leetcode/apps/api/src/srs.ts

/**
 * SRS domain: ET dates, the interval ladder, deterministic sampling, and the
 * one write path for attempts — ported from scripts/srs.ts, re-homed on D1.
 *
 * D1 is the single source of truth. The stage names the review a problem must
 * pass NEXT (`+2` = due 2 days after last clean solve). Passing advances
 * new → +2 → +5 → +10 → retired; any failure resets to +2. A problem's
 * FIRST-ever log enters the ladder at +2 regardless of result: a pass earns
 * a +2 review, a fail must be re-solved just as soon.
 *
 * All dates are America/New_York calendar strings (YYYY-MM-DD); arithmetic is
 * anchored at noon UTC so DST edges cannot shift a date. Never raw Date math.
 */
import scheduleJson from "../data/schedule.json";

// ── dates (America/New_York) ─────────────────────────────────────

export const CAMPAIGN_START = "2026-08-17";
export const CAMPAIGN_DAYS = 56;

const ET_DATE = new Intl.DateTimeFormat("en-CA", {
  timeZone: "America/New_York",
  dateStyle: "short",
});
const ET_HOUR = new Intl.DateTimeFormat("en-US", {
  timeZone: "America/New_York",
  hour: "numeric",
  hourCycle: "h23",
});

/** ET calendar date of an instant. */
export function etDate(now: Date): string {
  return ET_DATE.format(now);
}

/** ET hour 0–23 of an instant — the DST-proof cron guard. */
export function etHour(now: Date): number {
  return Number(ET_HOUR.format(now));
}

/** Noon-UTC anchor: date-only arithmetic immune to DST edges. */
function atNoon(date: string): Date {
  return new Date(`${date}T12:00:00Z`);
}

export function addDays(date: string, days: number): string {
  return new Date(atNoon(date).getTime() + days * 86_400_000).toISOString().slice(0, 10);
}

export function daysBetween(from: string, to: string): number {
  return Math.round((atNoon(to).getTime() - atNoon(from).getTime()) / 86_400_000);
}

/** 0 = Sunday … 6 = Saturday. */
export function weekdayOf(date: string): number {
  return atNoon(date).getUTCDay();
}

export function campaignDay(date: string): number {
  return daysBetween(CAMPAIGN_START, date) + 1;
}

/** 1-based campaign week (Mon–Sun), aligned to CAMPAIGN_START. */
export function campaignWeek(date: string): number {
  return Math.floor(daysBetween(CAMPAIGN_START, date) / 7) + 1;
}

/** Monday of the date's campaign week. */
export function weekMonday(date: string): string {
  return addDays(CAMPAIGN_START, (campaignWeek(date) - 1) * 7);
}

export function isoWeek(date: string): number {
  const d = atNoon(date);
  d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay() || 7));
  const jan1 = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
  return Math.ceil(((d.getTime() - jan1.getTime()) / 86_400_000 + 1) / 7);
}

export function prettyDate(date: string): string {
  return new Intl.DateTimeFormat("en-US", {
    timeZone: "UTC",
    weekday: "long",
    month: "long",
    day: "numeric",
  }).format(atNoon(date));
}

// ── the schedule (bundled; humans edit apps/api/data/schedule.json) ───

export const SCHEDULE: Record<string, number> = scheduleJson;

/** Week in which a topic was (or will be) taught. */
export function topicWeek(topic: number): number | undefined {
  for (const [date, t] of Object.entries(SCHEDULE)) {
    if (t === topic) return campaignWeek(date);
  }
  return undefined;
}

// ── deterministic sampling ───────────────────────────────────────

/** FNV-1a → mulberry32: seeded PRNG so re-runs pick identical problems. */
export function rng(seed: string): () => number {
  let h = 0x811c9dc5;
  for (let i = 0; i < seed.length; i++) {
    h ^= seed.charCodeAt(i);
    h = Math.imul(h, 0x01000193);
  }
  return () => {
    h = Math.imul(h ^ (h >>> 15), h | 1);
    h ^= h + Math.imul(h ^ (h >>> 7), h | 61);
    return ((h ^ (h >>> 14)) >>> 0) / 4294967296;
  };
}

