import { app } from 'electron'
import { join } from 'path'
import { appendFileSync, mkdirSync, renameSync, statSync } from 'fs'
import { LOG_MAX_BYTES } from './constants'

/**
 * One small leveled, file-backed logger for the whole app (audit CC8). Before this,
 * diagnostics were effectively invisible in a packaged build: main-process catches
 * did a bare `console.error` (unreachable — the app menu/DevTools are suppressed in
 * production, M31) and the renderer's `logError` (M29) wrote to a console nobody could
 * open. Every catch now routes here and lands in `<userData>/logs/aerofetch.log`, so a
 * field-reported failure is actually diagnosable.
 *
 * Scope: this is the diagnostic log. `errorlog.ts` remains separate — it's user-facing
 * *download* failure data shown in Settings → Diagnostics, not app logging. The
 * user-facing toast half of CC8 ties to the global status surface (UI25/UX9).
 *
 * All `app`/fs access is lazy and guarded so importing this module never touches
 * `app` before it's ready — and so it's a safe no-op under unit tests, where
 * `electron`'s `app` is undefined.
 */

export type LogLevel = 'error' | 'warn' | 'info' | 'debug'

const LEVELS: Record<LogLevel, number> = { error: 0, warn: 1, info: 2, debug: 3 }

// Write everything at or above this level to the file. `debug` is dev-only noise, so
// it never hits the file (but still prints to the dev console).
const FILE_THRESHOLD = LEVELS.info

/** Format one log line: `2026-07-01T08:30:00.000Z [ERROR] message`. Pure, for tests. */
export function formatLine(level: LogLevel, message: string, now: Date = new Date()): string {
  return `${now.toISOString()} [${level.toUpperCase()}] ${message}`
}

/** Combine an operation label with an optional error/detail into one message. Pure. */
export function composeMessage(label: string, detail?: unknown): string {
  if (detail === undefined) return label
  const text = detail instanceof Error ? detail.message : String(detail)
  return `${label}: ${text}`
}

// `undefined` = not yet resolved, `null` = resolution failed (no app/userData, e.g.
// under unit tests) so file logging is disabled; a string is the resolved path.
let cachedFile: string | null | undefined
function logFile(): string | null {
  if (cachedFile !== undefined) return cachedFile
  try {
    const dir = join(app.getPath('userData'), 'logs')
    mkdirSync(dir, { recursive: true })
    cachedFile = join(dir, 'aerofetch.log')
  } catch {
    // No app / userData available (unit test, or app not ready) — disable the file sink.
    cachedFile = null
  }
  return cachedFile
}

/** Dev mode, resolved lazily and guarded (app is undefined in the test env). */
function isDev(): boolean {
  try {
    return !app.isPackaged
  } catch {
    return false
  }
}

// Rotate to `<file>.1` (single generation) once the live file passes the cap, so the
// log can't grow without bound. Best-effort: any fs error here is swallowed — logging
// must never throw into a catch handler.
function rotateIfNeeded(file: string): void {
  try {
    if (statSync(file).size > LOG_MAX_BYTES) renameSync(file, `${file}.1`)
  } catch {
    /* file missing (first write) or rename race — ignore */
  }
}

function writeToFile(line: string): void {
  const file = logFile()
  if (!file) return
  rotateIfNeeded(file)
  try {
    appendFileSync(file, line + '\n')
  } catch {
    /* disk full / read-only profile — nothing more we can safely do */
  }
}

function emit(level: LogLevel, label: string, detail?: unknown): void {
  const message = composeMessage(label, detail)
  // Console in dev keeps the familiar `--inspect` workflow; the file is the sink that
  // survives into a packaged build.
  if (isDev()) {
    const fn = level === 'error' ? console.error : level === 'warn' ? console.warn : console.log
    fn(`[AeroFetch] ${message}`)
  }
  if (LEVELS[level] <= FILE_THRESHOLD) writeToFile(formatLine(level, message))
}

export const logger = {
  error: (label: string, detail?: unknown): void => emit('error', label, detail),
  warn: (label: string, detail?: unknown): void => emit('warn', label, detail),
  info: (label: string, detail?: unknown): void => emit('info', label, detail),
  debug: (label: string, detail?: unknown): void => emit('debug', label, detail)
}