import fs from "node:fs";
import os from "node:os";
import path from "node:path";

const DEFAULT_INSTANCE_ID = "default";
const INSTANCE_ID_RE = /^[a-zA-Z0-9_-]+$/;

// [nexus] Read ~/.nexus pointer file for custom home directory
function resolveNexusPointerFile(): string | null {
  const pointerPath = path.resolve(os.homedir(), ".nexus");
  try {
    const raw = fs.readFileSync(pointerPath, "utf-8").trim();
    if (raw.length > 0) {
      // Inline tilde expansion (expandHomePrefix is defined later in this file)
      const expanded = raw === "~" ? os.homedir()
        : raw.startsWith("~/") ? path.resolve(os.homedir(), raw.slice(2))
        : raw;
      return path.resolve(expanded);
    }
  } catch {
    // ~/.nexus does not exist or is unreadable — fall through
  }
  return null;
}

export function resolvePaperclipHomeDir(): string {
  // [nexus] Pointer-file: ~/.nexus overrides all other home resolution
  const nexusRoot = resolveNexusPointerFile();
  if (nexusRoot) return nexusRoot;

  const envHome = process.env.PAPERCLIP_HOME?.trim();
  if (envHome) return path.resolve(expandHomePrefix(envHome));
  return path.resolve(os.homedir(), ".paperclip");
}

export function resolvePaperclipInstanceId(override?: string): string {
  const raw = override?.trim() || process.env.PAPERCLIP_INSTANCE_ID?.trim() || DEFAULT_INSTANCE_ID;
  if (!INSTANCE_ID_RE.test(raw)) {
    throw new Error(
      `Invalid instance id '${raw}'. Allowed characters: letters, numbers, '_' and '-'.`,
    );
  }
  return raw;
}

export function resolvePaperclipInstanceRoot(instanceId?: string): string {
  const id = resolvePaperclipInstanceId(instanceId);
  return path.resolve(resolvePaperclipHomeDir(), "instances", id);
}

export function resolveDefaultConfigPath(instanceId?: string): string {
  return path.resolve(resolvePaperclipInstanceRoot(instanceId), "config.json");
}

export function resolveDefaultContextPath(): string {
  return path.resolve(resolvePaperclipHomeDir(), "context.json");
}

export function resolveDefaultCliAuthPath(): string {
  return path.resolve(resolvePaperclipHomeDir(), "auth.json");
}

export function resolveDefaultEmbeddedPostgresDir(instanceId?: string): string {
  return path.resolve(resolvePaperclipInstanceRoot(instanceId), "db");
}

export function resolveDefaultLogsDir(instanceId?: string): string {
  return path.resolve(resolvePaperclipInstanceRoot(instanceId), "logs");
}

export function resolveDefaultSecretsKeyFilePath(instanceId?: string): string {
  return path.resolve(resolvePaperclipInstanceRoot(instanceId), "secrets", "master.key");
}

export function resolveDefaultStorageDir(instanceId?: string): string {
  return path.resolve(resolvePaperclipInstanceRoot(instanceId), "data", "storage");
}

export function resolveDefaultBackupDir(instanceId?: string): string {
  return path.resolve(resolvePaperclipInstanceRoot(instanceId), "data", "backups");
}

export function expandHomePrefix(value: string): string {
  if (value === "~") return os.homedir();
  if (value.startsWith("~/")) return path.resolve(os.homedir(), value.slice(2));
  return value;
}

export function describeLocalInstancePaths(instanceId?: string) {
  const resolvedInstanceId = resolvePaperclipInstanceId(instanceId);
  const instanceRoot = resolvePaperclipInstanceRoot(resolvedInstanceId);
  return {
    homeDir: resolvePaperclipHomeDir(),
    instanceId: resolvedInstanceId,
    instanceRoot,
    configPath: resolveDefaultConfigPath(resolvedInstanceId),
    embeddedPostgresDataDir: resolveDefaultEmbeddedPostgresDir(resolvedInstanceId),
    backupDir: resolveDefaultBackupDir(resolvedInstanceId),
    logDir: resolveDefaultLogsDir(resolvedInstanceId),
    secretsKeyFilePath: resolveDefaultSecretsKeyFilePath(resolvedInstanceId),
    storageDir: resolveDefaultStorageDir(resolvedInstanceId),
  };
}