/**
 * The daily digest email — built from D1 + the bundled schedule, sent at
 * 8 AM ET via the send_email binding. Plain inline-styled HTML that renders
 * clean in Gmail mobile; always sent with a text alternative.
 *
 * Retrieval rules: review and drill lines carry number + difficulty only —
 * never the topic, never a solution link. The learning day's core list is
 * the only labeled section. Reviews + drills ≤ 6, reviews first, overflow
 * simply stays due (oldest tomorrow). Sunday is a two-line rest note.
 *
 * Idempotency: email_log keys sends by ET date — a same-day re-send is a
 * no-op unless forced; the body itself is deterministic (drill picks are
 * seeded by the date).
 */
import {
  CAMPAIGN_DAYS,
  type ProblemRow,
  SCHEDULE,
  addDays,
  campaignDay,
  campaignWeek,
  dueReviews,
  isoWeek,
  pickDrills,
  prettyDate,
  streak,
  weekdayOf,
} from "./srs.ts";
import { signLink } from "./links.ts";

const DAILY_CAP = 6;

function cap(difficulty: string): string {
  return difficulty[0]!.toUpperCase() + difficulty.slice(1);
}

export interface Digest {
  subject: string;
  html: string;
  text: string;
}

/** One-tap pass/fail anchor pair for a problem line. */
async function tapLinks(env: Env, lc: number, date: string): Promise<string> {
  const pass = await signLink(env.LINK_KEY, lc, "pass", date);
  const fail = await signLink(env.LINK_KEY, lc, "fail", date);
  const url = (r: string, sig: string) =>
    `${env.PUBLIC_URL}/log?p=${lc}&r=${r}&d=${date}&sig=${sig}`;
  return (
    `<a href="${url("pass", pass)}" style="color:#1a7f37;font-weight:bold">pass</a>` +
    ` · <a href="${url("fail", fail)}" style="color:#cf222e;font-weight:bold">fail</a>`
  );
}

export async function buildDigest(env: Env, date: string): Promise<Digest> {
  const db = env.DB;
  const day = campaignDay(date);
  const week = campaignWeek(date);
  const header = `${prettyDate(date)} — Day ${day}/${CAMPAIGN_DAYS} · Week ${week}`;

  if (weekdayOf(date) === 0) {
    return {
      subject: `SRS — ${prettyDate(date)} — rest day`,
      html: `<p><strong>${header}</strong></p><p>Rest day. Nothing is due. Overdue reviews moved to Monday.</p>`,
      text: `${header}\nRest day. Nothing is due. Overdue reviews moved to Monday.`,
    };
  }

  const due = await dueReviews(db, date);
  const reviews = due.slice(0, DAILY_CAP);
  const carried = due.length - reviews.length;
  const drills = await pickDrills(db, date, DAILY_CAP - reviews.length);
  const currentStreak = await streak(db, date);

  const htmlParts: string[] = [];
  const textParts: string[] = [];
  htmlParts.push(
    `<h2 style="margin:0 0 4px">${header}</h2>`,
    `<p style="margin:0 0 16px;color:#57606a">🔥 streak: ${currentStreak} day${currentStreak === 1 ? "" : "s"}</p>`,
  );
  textParts.push(header, `streak: ${currentStreak}`);

  // New topic — the only labeled section.
  const topicIssue = SCHEDULE[date];
  if (topicIssue !== undefined) {
    const topic = await db
      .prepare("SELECT name FROM topics WHERE issue = ?")
      .bind(topicIssue)
      .first<{ name: string }>();
    const { results: core } = await db
      .prepare(
        "SELECT * FROM problems WHERE topic_issue = ? AND set_label = 'core' ORDER BY lc_number",
      )
      .bind(topicIssue)
      .all<ProblemRow>();
    const links = core
      .map(
        (p) =>
          `<a href="https://github.com/${env.REPO}/issues/${p.issue}">LC ${p.lc_number}</a>${p.stage === "new" ? "" : " ✓"}`,
      )
      .join(" · ");
    htmlParts.push(
      `<h3 style="margin:16px 0 4px">New topic: ${topic?.name ?? `#${topicIssue}`}</h3>`,
      `<p style="margin:0">Core: ${links}</p>`,
    );
    textParts.push(`New topic: ${topic?.name ?? topicIssue} — core: ${core.map((p) => `LC ${p.lc_number}`).join(", ")}`);
  }

  // Reviews due — unlabeled lines, one-tap links.
  htmlParts.push(`<h3 style="margin:16px 0 4px">Reviews due (${reviews.length})</h3>`);
  textParts.push(`Reviews due (${reviews.length})`);
  if (reviews.length) {
    htmlParts.push(
      `<p style="margin:0 0 4px;color:#57606a">Solve each from scratch. Do not open your old solution first.</p>`,
    );
    for (const p of reviews) {
      htmlParts.push(
        `<p style="margin:2px 0">LC ${p.lc_number} — ${cap(p.difficulty)} — stage ${p.stage} — ${await tapLinks(env, p.lc_number, date)}</p>`,
      );
      textParts.push(`- LC ${p.lc_number} — ${cap(p.difficulty)} — stage ${p.stage}`);
    }
    if (carried > 0) {
      htmlParts.push(`<p style="margin:4px 0;color:#57606a">${carried} more carried to tomorrow (cap ${DAILY_CAP}).</p>`);
      textParts.push(`${carried} more carried to tomorrow.`);
    }
  } else {
    htmlParts.push(`<p style="margin:0">None.</p>`);
    textParts.push("none");
  }

