/**
 * Minimal Model Context Protocol (MCP) server over stdio.
 *
 * Implements just enough of the MCP JSON-RPC 2.0 surface to expose
 * a static list of read-only validation tools to a Claude Code client:
 *   - initialize
 *   - tools/list
 *   - tools/call
 *
 * We intentionally avoid the @modelcontextprotocol/sdk dependency here —
 * the KIS-TOiR optimization policy forbids adding new npm packages for
 * these lightweight validators, so we speak raw JSON-RPC on stdin/stdout
 * with Node built-ins only.
 *
 * Tool contract:
 *   {
 *     name: string,
 *     description: string,
 *     inputSchema?: JSONSchema,
 *     execute: (args: object) => Promise<{ success: boolean, result?, error? }>
 *   }
 *
 * execute() must be read-only, must resolve quickly (<500ms target), and
 * must NEVER throw — wrap failures in { success: false, error }.
 */

import readline from "node:readline";

const PROTOCOL_VERSION = "2024-11-05";

// JSON-RPC 2.0 constants
const JSONRPC_VERSION = "2.0";
const FIELD_JSONRPC = "jsonrpc";
const FIELD_ID = "id";
const FIELD_METHOD = "method";
const FIELD_PARAMS = "params";
const FIELD_RESULT = "result";
const FIELD_ERROR = "error";
const FIELD_CODE = "code";
const FIELD_MESSAGE = "message";
const FIELD_DATA = "data";
const CONTENT_TYPE_TEXT = "text";
const FIELD_CONTENT = "content";
const FIELD_TYPE = "type";
const FIELD_IS_ERROR = "isError";

function write(obj) {
  // MCP framing over stdio is line-delimited JSON (one JSON object per line).
  process.stdout.write(JSON.stringify(obj) + "\n");
}

function ok(id, result) {
  write({ [FIELD_JSONRPC]: JSONRPC_VERSION, [FIELD_ID]: id, [FIELD_RESULT]: result });
}

function fail(id, code, message, data) {
  write({
    [FIELD_JSONRPC]: JSONRPC_VERSION,
    [FIELD_ID]: id,
    [FIELD_ERROR]: { [FIELD_CODE]: code, [FIELD_MESSAGE]: message, [FIELD_DATA]: data },
  });
}

/**
 * Start an MCP server. Blocks on stdin until the parent process closes it.
 */
export function runMCPServer({ name, version = "0.1.0", tools }) {
  const byName = new Map(tools.map((t) => [t.name, t]));

  const rl = readline.createInterface({
    input: process.stdin,
    crlfDelay: Infinity,
  });

  // Log to stderr so we don't corrupt the stdout JSON-RPC channel.
  const log = (level, msg) =>
    process.stderr.write(`[${level}] ${name}: ${msg}\n`);

  log("INFO", `starting (tools=${tools.length})`);

  rl.on("line", async (line) => {
    const trimmed = line.trim();
    if (!trimmed) return;

    let req;
    try {
      req = JSON.parse(trimmed);
    } catch (e) {
      log("WARN", `invalid JSON on stdin: ${e.message}`);
      return;
    }

    const { id, method, params } = req;

    // Notifications have no id and expect no response.
    const isNotification = id === undefined || id === null;

    try {
      if (method === "initialize") {
        ok(id, {
          protocolVersion: PROTOCOL_VERSION,
          capabilities: { tools: { listChanged: false } },
          serverInfo: { name, version },
        });
        return;
      }

      if (method === "tools/list") {
        ok(id, {
          tools: tools.map((t) => ({
            name: t.name,
            description: t.description,
            inputSchema: t.inputSchema ?? {
              [FIELD_TYPE]: "object",
              properties: {},
              additionalProperties: true,
            },
          })),
        });
        return;
      }

      if (method === "tools/call") {
        const toolName = params?.name;
        const tool = byName.get(toolName);
        if (!tool) {
          fail(id, -32601, `Unknown tool: ${toolName}`);
          return;
        }

        let result;
        try {
          // Enforce per-tool timeout: tools MUST resolve within 2000ms.
          // Prevents hung tools from blocking the orchestrator.
          const toolTimeout = new Promise((_, reject) =>
            setTimeout(
              () => reject(new Error("Tool execution timeout (>2000ms)")),
              2000
            )
          );
          result = await Promise.race([
            tool.execute(params?.arguments ?? {}),
            toolTimeout,
          ]);
        } catch (e) {
          // Capture all errors (timeout, execution, etc.) uniformly.
          result = { success: false, error: e?.message ?? String(e) };
        }

        ok(id, {
          [FIELD_CONTENT]: [
            {
              [FIELD_TYPE]: CONTENT_TYPE_TEXT,
              text:
                typeof result === "string"
                  ? result
                  : JSON.stringify(result, null, 2),
            },
          ],
          [FIELD_IS_ERROR]: result?.success === false,
        });
        return;
      }

      if (method?.startsWith("notifications/")) {
        // Client lifecycle notifications (initialized, cancelled, ...).
        return;
      }

      if (!isNotification) {
        fail(id, -32601, `Method not found: ${method}`);
      }
    } catch (e) {
      log("ERROR", `handler crashed: ${e?.stack ?? e}`);
      if (!isNotification) {
        fail(id, -32603, "Internal server error", { message: e?.message });
      }
    }
  });

  rl.on("close", () => {
    log("INFO", "stdin closed, exiting");
    process.exit(0);
  });
}