import { detectMux as realDetectMux } from "../mux/index.ts";
import { emitDiagnostic } from "../diagnostics/diagnostics.ts";

const warnedInvalidValues = new Set<string>();

// Indirection: the auto-mode branch reads through a mutable reference so
// tests can swap in a stub detector. Production callers still resolve
// `realDetectMux`; keeping this module-scoped preserves zero-arg call sites.
let detectMuxImpl: () => boolean = realDetectMux;

export type BackendKind = "pane" | "headless";

export function selectBackend(): BackendKind {
  const raw = (process.env.PI_SUBAGENT_MODE ?? "auto").toLowerCase();
  if (raw === "pane") return "pane";
  if (raw === "headless") return "headless";
  if (raw !== "auto" && !warnedInvalidValues.has(raw)) {
    warnedInvalidValues.add(raw);
    emitDiagnostic({
      code: "invalid-subagent-mode",
      audience: { human: true },
      message:
        `[subagents] invalid PI_SUBAGENT_MODE="${raw}"; ` +
        `using auto (valid: pane | headless | auto)\n`,
    });
  }
  return detectMuxImpl() ? "pane" : "headless";
}

export const __test__ = {
  resetWarnedValues(): void {
    warnedInvalidValues.clear();
  },
  setDetectMux(fn: () => boolean): void {
    detectMuxImpl = fn;
  },
  restoreDetectMux(): void {
    detectMuxImpl = realDetectMux;
  },
};