import { createAttachConnection } from "./attach.js";
import type { AttachInfo } from "./paneTitle.js";
import type { PaneConnection } from "./pane.js";
import { createStructuredConnection } from "./structured.js";
import { setSessionFocus } from "./api.js";
import type { CatalogState, CatalogStore } from "./events.js";
import { sessionName, type SessionView, type Uuid } from "./types.js";

/**
 * How many *parked* (off-screen but still-connected) sessions to keep warm
 * beyond whatever is currently visible. Visible panes are never counted or
 * evicted — only this background buffer is bounded, so the browser doesn't hold
 * an unbounded fan of open sockets. Switching back to a recently-viewed session
 * is instant while it's still parked; older ones are evicted (WS closed) once
 * the buffer overflows.
 */
const MAX_PARKED = 6;

/** A session's view defaults to `"Terminal"` wherever it isn't carried —
 *  matching the wire default (`SessionView`'s `#[default]`, types.ts). Only
 *  meaningful once the session itself is known; an altogether-unknown
 *  session (no `AttachInfo`/`Session` at all) is a distinct, un-resolved
 *  state callers must check for separately, not "Terminal". */
function resolveView(view: SessionView | undefined): SessionView {
  return view ?? "Terminal";
}

export interface PoolOptions {
  /** Fired when the focused session changes (drives sidebar highlight + HUD). */
  onFocusChange?: (id: Uuid | null) => void;
  /** Fired when a view's explicit disconnect control is used; the embedder
   *  decides how to update the layout/route (then typically calls `close`). */
  onRequestClose?: (id: Uuid) => void;
  /** Fired when a connection's socket closes on its own (session ended/dropped),
   *  not via an intentional close/eviction. The embedder closes the pane. */
  onClosed?: (id: Uuid) => void;
  /** Forwarded to each connection: should a dropped socket be retried? */
  shouldReconnect?: (id: Uuid, code: number) => boolean;
  /** Threaded through to a structured connection so its rail can read live
   *  worktree/git-status data (#140). Terminal connections don't need it. */
  store?: CatalogStore;
}

/**
 * Owns the set of live attach connections, decoupled from the router/UI.
 *
 * Unlike the old single-active pool, several connections can be **mounted**
 * (visible) at once — one per leaf of the active tab's layout tree. The shell
 * reconciles the visible set with {@link setVisible}; sessions no longer shown
 * are **parked** in a hidden cache (still streaming) and bounded by an LRU. A
 * connection only dies on eviction, an explicit {@link close}, or {@link dispose}.
 */
export class ConnectionPool {
  /** All live connections. Insertion order doubles as LRU: oldest first. */
  private readonly cache = new Map<Uuid, PaneConnection>();
  /** The view each cached connection was actually created for — a deep link
   *  or a restored layout can `mount()` a session before the catalog
   *  snapshot names its view, defaulting it to `"Terminal"`; this is what
   *  lets a later reconciliation notice the resolved view disagrees (#140
   *  review). */
  private readonly connViews = new Map<Uuid, SessionView>();
  /** Sessions currently mounted into a visible pane (never evicted). */
  private readonly mounted = new Set<Uuid>();
  private readonly hidden: HTMLElement;
  private focusedId: Uuid | null = null;
  private readonly opts: PoolOptions;
  private readonly unsubscribeStore?: () => void;

  constructor(opts: PoolOptions = {}) {
    this.opts = opts;
    // Off-screen home for parked, still-connected terminals.
    this.hidden = document.createElement("div");
    this.hidden.className = "conn-cache";
    this.hidden.setAttribute("aria-hidden", "true");
    document.body.appendChild(this.hidden);
    // A session `mount()`ed before its catalog entry exists gets a
    // `"Terminal"` connection by default; once the snapshot actually names
    // the view, fix any connection that guessed wrong — nothing else
    // re-`mount()`s an already-open pane just because the catalog updated.
    this.unsubscribeStore = opts.store?.subscribe((state) => this.reconcileViews(state));
  }

  /** The focused session, or null when nothing is focused (welcome pane). */
  focused(): Uuid | null {
    return this.focusedId;
  }

