leetcode/apps/api/src/srs.ts

/**
 * SRS domain: ET dates, the work week, the interval ladder, concept
 * temperature, deterministic sampling, and the one write path for attempts.
 *
 * D1 is the single source of truth. The stage names the review a problem must
 * pass NEXT (`+3` = due 3 working days after the last clean solve). Passing
 * advances new → +3 → +7 → retired; any failure resets to +3. A problem's
 * FIRST-ever log enters the ladder at +3 regardless of result: a pass earns a
 * 3-day review, a fail must be re-solved just as soon. Two rungs, 3 and 7 —
 * the same numbers as the `work/3` and `work/7` buckets a re-solve is
 * scaffolded into, and as the review-issue windows in
 * apps/cli/spaced-repetition.ts.
 *
 * Sunday is off, structurally: every scheduled date comes out of
 * workingDay(), which never returns one. Nothing "falls on" the rest day and
 * gets swallowed or carried — it is simply never booked there.
 *
 * 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();
}

// ── the work week ────────────────────────────────────────────────

/**
 * Sunday, the one day the campaign never schedules: topics run Mon–Fri
 * (data/schedule.json), the gate is Saturday, and the review windows below are
 * picked so a solve comes back on a working day.
 */
const REST_DAY = 0;

export function isRestDay(date: string): boolean {
  return weekdayOf(date) === REST_DAY;
}

/**
 * The `offset`-th working day counting from `from` (0 = `from` itself), with
 * Sundays skipped — the ONE place a scheduled date is minted.
 *
 * Work may slide later, never earlier: pulling a review back to Saturday would
 * shorten the very interval it exists to test, so a Sunday landing becomes
 * Monday. With the +3/+7 ladder that only ever happens to a Thursday solve's
 * 3-day review; +7 lands on the solve's own weekday, which is never a Sunday.
 */
export function workingDay(from: string, offset = 0): string {
  let cursor = isRestDay(from) ? addDays(from, 1) : from;
  for (let i = 0; i < offset; i++) {
    cursor = addDays(cursor, 1);
    if (isRestDay(cursor)) cursor = addDays(cursor, 1);
  }
  return cursor;
}

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;

/** Date the schedule teaches a topic on, if it teaches it at all. */
export function topicDate(topic: number): string | undefined {
  for (const [date, t] of Object.entries(SCHEDULE)) {
    if (t === topic) return date;
  }
  return undefined;
}

/** Week in which a topic was (or will be) taught. */
export function topicWeek(topic: number): number | undefined {
  const date = topicDate(topic);
  return date === undefined ? undefined : campaignWeek(date);
}

// ── 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 ─────────────────────────────────────────────────────────

/**
 * Review windows in days. One vocabulary for the whole campaign: these are the
 * ladder rungs, the `work/<n>` buckets the picker scaffolds into, and the
 * windows the `Spaced Repetition — <date>` issues are built from.
 *
 * 3 and 7 keep the rest day free. +7 returns a solve to its own weekday, and
 * of the five learning weekdays only Thursday's +3 touches a Sunday — which
 * workingDay() slides to Monday.
 */
export const WINDOWS = [3, 7] as const;
export type Window = (typeof WINDOWS)[number];

export type Stage = "new" | `+${Window}` | "retired";
/** Chart / stats order, low to high. */
export const STAGES: Stage[] = ["new", "+3", "+7", "retired"];
export type Result = "pass" | "fail";
export type Kind = "first" | "review" | "drill" | "gate";

/**
 * Catalog vocabulary, carried by the `diff:*` / `set:*` issue labels and
 * stored verbatim in `problems`. Listed low to high / most to least required,
 * which is the order every chart, table and digest section prints them in.
 */
export const DIFFICULTIES = ["easy", "medium", "hard"] as const;
export type Difficulty = (typeof DIFFICULTIES)[number];
export const SETS = ["core", "optional", "deferred"] as const;
export type SetLabel = (typeof SETS)[number];

/**
 * How warm a concept is: the age of a topic's most recent attempt, bucketed by
 * the ladder's own windows so the two cannot drift. Touched inside the first
 * window it is `hot`, inside the second `fresh`, inside one more full second
 * window `fading`; older than that — or never attempted — it is `cold`.
 *
 * A read-side label only: nothing is scheduled off it. It answers the question
 * the ladder cannot, because the ladder tracks problems and a topic can go
 * quiet for a fortnight while not one of its problems comes due.
 */
export const TEMPERATURES = ["hot", "fresh", "fading", "cold"] as const;
export type Temperature = (typeof TEMPERATURES)[number];

/** Warmest band first, with the age it tolerates. Past the last one is cold. */
const TEMPERATURE_MAX_AGE: [Temperature, number][] = [
  ["hot", WINDOWS[0]],
  ["fresh", WINDOWS[1]],
  ["fading", WINDOWS[1] * 2],
];

/** Days since a topic's last attempt → its band; null (never) is cold. */
export function temperatureOf(age: number | null): Temperature {
  if (age === null) return "cold";
  for (const [temperature, max] of TEMPERATURE_MAX_AGE) {
    if (age <= max) return temperature;
  }
  return "cold";
}

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> = { "+3": 3, "+7": 7 };
const NEXT_STAGE: Record<string, Stage> = { new: "+3", "+3": "+7", "+7": "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>();
}

