import type {
  BackendEventPayload,
  BrowserEvent,
  FrontendEventPayload,
  MaglevdStatusPayload,
  VPPStatusPayload,
} from "../types";
import { fetchAllState } from "./rest";
import {
  applyBackendTransition,
  applyFrontendTransition,
  applyMaglevdStatus,
  applyVPPStatus,
  replaceAll,
} from "../stores/state";
import { pushEvent } from "../stores/events";

// openEventStream wires the SPA to /view/api/events. EventSource auto-
// reconnects with the Last-Event-ID header set, which the Go broker uses
// to replay events from its 30s ring buffer. A "resync" event tells us to
// refetch full state and redraw.
export function openEventStream(): EventSource {
  const es = new EventSource("/view/api/events");

  es.onmessage = (msg) => {
    try {
      const ev = JSON.parse(msg.data) as BrowserEvent;
      dispatch(ev);
    } catch (err) {
      console.error("sse parse error", err, msg.data);
    }
  };

  // "resync" is emitted as a named event so we can listen for it
  // without it going through the default onmessage dispatch.
  es.addEventListener("resync", async () => {
    try {
      const snaps = await fetchAllState();
      replaceAll(snaps);
    } catch (err) {
      console.error("resync refetch failed", err);
    }
  });

  es.onerror = (err) => {
    // EventSource handles reconnection on its own — just log.
    console.debug("sse error, browser will reconnect", err);
  };

  return es;
}

function dispatch(ev: BrowserEvent) {
  pushEvent(ev);
  switch (ev.type) {
    case "backend":
      // The reducer also recomputes effective weights across every
      // frontend so pool-failover transitions (primary ⇌ fallback)
      // reflect instantly, without waiting for the 30s refresh.
      applyBackendTransition(ev.maglevd, ev.payload as BackendEventPayload);
      break;
    case "frontend":
      applyFrontendTransition(ev.maglevd, ev.payload as FrontendEventPayload);
      break;
    case "maglevd-status":
      applyMaglevdStatus(ev.maglevd, ev.payload as MaglevdStatusPayload);
      break;
    case "vpp-status":
      applyVPPStatus(ev.maglevd, (ev.payload as VPPStatusPayload).state);
      break;
    case "log":
      // Log events are displayed in the DebugPanel but no longer
      // mutate the state tree. The previous vpp-lb-sync-as-*
      // routing was removed because a VIP-scoped event was being
      // naively written into every frontend that shared the
      // backend, corrupting effective_weight for frontends with
      // different per-pool configured weights. Backend state
      // changes (arriving via "backend" events above) are a
      // sufficient trigger for recomputing effective weights
      // locally, and the set-weight kebab action updates the
      // store directly via applyConfiguredWeight on its POST
      // success path.
      break;
  }
}