import { useCallback, useEffect, useRef, useState } from 'react';
import { CircleAlert, Loader2, RefreshCw } from 'lucide-react';
import { useClient } from 'hooks/useClient';
import { Button } from '@/components/ui/button';
import { readableError } from './errors';

/**
 * Building `derived/` from `originals/`, shown where those two folders are.
 *
 * ── Why it is not its own section ──
 *
 * It was, briefly. But a section called "Derived media" sitting beside the block that lists
 * `originals/` and `derived/` is the same subject described twice, and the reader has to work out that
 * the two are about one thing. The action belongs to the folders it acts on, so it lives under them.
 *
 * ── It only appears once there is somewhere to put the output ──
 *
 * No `media/`, or a partial one, means the button would fail on a missing directory. The Create button
 * above it is the thing to press first, and offering both at once is offering a choice where there is
 * an order.
 *
 * ── It polls, and only while something is running ──
 *
 * A full pass takes minutes, so the request that starts it returns immediately and this asks how it is
 * going. The interval is torn down when the answer says it has stopped, so an idle panel makes no
 * requests — which is what keeps a screen somebody left open from being a request a second all
 * afternoon.
 *
 * ── Built and skipped are counted separately ──
 *
 * Re-running is normal and cheap: everything already correct is skipped, because each derived folder
 * carries the size and mtime it was built from. "412 already current" is the system saying it had
 * nothing to do, which is a different and more useful answer than "412 done".
 *
 * ── Failures stay ──
 *
 * One bad file in four hundred is exactly what a progress bar hides, and it is the only part of this
 * anybody has to act on.
 */

type SyncError = { file: string; message: string };

type SyncState = {
  running: boolean;
  total: number | null;
  done: number;
  built: number;
  skipped: number;
  errors: SyncError[];
  current: string[];
  startedAt: string | null;
  finishedAt: string | null;
  fatal: string | null;
};

type Run = {
  startedAt: string;
  finishedAt: string;
  trigger: 'manual' | 'watcher';
  scope: string;
  total: number;
  built: number;
  skipped: number;
  errors: SyncError[];
  fatal: string | null;
};

type Reconciliation = { originals: number; current: number; missing: string[]; stale: string[] };

type Ledger = { runs: Run[]; reconciliation: Reconciliation };

/** How often to ask, while something is happening. Fast enough to look live, slow enough to be free. */
const POLL_MS = 1000;

/** Recent runs worth showing. Enough to see a pattern, few enough to stay a footnote. */
const SHOWN_RUNS = 5;

const clock = (iso: string) =>
  new Date(iso).toLocaleTimeString(undefined, { hour: '2-digit', minute: '2-digit' });

const day = (iso: string) => new Date(iso).toLocaleDateString(undefined, { day: 'numeric', month: 'short' });

/** `1m 12s`, or `8s`. Minutes matter here and hours have never happened. */
const took = (from: string, to: string) => {
  const seconds = Math.max(0, Math.round((Date.parse(to) - Date.parse(from)) / 1000));
  return seconds < 60 ? `${seconds}s` : `${Math.floor(seconds / 60)}m ${seconds % 60}s`;
};

export const MediaSync = ({ onSettled }: { onSettled?: (settled: boolean) => void }) => {
  const client = useClient();
  const clientRef = useRef(client);
  clientRef.current = client;

  const [state, setState] = useState<SyncState | null>(null);
  const [ledger, setLedger] = useState<Ledger | null>(null);
  const [busy, setBusy] = useState(false);
  const [error, setError] = useState<string | null>(null);

  const load = useCallback(async () => {
    try {
      setState(await clientRef.current.get<SyncState>('/fofos-admin/_config/media/sync'));
    } catch {
      // Owner-only. Root Location above has already said so once if that is the reason.
      setState(null);
    }
  }, []);

  /*
    The ledger is NOT polled.

    It walks the whole tree reading manifests, so asking for it once a second beside a progress bar
    would have every tick re-read the library. It is read on arrival and again when a run stops, which
    are the only two moments its answer can have changed.
  */
  const loadLedger = useCallback(async () => {
    try {
      setLedger(await clientRef.current.get<Ledger>('/fofos-admin/_config/media/ledger'));
    } catch {
      setLedger(null);
    }
  }, []);

  useEffect(() => {
    void load();
    void loadLedger();
  }, [load, loadLedger]);

  useEffect(() => {
    if (!state?.running) return;
    const timer = setInterval(() => void load(), POLL_MS);
    return () => clearInterval(timer);
  }, [state?.running, load]);