/**
 * What "due" means, in one place: not retired, scheduled on/before ?1, and
 * past its deferral if it has one. Every caller that asks "what is open on
 * this day" — the digest, the review issue, the docs queue chart — shares
 * this clause so they cannot answer differently.
 */
export const DUE_WHERE = `stage != 'retired' AND next_review IS NOT NULL AND next_review <= ?1
    AND (defer_until IS NULL OR defer_until <= ?1)`;

/** 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 ${DUE_WHERE} ORDER BY next_review, lc_number`)
    .bind(date)
    .all<ProblemRow>();
  return results;
}

/** One problem the ladder wants re-solved today. */
export interface DueRow {
  lc: number;
  issue: number;
  title: string;
  difficulty: string;
  /** The rung being settled: also the `work/<n>` bucket the re-solve goes in. */
  stage: Stage;
  bucket: number;
  /** Scheduled day; earlier than `date` when a day was missed. */
  due: string;
  /** Days late (0 = due today), so a renderer can say so. */
  late: number;
  /** Last logged attempt, and how long ago — the "last seen" line. */
  last_seen: string | null;
  days_since: number | null;
}

/**
 * Everything the ladder wants re-solved on `date`, oldest first.
 *
 * The read side of "which problems are open today": the same rows the digest
 * mails, shaped for whoever renders them (the daily Spaced Repetition issue).
 * Nothing here writes, so it is safe to call repeatedly, and there is no
 * second schedule to drift — D1's ladder is the schedule.
 */
export async function dueToday(db: D1Database, date: string): Promise<{ date: string; due: DueRow[] }> {
  const { results } = await db
    .prepare(
      `SELECT lc_number, issue, title, difficulty, stage, next_review,
              (SELECT MAX(a.date) FROM attempts a WHERE a.lc_number = problems.lc_number) AS last_seen
         FROM problems WHERE ${DUE_WHERE} ORDER BY next_review, lc_number`,
    )
    .bind(date)
    .all<{
      lc_number: number;
      issue: number;
      title: string;
      difficulty: string;
      stage: Stage;
      next_review: string;
      last_seen: string | null;
    }>();
  return {
    date,
    due: results.map((r) => ({
      lc: r.lc_number,
      issue: r.issue,
      title: r.title,
      difficulty: r.difficulty,
      stage: r.stage,
      // "+3" -> 3: the rung's number IS its work/ bucket.
      bucket: Number(r.stage.slice(1)),
      due: r.next_review,
      late: daysBetween(r.next_review, date),
      last_seen: r.last_seen,
      days_since: r.last_seen === null ? null : daysBetween(r.last_seen, date),
    })),
  };
}

/** Reviews surfaced per day; the digest levels everything past this forward. */
export const REVIEW_CAP = 3;

/**
 * Load-level an overloaded review queue: everything due beyond today's
 * REVIEW_CAP is pushed to a concrete future WORKING day — at most REVIEW_CAP
 * per day, oldest first — instead of piling up as "due today". Runs every
 * digest morning, so a future day that grows past the cap (spill plus newly
 * maturing reviews) is simply re-levelled when it arrives. Idempotent within
 * a date: after one pass at most REVIEW_CAP problems remain due today, so a
 * second pass moves nothing. The spill starts tomorrow, or Monday when
 * tomorrow is the rest day.
 */
export async function levelReviews(db: D1Database, date: string): Promise<number> {
  const overflow = (await dueReviews(db, date)).slice(REVIEW_CAP);
  if (overflow.length === 0) return 0;
  await db.batch(
    overflow.map((p, i) =>
      db
        .prepare("UPDATE problems SET next_review = ? WHERE lc_number = ?")
        .bind(workingDay(addDays(date, 1), Math.floor(i / REVIEW_CAP)), p.lc_number),
    ),
  );
  return overflow.length;
}

/**
 * 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, gate scoring, and solutions
 * landing in work/ 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.
 *
 * Commit idempotency needs no index either. A commit names the rung the file
 * it landed in settles — `work/1` settles `new`, `work/3` settles `+3`,
 * `work/7` settles `+7` — and the stage check below writes only when the
 * ladder is actually standing on that rung. So re-pushing a solution, or the
 * reconciler re-sending its whole implemented set on every push, moves
 * nothing: the rung it names has already been left behind.
 */
export async function logAttempt(
  db: D1Database,
  opts: {
    lc: number;
    date: string;
    result: Result;
    source: "email" | "webhook" | "commit";
    /** Commit only: the stage this file settles. Default `new` = first solve. */
    rung?: Stage;
    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` };
  // A commit reports a file, not an event: it may only settle the rung that
  // file's bucket IS. Any other stage means the push is old news (the ladder
  // already moved past it) or premature — no write either way, and a retired
  // problem can never be woken. Fresh reviews otherwise come from the digest
  // tap, a /done comment, or a gate.
  if (opts.source === "commit" && p.stage !== (opts.rung ?? "new")) {
    return {
      ...nothing,
      title: p.title,
      stage: p.stage,
      next_review: p.next_review,
      issue: p.issue,
      duplicate: true,
    };
  }
  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 +3 for pass AND fail.
  let stage: Stage;
  if (first) {
    stage = "+3";
  } else if (opts.result === "pass") {
    stage = NEXT_STAGE[p.stage] ?? "retired";
  } else {
    stage = "+3";
  }
  // workingDay(), not addDays(): a Thursday solve's +3 would land on the rest
  // day, and it takes Monday instead.
  const next = stage === "retired" ? null : workingDay(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,
  };
}