  // Blind drills — unlabeled.
  htmlParts.push(`<h3 style="margin:16px 0 4px">Blind drills (${drills.length})</h3>`);
  textParts.push(`Blind drills (${drills.length})`);
  if (drills.length) {
    htmlParts.push(
      `<p style="margin:0 0 4px;color:#57606a">No topic given. Name the pattern out loud before you code.</p>`,
    );
    for (const p of drills) {
      htmlParts.push(
        `<p style="margin:2px 0">LC ${p.lc_number} — ${cap(p.difficulty)} — ${await tapLinks(env, p.lc_number, date)}</p>`,
      );
      textParts.push(`- LC ${p.lc_number} — ${cap(p.difficulty)}`);
    }
  } else {
    htmlParts.push(`<p style="margin:0">None today.</p>`);
    textParts.push("none");
  }

  // Saturday: the review issue already exists (created at midnight ET).
  if (weekdayOf(date) === 6) {
    const gate = await db
      .prepare("SELECT issue FROM gates WHERE week = ? AND issue IS NOT NULL")
      .bind(isoWeek(date))
      .first<{ issue: number }>();
    if (gate) {
      htmlParts.push(
        `<h3 style="margin:16px 0 4px">Saturday gate</h3>`,
        `<p style="margin:0"><a href="https://github.com/${env.REPO}/issues/${gate.issue}">Review — Week ${campaignWeek(date)} (#${gate.issue})</a> — timed, blind, then close the issue.</p>`,
      );
      textParts.push(`Saturday gate: https://github.com/${env.REPO}/issues/${gate.issue}`);
    }
  }

  // Footer: yesterday's log, gate rate to date, progress page.
  const yesterday = addDays(date, -1);
  const { results: logged } = await db
    .prepare("SELECT lc_number, kind, result FROM attempts WHERE date = ? ORDER BY id")
    .bind(yesterday)
    .all<{ lc_number: number; kind: string; result: string }>();
  const lastGate = await db
    .prepare("SELECT pass_rate FROM gates WHERE pass_rate IS NOT NULL ORDER BY week DESC LIMIT 1")
    .first<{ pass_rate: number }>();
  const footerBits = [
    logged.length
      ? `Yesterday: ${logged.map((a) => `LC ${a.lc_number} ${a.result === "pass" ? "✅" : "❌"}`).join(" · ")}`
      : "Yesterday: no attempts logged.",
    lastGate ? `Gate pass rate: ${Math.round(lastGate.pass_rate * 100)}%` : "No gates scored yet.",
  ];
  htmlParts.push(
    `<hr style="border:none;border-top:1px solid #d0d7de;margin:16px 0">`,
    `<p style="margin:0;color:#57606a;font-size:13px">${footerBits.join(" · ")} · <a href="${env.DOCS_URL}/progress">progress page</a></p>`,
  );
  textParts.push(...footerBits, `progress: ${env.DOCS_URL}/progress`);

  const load = reviews.length + drills.length;
  return {
    subject: `SRS — ${prettyDate(date)} — ${load} on deck`,
    html: `<div style="font-family:-apple-system,Segoe UI,Helvetica,Arial,sans-serif;max-width:600px;margin:0 auto;padding:12px">${htmlParts.join("\n")}</div>`,
    text: textParts.join("\n"),
  };
}

export interface SendReport {
  sent: boolean;
  reason: string;
  digest: Digest;
}

/** Send today's digest exactly once per ET date (unless forced). */
export async function sendDigest(
  env: Env,
  date: string,
  opts: { force?: boolean; dry?: boolean } = {},
): Promise<SendReport> {
  const digest = await buildDigest(env, date);
  if (opts.dry) return { sent: false, reason: "dry run", digest };

  const already = await env.DB.prepare("SELECT sent_at FROM email_log WHERE date = ?")
    .bind(date)
    .first();
  if (already && !opts.force) return { sent: false, reason: "already sent today", digest };

  await env.EMAIL.send({
    to: env.TO_EMAIL,
    from: { email: env.FROM_EMAIL, name: "SRS" },
    subject: digest.subject,
    html: digest.html,
    text: digest.text,
  });
  await env.DB.prepare(
    "INSERT INTO email_log (date, sent_at) VALUES (?, ?) ON CONFLICT(date) DO UPDATE SET sent_at = excluded.sent_at",
  )
    .bind(date, new Date().toISOString())
    .run();
  return { sent: true, reason: already ? "forced re-send" : "sent", digest };
}