/**
 * Domain library for the spaced-repetition system (SRS).
 *
 * State model — `.github/srs/srs.json` is the single source of truth; the
 * GitHub Project fields are a best-effort mirror (see srs-project.ts).
 *
 * The stage names the review a problem must pass NEXT: a problem at `+2` has
 * a review due 2 days after its last clean solve. Passing advances
 * new → +2 → +5 → +10 → retired; any failure resets to +2. `new` is reserved
 * for deferred Hards that have never been solved — `defer_until` keeps them
 * out of the queue until Sep 28.
 *
 * All dates are America/New_York calendar dates (YYYY-MM-DD): the cron fires
 * at 10:00 UTC = 6 AM ET, and the campaign is lived in ET.
 */
import { join } from "node:path";

import type { GitHub } from "./github.ts";

const ROOT = join(import.meta.dir, "..");

// ── state ────────────────────────────────────────────────────────

export type Stage = "new" | "+2" | "+5" | "+10" | "retired";
export type Result = "pass" | "fail";

export interface Attempt {
  date: string;
  /** first = learning-day solve, drill = blind drill, gate = gate problem. */
  kind: "first" | "review" | "drill" | "gate";
  result: Result;
}

export interface ProblemState {
  issue: number;
  topic: number;
  difficulty: string;
  set: string;
  solved_on?: string;
  stage: Stage;
  next_review?: string;
  /** Deferred Hards stay out of the due queue until this date. */
  defer_until?: string;
  history: Attempt[];
}

export interface TopicState {
  /** Blind-drill recognition misses since the last boost reset. */
  misses: number;
  /** Set at gate close; scheduler injects extra drills; next gate clears it. */
  boost: boolean;
}

export interface GateState {
  week: number;
  issue: number;
  date: string;
  problems: number[];
  /** Problems that fell back to the core set (optional pool exhausted). */
  fallbacks: number[];
  results: Record<string, Result>;
  rate?: number;
}

export interface State {
  problems: Record<string, ProblemState>;
  topics: Record<string, TopicState>;
  drill_pool_used: number[];
  gates: GateState[];
}

export const STATE_PATH = process.env.SRS_STATE ?? join(ROOT, ".github", "srs", "srs.json");
export const SCHEDULE_PATH =
  process.env.SRS_SCHEDULE ?? join(ROOT, ".github", "srs", "schedule.json");

export async function loadState(): Promise<State> {
  return JSON.parse(await Bun.file(STATE_PATH).text());
}

export async function saveState(state: State): Promise<void> {
  await Bun.write(STATE_PATH, `${JSON.stringify(state, null, 2)}\n`);
}

/** Day (YYYY-MM-DD) → topic issue number. Human-edited when life happens. */
export async function loadSchedule(): Promise<Record<string, number>> {
  return JSON.parse(await Bun.file(SCHEDULE_PATH).text());
}

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

/** Campaign day 1 — Monday of Phase I week 1. Day 56 = Oct 11. */
export const CAMPAIGN_START = "2026-08-17";
export const CAMPAIGN_DAYS = 56;

/** Today's ET calendar date; SRS_TODAY overrides for tests and reruns. */
export function todayET(): string {
  if (process.env.SRS_TODAY) return process.env.SRS_TODAY;
  return new Intl.DateTimeFormat("en-CA", {
    timeZone: "America/New_York",
    dateStyle: "short",
  }).format(new Date());
}

/** 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();
}

/** 1-based campaign day; may exceed CAMPAIGN_DAYS after the camp ends. */
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;
}

export function isoWeek(date: string): number {
  const d = atNoon(date);
  // ISO 8601: week containing the year's first Thursday is week 1.
  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 interval ladder ──────────────────────────────────────────

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" };

