import { existsSync } from "node:fs";
import { join } from "node:path";
import { getAgentPath, readJsonFile } from "./agent-paths";

/**
 * Loaded config value plus the path or `defaults` marker that supplied it.
 * The `config` field is always the caller's merged output type, even when no file exists.
 * `source` is presentation/debug metadata and is not guaranteed to be a readable file when it equals `defaults`.
 */
export interface LoadedScopedConfig<T> {
  config: T;
  source: string;
}

/**
 * Resolves a config-relative path to project-local and agent-level candidates.
 * The result includes `.pi/<relativePath>` under `cwd` and the agent path from `getAgentPath`.
 * No filesystem checks are performed here, so callers can inspect or load candidates in their own order.
 */
export function resolveScopedPath(cwd: string, relativePath: string): { projectPath: string; agentPath: string } {
  return {
    projectPath: join(cwd, ".pi", relativePath),
    agentPath: getAgentPath(relativePath),
  };
}

/**
 * Loads the project-scoped config if present, otherwise the agent-scoped config, otherwise defaults.
 * The returned config is produced by the supplied merge function with parsed JSON or `null`.
 * This helper reads from disk and intentionally does not merge project and agent files together.
 */
export function loadScopedConfig<TInput, TOutput>(
  cwd: string,
  relativePath: string,
  defaults: TOutput,
  merge: (defaults: TOutput, input: TInput | null) => TOutput,
): LoadedScopedConfig<TOutput> {
  const { projectPath, agentPath } = resolveScopedPath(cwd, relativePath);

  if (existsSync(projectPath)) {
    return {
      config: merge(defaults, readJsonFile<TInput | null>(projectPath, null)),
      source: projectPath,
    };
  }

  if (existsSync(agentPath)) {
    return {
      config: merge(defaults, readJsonFile<TInput | null>(agentPath, null)),
      source: agentPath,
    };
  }

  return {
    config: merge(defaults, null),
    source: "defaults",
  };
}