  /** Back-compat alias for {@link focused} (used by the debug HUD). */
  active(): Uuid | null {
    return this.focusedId;
  }

  isMounted(id: Uuid): boolean {
    return this.mounted.has(id);
  }

  /**
   * Ensure `id` is live and shown inside `pane`, reusing its connection if one
   * exists (refreshing LRU recency). Does not touch other mounted panes.
   */
  mount(id: Uuid, pane: HTMLElement, info?: AttachInfo, opts?: { focus?: boolean }): void {
    let conn = this.cache.get(id);
    if (conn) {
      // `info === undefined` means "session not known yet", not "Terminal" —
      // never downgrade a resolved connection just because a later caller
      // couldn't resolve the session. Only a *resolved* view that disagrees
      // with what this connection was built for forces a replace.
      if (info !== undefined && resolveView(info.view) !== this.connViews.get(id)) {
        conn = this.replace(id, info);
      } else {
        this.cache.delete(id);
        this.cache.set(id, conn); // LRU refresh
        if (info) conn.updateInfo(info);
      }
    } else {
      conn = this.create(id, info);
      this.cache.set(id, conn);
    }
    if (conn.el.parentElement !== pane) {
      pane.replaceChildren(conn.el);
      conn.onMount(opts);
    } else if (!this.mounted.has(id)) {
      conn.onMount(opts);
    }
    this.mounted.add(id);
    this.evict();
  }

  private create(id: Uuid, info?: AttachInfo): PaneConnection {
    const view = resolveView(info?.view);
    this.connViews.set(id, view);
    const connOpts = {
      onRequestClose: (sid: Uuid) => this.opts.onRequestClose?.(sid),
      onClosed: (sid: Uuid) => this.opts.onClosed?.(sid),
      shouldReconnect: (sid: Uuid, code: number) => this.opts.shouldReconnect?.(sid, code) ?? false,
    };
    return view === "Structured"
      ? createStructuredConnection(id, connOpts, info, this.opts.store)
      : createAttachConnection(id, connOpts, info);
  }

  /** Dispose whatever is cached for `id` and cache a freshly created
   *  connection in its place. Does not touch the DOM or `mounted` — callers
   *  place the new element themselves. */
  private replace(id: Uuid, info?: AttachInfo): PaneConnection {
    this.cache.get(id)?.dispose();
    const fresh = this.create(id, info);
    this.cache.set(id, fresh);
    return fresh;
  }

  /** Proactively fix any cached connection whose view no longer matches the
   *  catalog — the only path that repairs a pane nothing will `mount()`
   *  again on its own (parked, or mounted but not currently being
   *  re-rendered by anything else). A session absent from `state` (not yet
   *  known, or removed) is left alone — that's `main.ts`'s
   *  `closePane`/eviction concern, not a view mismatch. */
  private reconcileViews(state: CatalogState): void {
    for (const id of [...this.cache.keys()]) {
      const session = state.sessions.get(id);
      if (session === undefined || resolveView(session.view) === this.connViews.get(id)) continue;
      const existing = this.cache.get(id)!;
      const parent = existing.el.parentElement;
      const wasMounted = this.mounted.has(id);
      // Capture before anything below touches the DOM: dispose()/el.remove()
      // detach `existing.el`, and once a focused descendant is detached the
      // browser moves `document.activeElement` to `<body>` — too late to
      // tell whether the keyboard was actually in this pane.
      const hadDomFocus = existing.el.contains(document.activeElement);
      const info: AttachInfo = { name: sessionName(session), command: session.command, view: session.view };
      const fresh = this.replace(id, info);
      existing.el.remove(); // don't rely on dispose() to clear the DOM
      if (parent && parent !== this.hidden) {
        parent.replaceChildren(fresh.el);
      } else {
        // Parked: `this.hidden` holds every parked connection's element as
        // siblings — append, never replaceChildren, or they'd all vanish.
        this.hidden.appendChild(fresh.el);
      }
      if (wasMounted) {
        // `focusedId` names the session the pool was last told to focus —
        // but a same-tab sidebar preview moves the keyboard into the
        // sidebar without clearing it (multiview.ts skips `setFocus` while
        // previewing), so `focusedId` alone can point at a pane the
        // keyboard already left. Only re-focus when the DOM agrees it
        // never did.
        fresh.onMount({ focus: this.focusedId === id && hadDomFocus });
      } else {
        fresh.onUnmount();
      }
    }
  }

