// Per-Repo Config - Loads `.pi-lsp.json` from the nearest ancestor of a
// given path and merges it with the built-in server registry. Config is a
// JSON file shaped like:
//
//   {
//     "servers": [
//       { "id": "rust-analyzer", "match": ["rs"], "command": "rust-analyzer",
//         "args": [], "rootMarkers": ["Cargo.toml"], "languageId": "rust" },
//       { "id": "gopls", "args": ["-remote=auto", "-vv"] }
//     ],
//     "disable": ["oxlint"]
//   }
//
// Merge Semantics:
//   - Entries whose `id` matches a built-in shallow-merge their fields (user wins).
//   - Entries with a new `id` append, and must include all required fields.
//   - The `disable` list filters the merged result by id.
//
// Caching: Resolved lists are cached per config-file path and invalidated by
// mtime. If no config file exists, the built-in registry is returned.
import * as fs from "node:fs";
import * as path from "node:path";
import { servers as builtinServers } from "../server.ts";
import type { ServerConfig } from "./types.ts";

const CONFIG_FILE = ".pi-lsp.json";

interface PiLspConfig {
  servers?: Array<Partial<ServerConfig> & { id: string }>;
  disable?: string[];
}

interface CacheEntry {
  mtimeMs: number;
  resolved: ServerConfig[];
}

const cache = new Map<string, CacheEntry>();

// Find Config File - Walks upward from `fromDir` looking for `.pi-lsp.json`.
// Returns the absolute path or null if none found before the filesystem root.
function findConfigFile(fromDir: string): string | null {
  let dir = path.resolve(fromDir);
  const { root } = path.parse(dir);
  while (true) {
    const candidate = path.join(dir, CONFIG_FILE);
    if (fs.existsSync(candidate)) return candidate;
    if (dir === root) return null;
    dir = path.dirname(dir);
  }
}

// Merge Config - Built-ins by id, user overrides shallow-merge in, new ids
// append (with required-field validation), and `disable` filters the result.
function mergeConfig(config: PiLspConfig, sourcePath: string): ServerConfig[] {
  const byId = new Map<string, ServerConfig>(
    builtinServers.map((s) => [s.id, { ...s }]),
  );

  // Apply Overrides And New Servers
  for (const override of config.servers ?? []) {
    if (!override.id || typeof override.id !== "string") {
      throw new Error(
        `pi-lsp config (${sourcePath}): every entry in "servers" must have a string "id"`,
      );
    }
    const existing = byId.get(override.id);
    if (existing) {
      byId.set(override.id, { ...existing, ...override } as ServerConfig);
    } else {
      // New Server - Must include all required spawn fields.
      const required = ["match", "command", "args", "rootMarkers"] as const;
      const missing = required.filter((k) => override[k] === undefined);
      if (missing.length > 0) {
        throw new Error(
          `pi-lsp config (${sourcePath}): new server "${override.id}" is missing required field(s): ${missing.join(", ")}`,
        );
      }
      byId.set(override.id, override as ServerConfig);
    }
  }

  // Apply Disable Filter
  const disabled = new Set(config.disable ?? []);
  return Array.from(byId.values()).filter((s) => !disabled.has(s.id));
}

// Get Servers For Path - Returns the merged ServerConfig list applicable to
// `fromDir` (or the directory containing a file path). Cached per config-file
// path with mtime invalidation. On parse/merge failure, logs to stderr and
// falls back to the built-in registry so a broken config never breaks LSP.
export function getServersForPath(fromPath: string): ServerConfig[] {
  // Resolve To Directory - Accept either a file path or a directory.
  let dir = path.resolve(fromPath);
  try {
    const st = fs.statSync(dir);
    if (!st.isDirectory()) dir = path.dirname(dir);
  } catch {
    dir = path.dirname(dir);
  }

  const configPath = findConfigFile(dir);
  if (!configPath) return builtinServers;

  let mtimeMs: number;
  try {
    mtimeMs = fs.statSync(configPath).mtimeMs;
  } catch {
    return builtinServers;
  }

  const cached = cache.get(configPath);
  if (cached && cached.mtimeMs === mtimeMs) return cached.resolved;

  let parsed: PiLspConfig;
  try {
    const raw = fs.readFileSync(configPath, "utf8");
    parsed = JSON.parse(raw);
  } catch (err) {
    process.stderr.write(
      `pi-lsp: failed to read/parse ${configPath}: ${(err as Error).message}\n`,
    );
    return builtinServers;
  }

  let resolved: ServerConfig[];
  try {
    resolved = mergeConfig(parsed, configPath);
  } catch (err) {
    process.stderr.write(`pi-lsp: ${(err as Error).message}\n`);
    return builtinServers;
  }

  cache.set(configPath, { mtimeMs, resolved });
  return resolved;
}

// Clear Config Cache - Test hook; not used in production.
export function clearConfigCache(): void {
  cache.clear();
}