled (the response is already written); a genuine fault returns
// the raw error so the door answers 500.
func (a *Actions) resolveDecision(w http.ResponseWriter, r *http.Request, actorID int64) (Request, users.User, users.User, error) {
	ctx := r.Context()
	approver, err := a.d.Users.Get(ctx, actorID)
	if errors.Is(err, users.ErrNotFound) {
		writeError(w, http.StatusForbidden, "forbidden")
		return Request{}, users.User{}, users.User{}, errHandled
	}
	if err != nil {
		return Request{}, users.User{}, users.User{}, err
	}

	id, err := strconv.ParseInt(strings.TrimSpace(r.FormValue("request")), 10, 64)
	if err != nil || id <= 0 {
		writeError(w, http.StatusBadRequest, "bad request")
		return Request{}, users.User{}, users.User{}, errHandled
	}
	req, err := a.d.Requests.Get(ctx, id)
	if errors.Is(err, ErrNotFound) {
		writeError(w, http.StatusNotFound, "not found")
		return Request{}, users.User{}, users.User{}, errHandled
	}
	if err != nil {
		return Request{}, users.User{}, users.User{}, err
	}
	if req.Status != StatusPending {
		writeError(w, http.StatusConflict, "the request is no longer pending")
		return Request{}, users.User{}, users.User{}, errHandled
	}
	if !req.ExpiredAt(a.d.PendingTTL).After(a.d.Now().UTC()) {
		_ = a.d.Requests.Decide(ctx, req.ID, 0, StatusExpired)
		writeError(w, http.StatusGone, "the request has expired")
		return Request{}, users.User{}, users.User{}, errHandled
	}

	requester, err := a.d.Users.Get(ctx, req.UserID)
	if errors.Is(err, users.ErrNotFound) {
		writeError(w, http.StatusNotFound, "not found")
		return Request{}, users.User{}, users.User{}, errHandled
	}
	if err != nil {
		return Request{}, users.User{}, users.User{}, err
	}
	// AC-2 / R-7: the authority is the CHAIN — the approver must be an ancestor
	// (on the requester's manager line, upward) or the admin, and never the
	// requester themselves.
	if !users.CanApprove(approver, requester, a.isAncestor(ctx, approver.ID, requester.ID)) {
		a.record(ctx, "recovery.forbidden", requester.ID, fmt.Sprintf("request:%d actor:%d", req.ID, actorID))
		writeError(w, http.StatusForbidden, "forbidden")
		return Request{}, users.User{}, users.User{}, errHandled
	}
	return req, requester, approver, nil
}

// isAncestor reports whether ancestorID sits on requesterID's manager line,
// upward (R-7). It walks the chain, following manager_user_id links, and fails
// closed on a broken lookup, an absent link, or a cycle.
func (a *Actions) isAncestor(ctx context.Context, ancestorID, requesterID int64) bool {
	if ancestorID == 0 || requesterID == 0 {
		return false
	}
	seen := map[int64]bool{}
	cur := requesterID
	for depth := 0; depth < maxChainDepth; depth++ {
		if seen[cur] {
			return false // a cycle in the chain — fail closed
		}
		seen[cur] = true
		mgr, err := a.d.Users.Manager(ctx, cur)
		if err != nil || !mgr.Set {
			return false
		}
		if mgr.Value == ancestorID {
			return true
		}
		cur = mgr.Value
	}
	return false
}

// ── email bodies ────────────────────────────────────────────────────────────

func (a *Actions) approverBody(requester users.User, req Request) string {
	return fmt.Sprintf(`A user has requested access recovery.

User:  %s
Email: %s

You are their manager. Sign in to MyDay and approve or deny request #%d.
If this request is unexpected, deny it.

%s
`, requester.DisplayName, requester.Email, req.ID, a.baseURL())
}

func (a *Actions) employeeBody(requester users.User, req Request) string {
	return fmt.Sprintf(`Hello %s,

Your access recovery request (#%d) has been submitted for approval.
Your manager must approve it before a new access link is issued.

%s
`, requester.DisplayName, req.ID, a.baseURL())
}

func (a *Actions) enrolmentBody(requester users.User, guid string) string {
	return fmt.Sprintf(`Hello %s,

Your access recovery was approved. Your old access link is now disabled.

Enrol your new passkey and authenticator here:
%s/%s

%s
`, requester.DisplayName, strings.TrimRight(a.baseURL(), "/"), guid, a.baseURL())
}

func (a *Actions) baseURL() string { return strings.TrimRight(a.d.BaseURL, "/") }

func (a *Actions) send(ctx context.Context, m mail.Message) {
	if err := a.d.Mailer.Send(ctx, m); err != nil {
		a.d.Logger.Warn("recovery: mail delivery failed", "to", m.To, "error", err)
	}
}

// record writes one audit row for the flow (A11). The door has already attached
// the actor to the context for the session-gated decisions.
func (a *Actions) record(ctx context.Context, action string, subjectUser int64, detail string) {
	if a.d.Audit == nil {
		return
	}
	a.d.Audit.Record(ctx, audit.Event{
		Action:  action,
		Subject: fmt.Sprintf("user:%d", subjectUser),
		Detail:  detail,
	})
}

// ── rendering helpers ───────────────────────────────────────────────────────

func accessPage(token string) web.Page {
	c := html.EscapeString(token)
	return web.Page{
		Title: "Account access",
		Guest: true,
		CSRF:  token,
		Content: web.Markup(`<h1>Account access</h1>
<p>Enter the email address on record. If we find it, your manager will be asked to approve a new access link.</p>
<form method="post" action="/access?do=auth&amp;action=recover">
<input type="hidden" name="csrf" value="` + c + `">
<label>Email <input name="email" type="email" autocomplete="email" required></label>
<button type="submit">Request access recovery</button></form>`),
	}
}

// noticePage is the generic outcome page (AC-4).
func noticePage() web.Page {
	return web.Page{
		Title: "Account access",
		Guest: true,
		Content: web.Markup(`<h1>Account access</h1>
<p>` + genericNotice + `</p>`),
	}
}

func renderHTML(w http.ResponseWriter, status int, page web.Page) error {
	w.Header().Set("Cache-Control", "no-store")
	return web.Render(w, status, page)
}

func writeJSON(w http.ResponseWriter, status int, body string) {
	w.Header().Set("Cache-Control", "no-store")
	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	w.WriteHeader(status)
	_, _ = io.WriteString(w, body)
}

func writeError(w http.ResponseWriter, status int, msg string) {
	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	w.Header().Set("X-Content-Type-Options", "nosniff")
	w.WriteHeader(status)
	_, _ = io.WriteString(w, msg+"\n")
}

// clientIP returns the resolved client IP the door attached to the context
// (A6) — empty when unavailable.
func clientIP(ctx context.Context) string {
	if m, ok := audit.MetaFrom(ctx); ok {
		return m.ClientIP
	}
	return ""
}