/**
 * Composition root.
 *
 * The only module permitted to import from every layer, and the only place
 * where the pure core is joined to the impure edge. Everything above this file
 * is testable without a browser; everything below it is a browser detail.
 *
 * Read this file to understand the dataflow. It is deliberately linear:
 *
 *   load corpus  ->  restore state  ->  render  ->  observe  ->  re-rank  ->  render
 *                                          ^                                    |
 *                                          +------------------------------------+
 */

import { loadCorpus } from '../adapters/corpus.js';
import { createStatePort } from '../adapters/storage.js';
import { createObserver } from '../adapters/observer.js';
import { rank, applyEvent, recordImpressions, initialState, DEFAULT_CONFIG } from '../core/rank/rank.js';
import { applyOrder, bindWhy } from '../ui/feed.js';
import { createTerrain } from '../ui/terrain.js';

/** Re-rank at most this often. Re-ordering under the reader's cursor is hostile. */
const RERANK_IDLE_MS = 2600;

/** Yield to the main thread, with a fallback for engines without the Scheduler API. */
async function yieldToMain() {
  if ('scheduler' in globalThis && 'yield' in globalThis.scheduler) {
    await globalThis.scheduler.yield();
  } else {
    await new Promise((r) => setTimeout(r, 0));
  }
}

async function boot() {
  const feedEl = document.querySelector('[data-feed]');
  if (!feedEl) return;

  const { items, dim } = await loadCorpus(
    feedEl.dataset.corpus ?? '/content/corpus.json',
    feedEl.dataset.embeddings ?? '/content/embeddings.bin',
  );

  const index = new Map(items.map((i) => [i.id, i]));
  const indexObj = Object.fromEntries(index);
  const topics = [...new Set(items.map((i) => i.topic))].sort();

  const store = createStatePort(dim);
  let state = store.load() ?? initialState(dim);

  // Session seed: stable for this tab so a re-render is not a re-shuffle, but
  // different between visitors so the site is not identical for everyone.
  const seed = (() => {
    try {
      const k = 'ftj.seed';
      const found = sessionStorage.getItem(k);
      if (found) return Number(found);
      const s = (Date.now() ^ (Math.random() * 0xffffffff)) >>> 0;
      sessionStorage.setItem(k, String(s));
      return s;
    } catch { return 20260907; }
  })();

  let round = 0;
  let dirty = false;
  let timer = 0;

  // The terrain needs the projected manifold — the 2D shape of the corpus.
  // It is a separate fetch because the feed is fully usable without it: if this
  // request fails the page loses its drawing, not its content.
  const canvas = document.querySelector('[data-instrument]');
  let terrain = null;
  let manifold = null;
  if (canvas) {
    try {
      const res = await fetch(feedEl.dataset.manifold ?? '/content/manifold.json', { credentials: 'omit' });
      if (res.ok) {
        manifold = await res.json();
        terrain = createTerrain(canvas, manifold);
      }
    } catch {
      /* No drawing. The document stands on its own. */
    }
  }
  /** itemId -> its position on the manifold. */
  const place = new Map((manifold?.points ?? []).map((p) => [p.id, p]));

  /** Posterior means, normalised — "how much does the model like each topic". */
  function topicMix() {
    const means = topics.map((t) => {
      const p = state.topics[t] ?? { alpha: DEFAULT_CONFIG.priorAlpha, beta: DEFAULT_CONFIG.priorBeta };
      return p.alpha / (p.alpha + p.beta);
    });
    const sum = means.reduce((s, m) => s + m, 0) || 1;
    return means.map((m) => m / sum);
  }

  /**
   * Where on the manifold should the light fall?
   *
   * Not a projection of the interest vector — projecting a 384-dimensional
   * vector into a 2D layout that was fitted to *distances* would put the light
   * somewhere arbitrary. Instead, take the terrain positions of the entries the
   * model currently ranks highest and use their weighted centroid. The light
   * lands where the work it is recommending actually sits, which is both
   * correct and legible: you can see it move to the region it is drawn to.
   *
   * @param {Array<{item: object}>} ranked
   */
  function focusOf(ranked) {
    let sx = 0; let sy = 0; let sw = 0;
    for (let i = 0; i < Math.min(6, ranked.length); i += 1) {
      const p = place.get(ranked[i].item.id);
      if (!p) continue;
      const w = Math.exp(-i / 2.5);      // the top entry dominates
      sx += p.x * w; sy += p.y * w; sw += w;
    }
    if (sw === 0) return null;
    return { x: sx / sw, y: sy / sw };
  }

  async function rerank({ animate = true } = {}) {
    const ranked = rank(items, state, { nowMs: Date.now(), seed, round, limit: items.length });
    round += 1;
    await yieldToMain();
    applyOrder(feedEl, ranked, { animate });
    state = recordImpressions(state, ranked.slice(0, 6).map((r) => r.item.id));
    store.save(state);
    const focus = focusOf(ranked);
    if (focus) terrain?.update({ focus });
    renderState();
  }

  const stateEl = document.querySelector('[data-state-topics]');
  function renderState() {
    if (!stateEl) return;
    const mix = topicMix();
    stateEl.innerHTML = topics.map((t, i) => {
      const p = state.topics[t] ?? { alpha: 1, beta: 1 };
      const evidence = p.alpha + p.beta - 2;
      return `<div class="belief__t">
        <span class="belief__n"><span>${t}</span><b>${(mix[i] * 100).toFixed(0)}%</b></span>
        <span class="bar"><i style="--v:${mix[i].toFixed(3)}"></i></span>
        <span class="belief__n" style="color:var(--ink-faint)"><span>${evidence > 0 ? `${evidence.toFixed(1)} observations` : 'no evidence yet'}</span></span>
      </div>`;
    }).join('');
  }

  function scheduleRerank() {
    dirty = true;
    clearTimeout(timer);
    timer = setTimeout(() => { if (dirty) { dirty = false; rerank(); } }, RERANK_IDLE_MS);
  }

  const observer = createObserver({
    root: feedEl,
    index,
    onSignal(ev) {
      state = applyEvent(state, ev, indexObj);
      scheduleRerank();
    },
  });
  for (const el of feedEl.querySelectorAll('[data-item-id]')) observer.observe(el);

  bindWhy(feedEl);
  document.documentElement.classList.add('is-live');

  // First pass is not animated: the reader has not asked for anything yet, and
  // a page that rearranges itself on arrival is disorienting rather than clever.
  await rerank({ animate: false });

  const reset = document.querySelector('[data-reset]');
  reset?.addEventListener('click', () => {
    store.clear();
    state = initialState(dim);
    round = 0;
    rerank();
  });
}

boot().catch((err) => {
  // A failure here must leave a readable document behind, not a blank page.
  console.error('[feedthejoe] runtime failed; static document remains intact', err);
});