/**
 * src/config.ts — T4: environment config loader
 *
 * Reads env vars once at startup, validates them, derives allowedOrigins from
 * local NIC IPs (M1 — NOT from bindHost; 0.0.0.0 is never a valid Origin),
 * and returns a frozen Config object.
 */

import os from 'node:os'
import type { Config, EnvLike } from './types.js'
import { tmuxAvailable } from './session/tmux.js'

/** USE_TMUX: '1'/'true'/'on' → on, '0'/'false'/'off' → off, else auto-detect tmux. */
function resolveUseTmux(raw: string | undefined): boolean {
  const v = raw?.trim().toLowerCase()
  if (v === '1' || v === 'true' || v === 'on') return true
  if (v === '0' || v === 'false' || v === 'off') return false
  return tmuxAvailable() // unset / 'auto'
}

// ── constants ─────────────────────────────────────────────────────────────────

const DEFAULT_PORT = 3000
const DEFAULT_BIND_HOST = '0.0.0.0'
const DEFAULT_IDLE_TTL_SEC = 24 * 60 * 60 // 24 hours in seconds
const DEFAULT_SCROLLBACK_BYTES = 2 * 1024 * 1024 // 2 MB
const DEFAULT_MAX_PAYLOAD_BYTES = 1 * 1024 * 1024 // 1 MB
const DEFAULT_WS_PATH = '/term'

// ── helpers ───────────────────────────────────────────────────────────────────

function parsePositiveInt(
  raw: string | undefined,
  label: string,
  fallback: number,
): number {
  if (raw === undefined) return fallback
  const n = Number(raw)
  if (!Number.isFinite(n) || !Number.isInteger(n) || n < 0) {
    throw new Error(
      `Invalid config: ${label}=${JSON.stringify(raw)} — must be a non-negative integer`,
    )
  }
  return n
}

function parsePort(raw: string | undefined): number {
  if (raw === undefined) return DEFAULT_PORT
  const n = Number(raw)
  if (!Number.isFinite(n) || !Number.isInteger(n) || n < 1 || n > 65535) {
    throw new Error(
      `Invalid config: PORT=${JSON.stringify(raw)} — must be an integer 1–65535`,
    )
  }
  return n
}

function parseIdleTtl(raw: string | undefined): number {
  if (raw === undefined) return DEFAULT_IDLE_TTL_SEC * 1000
  const n = Number(raw)
  if (!Number.isFinite(n) || !Number.isInteger(n) || n < 0) {
    throw new Error(
      `Invalid config: IDLE_TTL=${JSON.stringify(raw)} — must be a non-negative integer (seconds)`,
    )
  }
  return n * 1000
}

/**
 * Derive allowedOrigins from host NIC IPs (M1).
 *
 * Rules:
 *  - Enumerate os.networkInterfaces(), pick IPv4 entries.
 *  - Skip internal (loopback) entries — 127.0.0.1 / localhost are added explicitly.
 *  - For every qualifying address, produce both http:// and https:// origins.
 *  - Append ALLOWED_ORIGINS env-var items (comma-separated).
 *  - Deduplicate.
 *  - Never include 0.0.0.0 (wildcard listen address, never a browser Origin).
 */
function deriveAllowedOrigins(port: number, extraEnv: string | undefined): readonly string[] {
  const set = new Set<string>()

  // 1. Always include localhost / loopback
  const loopbackHosts = ['localhost', '127.0.0.1']
  for (const host of loopbackHosts) {
    set.add(`http://${host}:${port}`)
    set.add(`https://${host}:${port}`)
  }

  // 2. Enumerate NIC IPv4 addresses — skip internal (loopback already handled above)
  const ifaces = os.networkInterfaces()
  for (const ifaceList of Object.values(ifaces)) {
    if (!ifaceList) continue
    for (const entry of ifaceList) {
      // Only IPv4, non-internal entries; never 0.0.0.0
      if (
        entry.family === 'IPv4' &&
        !entry.internal &&
        entry.address !== '0.0.0.0'
      ) {
        set.add(`http://${entry.address}:${port}`)
        set.add(`https://${entry.address}:${port}`)
      }
    }
  }

  // 3. Merge ALLOWED_ORIGINS env var (comma-separated list of origin strings)
  if (extraEnv) {
    for (const raw of extraEnv.split(',')) {
      const trimmed = raw.trim()
      if (trimmed) set.add(trimmed)
    }
  }

  return Object.freeze([...set])
}

// ── loadConfig ────────────────────────────────────────────────────────────────

/**
 * Read and validate environment variables; fill in defaults for missing keys.
 * Invalid values (port out of range, TTL non-integer, etc.) throw immediately
 * so the server never starts in a misconfigured state (fail-fast).
 *
 * The returned Config is Object.freeze'd — immutable after construction.
 */
export function loadConfig(env: EnvLike): Config {
  const port = parsePort(env['PORT'])

  const bindHost = env['BIND_HOST'] ?? DEFAULT_BIND_HOST

  const shellPath =
    env['SHELL_PATH'] ?? (process.env['SHELL'] as string | undefined) ?? '/bin/zsh'

  const homeDir = os.homedir()

  const idleTtlMs = parseIdleTtl(env['IDLE_TTL'])

  const scrollbackBytes = parsePositiveInt(
    env['SCROLLBACK_BYTES'],
    'SCROLLBACK_BYTES',
    DEFAULT_SCROLLBACK_BYTES,
  )

  const maxPayloadBytes = parsePositiveInt(
    env['MAX_PAYLOAD_BYTES'],
    'MAX_PAYLOAD_BYTES',
    DEFAULT_MAX_PAYLOAD_BYTES,
  )

  const wsPath = env['WS_PATH'] ?? DEFAULT_WS_PATH

  const useTmux = resolveUseTmux(env['USE_TMUX'])

  const allowedOrigins = deriveAllowedOrigins(port, env['ALLOWED_ORIGINS'])

  return Object.freeze({
    port,
    bindHost,
    shellPath,
    homeDir,
    idleTtlMs,
    scrollbackBytes,
    maxPayloadBytes,
    wsPath,
    useTmux,
    allowedOrigins,
  } satisfies Config)
}