// Identity resolution for the MCP Watcher.
//
// Mirrors the layered resolution shipped in agent-ping (PR #1). The CLI
// has four layers (--as flag, env, $CLAUDE_HOME/ping-agent, ~/.ping-agent);
// the watcher subprocess has three (no argv override useful in MCP context).
//
// Spec: agent-ping spec §9; agent-watcher spec §4.4.

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

export interface ResolvedIdentity {
  agent: string;
  source: string;
}

/**
 * Resolve this process's agent identity.
 * Layers, first match wins:
 *   1. PING_AGENT_IDENTITY env var
 *   2. $CLAUDE_HOME/ping-agent (if CLAUDE_HOME is set and file exists)
 *   3. ~/.ping-agent
 *
 * Throws if no layer resolves.
 */
export function resolveIdentity(env: NodeJS.ProcessEnv = process.env): ResolvedIdentity {
  const envId = (env.PING_AGENT_IDENTITY ?? "").trim();
  if (envId) {
    return { agent: envId, source: "PING_AGENT_IDENTITY env var" };
  }

  const claudeHome = (env.CLAUDE_HOME ?? "").trim();
  if (claudeHome) {
    const f = join(claudeHome, "ping-agent");
    if (existsSync(f)) {
      const name = readFileSync(f, "utf8").trim();
      if (name) {
        return { agent: name, source: f };
      }
    }
  }

  const home = env.HOME ?? homedir();
  const defaultFile = join(home, ".ping-agent");
  if (existsSync(defaultFile)) {
    const name = readFileSync(defaultFile, "utf8").trim();
    if (name) {
      return { agent: name, source: defaultFile };
    }
  }

  throw new Error(
    `agent-watcher-mcp: no identity resolved. Set PING_AGENT_IDENTITY, ` +
      `$CLAUDE_HOME/ping-agent, or ${defaultFile}.`,
  );
}