// WebGL renderer policy for attached terminals. Split out of attach.ts so it
// can be tested without a DOM or an xterm instance (same reasoning as
// reconnect.ts).
//
// xterm's default renderer builds DOM nodes per cell, which is CPU-bound —
// exactly the wrong bottleneck on a phone, where a full-screen repaint from an
// agent dumping output visibly stutters. The WebGL addon moves glyph drawing to
// a GPU texture atlas instead.
//
// The cost is that a WebGL context is a scarce, revocable resource:
//
//   * Browsers cap live contexts (~16 on desktop Chrome, fewer on Android) and
//     silently kill the oldest past the cap. `ConnectionPool` keeps MAX_PARKED
//     parked connections alive on top of the mounted ones, so handing every
//     connection its own context would blow that budget on its own and bring a
//     parked terminal back blank.
//   * Android drops contexts as routine housekeeping — backgrounding the tab,
//     locking the screen, memory pressure.
//
// So the context follows the *view*, not the connection: acquired on mount,
// released on park. Losses count against a shared budget, because a device that
// keeps dropping contexts is telling us it cannot sustain one.

/** Context losses tolerated before the page gives up and stays on the DOM
 *  renderer. One loss is ordinary (a tab switch); repeated losses mean the
 *  device is thrashing, and re-acquiring only makes it worse. */
export const MAX_CONTEXT_LOSSES = 2;

/** Can this browser hand out a WebGL2 context at all? */
export function probeWebgl2(): boolean {
  return document.createElement("canvas").getContext("webgl2") !== null;
}

/**
 * Whether an attached terminal should try the WebGL renderer right now.
 *
 * Shared by every connection on the page: the context budget and the "this
 * device cannot cope" signal are both global, not per-terminal.
 */
export class WebglPolicy {
  private supported: boolean | null = null;
  private losses = 0;

  constructor(private readonly probe: () => boolean = probeWebgl2) {}

  /** True while WebGL is both available and still worth attempting. The probe
   *  runs at most once — support does not change under us, and creating
   *  throwaway contexts to re-ask would itself consume the budget. */
  enabled(): boolean {
    if (this.losses >= MAX_CONTEXT_LOSSES) return false;
    if (this.supported === null) {
      try {
        this.supported = this.probe();
      } catch {
        // A browser that throws rather than returning null has no usable
        // context either way.
        this.supported = false;
      }
    }
    return this.supported;
  }

  /** Record a lost context. Past the budget, `enabled()` stays false for the
   *  life of the page. */
  noteContextLoss(): void {
    this.losses += 1;
  }

  /** Losses recorded so far — for the debug HUD and tests. */
  contextLosses(): number {
    return this.losses;
  }
}