import { existsSync, readFileSync } from "node:fs";
import { join } from "node:path";

import { getAgentDir } from "@earendil-works/pi-coding-agent";

import type { RunnerBackend } from "./types.js";

export interface RunnerConfig {
  runnerBackend: RunnerBackend;
}

const DEFAULT_RUNNER_CONFIG: RunnerConfig = {
  runnerBackend: "in-process",
};

function parseRunnerBackend(value: unknown, path: string): RunnerBackend {
  if (value === "in-process" || value === "herdr") {
    return value;
  }
  throw new Error(
    `Invalid runnerBackend in ${path}: expected "in-process" or "herdr".`
  );
}

function readConfigFile(path: string): RunnerConfig | undefined {
  if (!existsSync(path)) {
    return undefined;
  }

  let raw: unknown;
  try {
    raw = JSON.parse(readFileSync(path, "utf-8"));
  } catch (err) {
    throw new Error(
      `Invalid runner config JSON in ${path}: ${err instanceof Error ? err.message : String(err)}`
    );
  }

  if (raw == null || typeof raw !== "object" || Array.isArray(raw)) {
    throw new Error(
      `Invalid runner config in ${path}: expected a JSON object.`
    );
  }

  const config = raw as { runnerBackend?: unknown };
  if (config.runnerBackend == null) {
    return undefined;
  }

  return {
    runnerBackend: parseRunnerBackend(config.runnerBackend, path),
  };
}

export function resolveRunnerConfig(cwd: string): RunnerConfig {
  const userPath = join(getAgentDir(), "subagents", "config.json");
  const projectPath = join(cwd, ".pi", "subagents", "config.json");

  return {
    ...DEFAULT_RUNNER_CONFIG,
    ...readConfigFile(userPath),
    ...readConfigFile(projectPath),
  };
}