  /** Park `id` back into the hidden cache (still connected) and stop showing it. */
  unmount(id: Uuid): void {
    if (!this.mounted.delete(id)) return;
    const conn = this.cache.get(id);
    if (conn) {
      this.hidden.appendChild(conn.el); // stays alive, just off-screen
      conn.onUnmount();
    }
  }

  /**
   * Reconcile the visible set to exactly `entries` (in render order): park any
   * mounted session not listed, mount/refresh each listed one in its pane.
   * `focus: false` shows the panes without taking the keyboard (preview).
   */
  setVisible(
    entries: { id: Uuid; pane: HTMLElement; info?: AttachInfo }[],
    opts?: { focus?: boolean },
  ): void {
    const next = new Set(entries.map((e) => e.id));
    for (const id of [...this.mounted]) {
      if (!next.has(id)) this.unmount(id);
    }
    for (const e of entries) this.mount(e.id, e.pane, e.info, opts);
  }

  /** Also tells the daemon which pane is focused (docs/SIDEBAR.md's
   *  `AwaitingInput` auto-clear) — the sidebar highlight never calls this,
   *  only a pane actually becoming the visible view. Best-effort: a failed
   *  report only delays the glow clearing. */
  setFocus(id: Uuid | null): void {
    if (id === this.focusedId) return;
    const previous = this.focusedId;
    this.focusedId = id;
    if (previous !== null) void setSessionFocus(previous, false).catch(() => {});
    if (id !== null) void setSessionFocus(id, true).catch(() => {});
    this.opts.onFocusChange?.(id);
  }

  /** Compat shim: show `id` alone in `pane` and focus it; park everything else. */
  activate(id: Uuid, pane: HTMLElement, info?: AttachInfo): void {
    this.setVisible([{ id, pane, info }]);
    this.setFocus(id);
  }

  /** Compat shim: park all visible panes and clear focus (e.g. welcome pane). */
  parkActive(): void {
    for (const id of [...this.mounted]) this.unmount(id);
    this.setFocus(null);
  }

  /** Put the keyboard back into the focused session's terminal. No-op when
   *  nothing is focused (welcome pane) or its connection is gone. */
  focusTerminal(): void {
    if (this.focusedId === null) return;
    this.cache.get(this.focusedId)?.focusTerminal();
  }

  /** Type raw bytes into the focused session's PTY. Returns false when there is
   *  nothing focused, or its connection is gone — the leader's literal Ctrl-K
   *  is a no-op over the welcome pane rather than an error
   *  (docs/COMMAND-BAR.md). */
  sendToFocused(data: string): boolean {
    if (this.focusedId === null) return false;
    const conn = this.cache.get(this.focusedId);
    if (!conn) return false;
    conn.sendInput(data);
    return true;
  }

  /** Fully disconnect + forget a session. */
  close(id: Uuid): void {
    const conn = this.cache.get(id);
    if (!conn) return;
    conn.dispose();
    this.cache.delete(id);
    this.connViews.delete(id);
    this.mounted.delete(id);
    if (this.focusedId === id) this.setFocus(null);
  }

  dispose(): void {
    this.unsubscribeStore?.();
    for (const conn of this.cache.values()) conn.dispose();
    this.cache.clear();
    this.connViews.clear();
    this.mounted.clear();
    this.hidden.remove();
  }

  /** Drop least-recently-viewed *parked* connections beyond the buffer cap. */
  private evict(): void {
    while (this.cache.size - this.mounted.size > MAX_PARKED) {
      let victim: Uuid | null = null;
      for (const id of this.cache.keys()) {
        if (!this.mounted.has(id)) {
          victim = id;
          break;
        }
      }
      if (victim === null) break;
      this.close(victim);
    }
  }
}