// ── The email DeliveryChannel: addressFor + deliver ────────────────────────
//
// ENGAGEMENT.md Phase 6. Phase 3 declared this channel and deliberately left it
// behaviourless ("declaring a function nothing calls freezes a signature before
// anything has tried to use it"); this is the phase that has something to try it
// with, and the signature survived unchanged.
//
// **What it does is four lookups and one send**, and the order matters because
// each step is a way the mail should not go out:
//
//   1. the address       — re-checked for `status = 'active'`, because a delayed
//                          row can outlive the account it was queued for
//   2. the rule          — for its per-channel template key; the outbox row
//                          carries `rule_id` and FK CASCADE guarantees it exists
//   3. the values        — the payload snapshot, plus §4.6.1's structural
//                          projection, plus this recipient's unsubscribe link
//   4. the template      — `renderByKey`, which falls back to the shipped seed
//                          rather than failing, and refuses a draft
//   5. the send          — `mailer.sendNotification`, which classifies rather
//                          than throwing
//
// **It never throws**, and that is a stronger statement than the worker's
// `try/catch` around it: a throw would be read as a transient failure and retried
// five times, so an unrenderable template would become five identical failures in
// the send log instead of one honest terminal row.
//
// **The unsubscribe link is per recipient and is built from `scope_key`, never
// from `subject_key`.** They differ for every Team event: the subject is
// `teamName` (a display string the cooldown keys on) and the scope is `team:12`.
// A Team renamed between the mail and the click must not orphan the link in it.

const rulesDb = require('../model/engagement/engagementRules.db')
const recipients = require('../model/engagement/engagementRecipients.db')
const templates = require('./templates')
const projection = require('./projection')
const suppressions = require('./suppressions')
const settings = require('../model/settings/settings.model')
const unsubscribeToken = require('../utils/unsubscribeToken')
const log = require('../utils/logger')('engagement')

// **Required lazily, and it is a real cycle rather than a style preference.**
// `engagement/index.js` requires `coreChannels`, which requires this file; and
// `utils/mailer` requires `engagement/index` for the transport registry. A
// top-level `require('../utils/mailer')` here therefore resolves while
// `engagement/index` is mid-evaluation, so mailer would capture `{}` for
// `transports` and every send would fail on `transports.get is not a function` —
// at send time, on a deployment, with the boot log clean. Resolved at call time
// instead, by which point both modules are fully evaluated.
const mailer = () => require('../utils/mailer')

// The template a rule renders through when it names none. §4.6.1 property 1: a
// new trigger must be mailable with no authoring at all, and this plus
// `projection.project` is that property's implementation.
const DEFAULT_TEMPLATE = 'notify.event'

const baseUrl = () => templates.baseUrl()

// Re-exported rather than defined here since Phase 9: the send log's hash and
// the suppression list's key have to be the same function or a bounce never finds
// the row it belongs to. `suppressions.js` owns it, next to the masking.
const hashAddress = suppressions.hashAddress

/**
 * The two unsubscribe URLs for one recipient of one scope, or nulls.
 *
 * TWO urls from one token, and they are not interchangeable. `unsubscribeUrl` is
 * the human one that goes in the mail body: the site's own page, which explains
 * what is about to happen and POSTs once a person has read it. `unsubscribeApiUrl`
 * is the machine one for the `List-Unsubscribe` header, where RFC 8058 says a
 * client may POST without showing anybody anything — so it has to be an endpoint,
 * not a page. The API route answers GET on the same path with a redirect to the
 * page, which covers clients that render the header as an ordinary link.
 *
 * A scope the token format cannot carry yields nulls rather than an exception:
 * the mail is worth sending without a one-click unsubscribe, and the recipient
 * still has the preferences screen. It is logged because it is a programming
 * error in whatever chose the scope key.
 */
function unsubscribeUrls(userId, scopeKey) {
  try {
    const token = unsubscribeToken.sign(userId, 'email', scopeKey || '')
    const base = baseUrl()
    return {
      unsubscribeUrl: `${base}/unsubscribe/${token}`,
      unsubscribeApiUrl: `${base}/api/v1/public/engagement/unsubscribe/${token}`,
    }
  } catch (err) {
    log.warn('could not build an unsubscribe link', { scope: scopeKey, message: err.message })
    return { unsubscribeUrl: null, unsubscribeApiUrl: null }
  }
}

/** Where this channel would send to, or null. */
const addressFor = (userId) => recipients.addressFor(userId)