/** Advance on a pass; returns false when the problem was already retired. */
export function pass(p: ProblemState, today: string): boolean {
  const next = NEXT_STAGE[p.stage];
  if (!next) return false;
  p.stage = next;
  p.solved_on = today;
  if (next === "retired") {
    delete p.next_review;
  } else {
    p.next_review = addDays(today, INTERVAL[next]!);
  }
  return true;
}

/** Reset on a failure: back to the +2 stage, due again in 2 days. */
export function fail(p: ProblemState, today: string): void {
  p.stage = "+2";
  p.next_review = addDays(today, 2);
}

/** Reviews due today or earlier, oldest first — the overflow carry order. */
export function dueReviews(state: State, today: string): [string, ProblemState][] {
  return Object.entries(state.problems)
    .filter(
      ([, p]) =>
        p.stage !== "retired" &&
        p.next_review !== undefined &&
        p.next_review <= today &&
        (p.defer_until === undefined || p.defer_until <= today),
    )
    .sort(([a, pa], [b, pb]) =>
      pa.next_review === pb.next_review
        ? Number(a) - Number(b)
        : pa.next_review! < pb.next_review!
          ? -1
          : 1,
    );
}

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

/** FNV-1a → mulberry32: a 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;
  };
}

/** Take 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);
}

// ── the live catalog (topics + problem sub-issues) ───────────────

export interface CatalogProblem {
  lc: number;
  issue: number;
  name: string;
  difficulty: "Easy" | "Medium" | "Hard";
  set: "core" | "optional" | "deferred";
  topic: number;
  open: boolean;
  url: string;
}

export interface CatalogTopic {
  issue: number;
  /** e.g. "Topic 03 — Two Pointers". */
  title: string;
  /** Bare name, e.g. "Two Pointers". */
  name: string;
  milestone: number | undefined;
}

export interface Catalog {
  topics: Map<number, CatalogTopic>;
  problems: Map<number, CatalogProblem>;
  byIssue: Map<number, CatalogProblem>;
}

const TITLE_RE = /^LC (\d+) · (.+) · (Easy|Medium|Hard) · (core|optional|deferred)$/;

/**
 * Walk every `topic` issue and its sub-issues into one lookup structure.
 * The sub-issue linkage is authoritative: every problem hangs off exactly
 * one topic (verified against the full issue inventory).
 */
export async function fetchCatalog(gh: GitHub): Promise<Catalog> {
  const topics = new Map<number, CatalogTopic>();
  for await (const raw of gh.list(`/repos/${gh.repo}/issues?labels=topic&state=all`)) {
    const t = raw as {
      number: number;
      title: string;
      pull_request?: unknown;
      milestone?: { number: number } | null;
    };
    if (t.pull_request) continue;
    const name = t.title.replace(/^Topic \d+ — /, "");
    topics.set(t.number, {
      issue: t.number,
      title: t.title,
      name,
      milestone: t.milestone?.number,
    });
  }

  const problems = new Map<number, CatalogProblem>();
  const byIssue = new Map<number, CatalogProblem>();
  for (const topic of topics.keys()) {
    for await (const raw of gh.list(`/repos/${gh.repo}/issues/${topic}/sub_issues`)) {
      const s = raw as { number: number; title: string; state: string; body?: string };
      const m = s.title.match(TITLE_RE);
      if (!m) continue; // non-curriculum sub-issue
      const url = (s.body ?? "").match(/https:\/\/leetcode\.com\/problems\/[a-z0-9-]+\/?/)?.[0];
      const p: CatalogProblem = {
        lc: Number(m[1]),
        issue: s.number,
        name: m[2]!,
        difficulty: m[3] as CatalogProblem["difficulty"],
        set: m[4] as CatalogProblem["set"],
        topic,
        open: s.state === "open",
        url: url ?? "",
      };
      problems.set(p.lc, p);
      byIssue.set(p.issue, p);
    }
  }
  return { topics, problems, byIssue };
}

// ── shared constants ─────────────────────────────────────────────

export const TODAY_TITLE = "📋 Today";