/** Up to n elements, Fisher–Yates order driven by the seeded PRNG. */
export function sample<T>(pool: T[], n: number, random: () => number): T[] {
  const copy = [...pool];
  for (let i = copy.length - 1; i > 0; i--) {
    const j = Math.floor(random() * (i + 1));
    [copy[i], copy[j]] = [copy[j]!, copy[i]!];
  }
  return copy.slice(0, n);
}

// ── rows ─────────────────────────────────────────────────────────

export type Stage = "new" | "+2" | "+5" | "+10" | "retired";
export type Result = "pass" | "fail";
export type Kind = "first" | "review" | "drill" | "gate";

export interface ProblemRow {
  lc_number: number;
  issue: number;
  topic_issue: number;
  title: string;
  difficulty: string;
  set_label: string;
  stage: Stage;
  next_review: string | null;
  defer_until: string | null;
}

export const INTERVAL: Record<string, number> = { "+2": 2, "+5": 5, "+10": 10 };
const NEXT_STAGE: Record<string, Stage> = { new: "+2", "+2": "+5", "+5": "+10", "+10": "retired" };

// ── queries ──────────────────────────────────────────────────────

export async function getProblem(db: D1Database, lc: number): Promise<ProblemRow | null> {
  return db.prepare("SELECT * FROM problems WHERE lc_number = ?").bind(lc).first<ProblemRow>();
}

/** Reviews due on/before `date`, oldest first — the overflow carry order. */
export async function dueReviews(db: D1Database, date: string): Promise<ProblemRow[]> {
  const { results } = await db
    .prepare(
      `SELECT * FROM problems
       WHERE stage != 'retired' AND next_review IS NOT NULL AND next_review <= ?1
         AND (defer_until IS NULL OR defer_until <= ?1)
       ORDER BY next_review, lc_number`,
    )
    .bind(date)
    .all<ProblemRow>();
  return results;
}

/**
 * Blind drills: unsolved optional problems from topics ALREADY LEARNED —
 * scheduled in an earlier week (the current week's optional pool is reserved
 * for Saturday's gate) AND showing learning evidence: at least one of the
 * topic's core problems has entered the ladder. A skipped learning day never
 * feeds drills just because its calendar week lapsed. Never repeated,
 * seeded by date. Boosted topics contribute up to 2 extra.
 */
export async function pickDrills(
  db: D1Database,
  date: string,
  budget: number,
): Promise<ProblemRow[]> {
  if (budget <= 0) return [];
  const week = campaignWeek(date);
  const { results } = await db
    .prepare(
      `SELECT p.*, t.boost AS boost FROM problems p
       JOIN topics t ON t.issue = p.topic_issue
       WHERE p.set_label = 'optional' AND p.stage = 'new'
         AND p.lc_number NOT IN (SELECT lc_number FROM drill_pool_used)
         AND NOT EXISTS (SELECT 1 FROM attempts a WHERE a.lc_number = p.lc_number)
         AND EXISTS (SELECT 1 FROM problems c
                     WHERE c.topic_issue = p.topic_issue
                       AND c.set_label = 'core' AND c.stage != 'new')
       ORDER BY p.lc_number`,
    )
    .all<ProblemRow & { boost: number }>();
  const pool = results.filter((p) => {
    const w = topicWeek(p.topic_issue);
    return w !== undefined && w < week;
  });
  const boosted = pool.filter((p) => p.boost === 1);
  const regular = pool.filter((p) => p.boost !== 1);
  const boostPicks = sample(boosted, Math.min(2, budget), rng(`boost-${date}`));
  const regularPicks = sample(
    regular,
    Math.min(2, Math.max(0, budget - boostPicks.length)),
    rng(`drill-${date}`),
  );
  return [...boostPicks, ...regularPicks].slice(0, budget);
}