/**
 * Narrow an audience to the users this channel may write an outbox row for
 * (Phase 9, decision 4).
 *
 * **One gate, and it is the Phase 1b verification setting.** With
 * `email_verification_required` on, a user whose address is unverified is
 * excluded here rather than refused at delivery, and the org lead settled it that
 * way for two reasons. It is a STANDING property — unlike a suppression, which
 * can appear inside a `delay_seconds` window and therefore has to be re-checked
 * at send time — so the outbox row would be written only to be thrown away. And a
 * deployment that upgraded before verifying anybody has an audience that is
 * almost entirely unverified: excluding at delivery would write a `suppressed`
 * row per person per rule firing, which is a send log nobody can read.
 *
 * The count comes back so the admin reach preview can say "1,204 excluded:
 * unverified" instead of quietly promising a number the engine will not deliver.
 *
 * **It fails OPEN, and the try/catch is load-bearing rather than defensive
 * habit.** `settings.isEmailVerificationRequired` swallows its own errors and
 * answers `off`, but `unverifiedAmong` does not, and an uncaught throw here does
 * not fail one recipient — `applyRule` awaits this before the per-user loop, so
 * it would abandon the whole rule for every channel it names. A database having
 * a bad minute would become a rule that silently sent nothing, with a clean send
 * log and nothing in the outbox to retry. Same direction as the suppression
 * check, for the same reason: the recoverable mistake is mail going out, not mail
 * silently stopping.
 */
async function eligible(userIds) {
  const list = userIds || []
  if (!list.length) return { userIds: [], excluded: {} }
  try {
    if (!(await settings.isEmailVerificationRequired())) {
      return { userIds: list.slice(), excluded: {} }
    }
    const unverified = await recipients.unverifiedAmong(list)
    if (!unverified.size) return { userIds: list.slice(), excluded: {} }
    return {
      userIds: list.filter((id) => !unverified.has(Number(id))),
      excluded: { unverified: unverified.size },
    }
  } catch (err) {
    log.error('verification gate could not be evaluated; not excluding anyone', { message: err.message })
    return { userIds: list.slice(), excluded: {} }
  }
}

/**
 * Deliver one claimed outbox row.
 *
 * @returns {Promise<{ok: boolean, retry?: boolean, transport?: string, detail?: string}>}
 */
async function deliver(row) {
  try {
    const to = await addressFor(row.user_id)
    if (!to) {
      // Terminal. Retrying does not give somebody an address, and a banned
      // account is not going to be un-banned by a five-minute backoff.
      return { ok: false, detail: 'no deliverable address for this user' }
    }

    const rule = await rulesDb.getById(row.rule_id)
    const key = (rule && rule.template_keys && rule.template_keys.email) || DEFAULT_TEMPLATE

    // Once, not once per use: the body's link and the header's must be the same
    // token, or a client that offers both offers two different unsubscribes.
    const unsub = unsubscribeUrls(row.user_id, row.scope_key)
    const values = projection.project(row.trigger_id, row.payload || {}, unsub)

    const rendered = await templates.renderByKey(key, values)
    if (!rendered) {
      // Neither a usable row nor a shipped seed. Terminal, and it names the key:
      // the operator deleted a template a rule points at, which the admin surface
      // refuses with a 409 — so reaching here means it happened out of band.
      return { ok: false, detail: `no template and no shipped default for "${key}"` }
    }
    if (rendered.missing.length) {
      // Not a refusal: an optional variable a trigger chose not to supply renders
      // as nothing by design. Logged with NAMES ONLY, never values — the same
      // rule the emit and dispatch log lines follow.
      log.debug('template variables had no value', { key, missing: rendered.missing })
    }

    // **The suppression check is HERE and not at enqueue** (Phase 9). An outbox
    // row can sit through a `delay_seconds` grace window, and an address can hard
    // bounce inside it — so the only check that can be correct is the one taken
    // immediately before the transport call. It is also the check the acceptance
    // criterion describes: a `suppressed` row in the send log, and no transport
    // call at all.
    const blocked = await suppressions.isSuppressed(to.address)
    if (blocked) {
      return {
        ok: false,
        suppressed: true,
        detail: `address is suppressed (${blocked.reason})`,
        addressHash: hashAddress(to.address),
      }
    }

    const result = await mailer().sendNotification({ to: to.address, rendered, ...unsub })

    // A failed send is where a hard bounce enters the system on SMTP, and the
    // reason it is worth catching rather than waiting for an API provider: a
    // single-recipient send refused at RCPT TO is a synchronous 5.1.1, which is
    // the most valuable deliverability signal there is and it was already being
    // thrown away. `considerFailure` is narrow — see bounceClassify.js — and its
    // note goes into the log on BOTH outcomes, so "this failed and was not
    // suppressed" says why.
    if (result && !result.ok && result.smtp) {
      const verdict = await suppressions.considerFailure({ address: to.address, error: result.smtp })
      // A bounce is terminal by definition. Overriding `retry` matters because
      // `PERMANENT_CODES` does not contain every code that can carry a 5.1.x, so
      // without this a genuine dead mailbox could still be retried four more
      // times — each one another refusal on our record with the relay.
      if (verdict.suppressed) {
        return {
          ok: false,
          retry: false,
          // `engagement_sends.status` has carried 'bounced' since §4.5 and
          // nothing wrote it until here, so the Send Log's "Bounced" filter
          // matched nothing — the live rig is what showed that. It is a distinct
          // status rather than a flavour of 'failed' because the two need
          // different actions: a failure means look at the relay, and a bounce
          // means that person's address is gone.
          bounced: true,
          transport: result.transport,
          detail: `${result.detail} — ${verdict.note}`,
          addressHash: hashAddress(to.address),
        }
      }
      return { ...result, detail: `${result.detail} — ${verdict.note}`, addressHash: hashAddress(to.address) }
    }

    // The send log stores a sha256 of the address and never the address itself
    // (schema.sql): enough to correlate a bounce, useless as a mailing list.
    // Attached on every outcome, because a failure is exactly the row a bounce
    // would need to be matched against.
    return { ...result, addressHash: hashAddress(to.address) }
  } catch (err) {
    // See the header: a throw here would be retried as if it were the relay's
    // fault. Classified as terminal instead, with the reason in the send log.
    log.error('email delivery failed', { outbox: row.id, message: err.message })
    return { ok: false, detail: `delivery error: ${err.message}` }
  }
}

module.exports = { addressFor, eligible, deliver, unsubscribeUrls, hashAddress, DEFAULT_TEMPLATE }