  // Re-read when a run STOPS. `running` going true→false is the edge, so a finished sync updates what
  // is left to do without the panel being reopened.
  const wasRunning = useRef(false);
  useEffect(() => {
    const running = state?.running ?? false;
    if (wasRunning.current && !running) void loadLedger();
    wasRunning.current = running;
  }, [state?.running, loadLedger]);

  const start = async () => {
    setBusy(true);
    setError(null);
    try {
      const result = await clientRef.current.post<{ started: boolean; state: SyncState }>(
        '/fofos-admin/_config/media/sync',
        {},
      );
      setState(result.state);
    } catch (caught) {
      setError(readableError(caught));
    } finally {
      setBusy(false);
    }
  };

  const running = state?.running ?? false;
  const total = state?.total ?? null;
  const done = state?.done ?? 0;
  // Null total means the walk has not finished, so there is nothing honest to divide by yet.
  const percent = total && total > 0 ? Math.round((done / total) * 100) : null;
  const reconciliation = ledger?.reconciliation ?? null;
  const outstanding = (reconciliation?.missing.length ?? 0) + (reconciliation?.stale.length ?? 0);

  /*
    One bit, upward: is there anything left to do?

    Reported rather than lifted, because everything else about this — the polling, the run history, the
    errors — belongs here and would only be harder to read in a parent. What the parent needs is the
    single fact it cannot compute: whether this part of the setup is finished.
  */
  useEffect(() => {
    if (reconciliation) onSettled?.(outstanding === 0);
  }, [reconciliation, outstanding, onSettled]);

  return (
    <div className="flex flex-col gap-2 pt-1">
      <div className="flex items-center justify-between gap-3">
        {/* Not what the last run did — what is TRUE NOW. A run's counts age the moment somebody uploads
            a file, and "62 built" three days ago says nothing about whether derived/ is current. This is
            recomputed from the filesystem, using the same freshness test the sync itself uses. */}
        <p className="min-w-0 flex-1 truncate text-xs text-muted-foreground">
          {running
            ? total === null
              ? 'Reading originals…'
              : `${done} of ${total}${state?.current.length ? ` · ${state.current.join(', ')}` : ''}`
            : reconciliation
              ? outstanding === 0
                ? `${reconciliation.current} of ${reconciliation.originals} current`
                : [
                    reconciliation.missing.length ? `${reconciliation.missing.length} missing` : '',
                    reconciliation.stale.length ? `${reconciliation.stale.length} changed` : '',
                  ]
                    .filter(Boolean)
                    .join(' · ')
              : 'Web-ready copies of everything in originals.'}
        </p>

        <Button variant="outline" size="sm" className="shrink-0" onClick={() => void start()} disabled={busy || running}>
          {running ? <Loader2 className="size-3.5 animate-spin" /> : <RefreshCw className="size-3.5" />}
          {running ? `Syncing${percent === null ? '' : ` ${percent}%`}` : outstanding > 0 ? `Sync ${outstanding}` : 'Sync now'}
        </Button>
      </div>

      {running && (
        <div className="h-1 w-full overflow-hidden rounded-full bg-muted">
          {/* Indeterminate until the walk has counted: a bar that starts full and shrinks is worse than
              one that has not started. */}
          <div
            className="h-full rounded-full bg-primary transition-[width] duration-500"
            style={{ width: `${percent ?? 0}%` }}
          />
        </div>
      )}

      {state?.fatal && (
        <p className="flex items-start gap-1.5 text-xs text-destructive">
          <CircleAlert className="mt-px size-3.5 shrink-0" />
          {state.fatal}
        </p>
      )}

      {state && state.errors.length > 0 && (
        <ul className="max-h-32 space-y-1 overflow-auto rounded border border-border/60 p-2">
          {state.errors.map((failure) => (
            <li key={failure.file} className="text-xs">
              <span className="font-medium">{failure.file}</span>
              <span className="text-muted-foreground"> — {failure.message}</span>
            </li>
          ))}
        </ul>
      )}

      {error && <p className="text-xs text-destructive">{error}</p>}

      {/*
        The ledger. Survives a restart, which the live state above does not — and it is the only place a
        failure from a run nobody watched is ever recorded, since a file that did not build leaves no
        folder and no manifest behind it.
      */}
      {ledger && ledger.runs.length > 0 && (
        <ul className="space-y-0.5 pt-1">
          {ledger.runs.slice(0, SHOWN_RUNS).map((run) => (
            <li key={run.finishedAt} className="flex items-baseline gap-2 text-xs text-muted-foreground/70">
              <span className="shrink-0 font-mono">
                {day(run.finishedAt)} {clock(run.finishedAt)}
              </span>
              <span className="min-w-0 flex-1 truncate">
                {run.scope ? `${run.scope} · ` : ''}
                {run.built} built
                {run.skipped ? `, ${run.skipped} current` : ''}
                {run.errors.length ? `, ${run.errors.length} failed` : ''}
                {run.fatal ? ` — ${run.fatal}` : ''}
                {run.trigger === 'watcher' ? ' · on upload' : ''}
              </span>
              <span className="shrink-0 font-mono">{took(run.startedAt, run.finishedAt)}</span>
            </li>
          ))}
        </ul>
      )}
    </div>
  );
};