/** Consecutive days with ≥1 attempt, ending today or yesterday. */
export async function streak(db: D1Database, today: string): Promise<number> {
  const { results } = await db
    .prepare("SELECT DISTINCT date FROM attempts ORDER BY date DESC LIMIT 90")
    .all<{ date: string }>();
  const days = new Set(results.map((r) => r.date));
  let cursor = days.has(today) ? today : addDays(today, -1);
  let n = 0;
  while (days.has(cursor)) {
    n++;
    cursor = addDays(cursor, -1);
  }
  return n;
}

// ── the one write path ───────────────────────────────────────────

export interface LogOutcome {
  lc: number;
  title: string;
  kind: Kind;
  result: Result;
  stage: Stage;
  next_review: string | null;
  /** First-ever attempt — caller closes the sub-issue on pass. */
  first: boolean;
  issue: number;
  /** Email one-tap replay: nothing changed. */
  duplicate: boolean;
  error?: string;
}

/**
 * Record an attempt and move the ladder. Ladder semantics live here and only
 * here — email one-taps, webhook /done lines, and gate scoring all converge.
 *
 * Email idempotency comes from the partial unique index on
 * (lc_number, date, kind) WHERE source='email': a replayed link inserts
 * nothing and must not touch the ladder. Webhook corrections (pass then fail
 * on the same day) remain legal — every webhook attempt appends.
 */
export async function logAttempt(
  db: D1Database,
  opts: { lc: number; date: string; result: Result; source: "email" | "webhook"; gate?: boolean },
): Promise<LogOutcome> {
  const p = await getProblem(db, opts.lc);
  const nothing: LogOutcome = {
    lc: opts.lc,
    title: "",
    kind: "review",
    result: opts.result,
    stage: "new",
    next_review: null,
    first: false,
    issue: 0,
    duplicate: false,
  };
  if (!p) return { ...nothing, error: `LC ${opts.lc} is not in the curriculum` };
  if (p.stage === "retired") {
    return { ...nothing, title: p.title, issue: p.issue, error: `LC ${opts.lc} is already retired` };
  }

  const attempted = await db
    .prepare("SELECT 1 AS x FROM attempts WHERE lc_number = ? LIMIT 1")
    .bind(opts.lc)
    .first();
  const first = !attempted && p.stage === "new";
  const kind: Kind = opts.gate ? "gate" : first ? (p.set_label === "optional" ? "drill" : "first") : "review";

  const inserted = await db
    .prepare(
      `INSERT INTO attempts (lc_number, date, kind, result, source) VALUES (?, ?, ?, ?, ?)
       ON CONFLICT DO NOTHING`,
    )
    .bind(opts.lc, opts.date, kind, opts.result, opts.source)
    .run();
  if (opts.source === "email" && inserted.meta.changes === 0) {
    return { ...nothing, title: p.title, kind, stage: p.stage, next_review: p.next_review, issue: p.issue, duplicate: true };
  }

  // Ladder move. First-ever logs enter at +2 for pass AND fail.
  let stage: Stage;
  if (first) {
    stage = "+2";
  } else if (opts.result === "pass") {
    stage = NEXT_STAGE[p.stage] ?? "retired";
  } else {
    stage = "+2";
  }
  const next = stage === "retired" ? null : addDays(opts.date, INTERVAL[stage]!);

  const writes = [
    db.prepare(
      "UPDATE problems SET stage = ?, next_review = ?, defer_until = NULL WHERE lc_number = ?",
    ).bind(stage, next, opts.lc),
  ];
  if (first && kind === "drill") {
    writes.push(
      db.prepare("INSERT OR IGNORE INTO drill_pool_used (lc_number) VALUES (?)").bind(opts.lc),
    );
    if (opts.result === "fail") {
      writes.push(
        db.prepare("UPDATE topics SET misses = misses + 1 WHERE issue = ?").bind(p.topic_issue),
      );
    }
  }
  await db.batch(writes);

  return {
    lc: opts.lc,
    title: p.title,
    kind,
    result: opts.result,
    stage,
    next_review: next,
    first,
    issue: p.issue,
    duplicate: false,
  };
}