import { appendFileSync, mkdirSync, renameSync, statSync, unlinkSync } from 'node:fs'
import { join } from 'node:path'

/**
 * The app's one log sink: a plain-text file under the OS log dir, written
 * SYNCHRONOUSLY so a line survives the process dying moments later.
 *
 * Why a file at all: `console.*` goes nowhere in real use. Helder runs one
 * process per project window, and every window past the first is spawned by
 * `spawnInstance()` with `stdio: 'ignore'` — its output is discarded. Launched
 * from Finder there's no terminal attached either. Before this, a crash left
 * literally no trace; that's what made "it crashed sometimes" undebuggable.
 *
 * This module deliberately does NOT import electron, so it stays unit-testable.
 * `initLogger()` is handed the directory by the caller (see diagnostics.ts).
 */

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

const LEVELS: Record<LogLevel, number> = { debug: 10, info: 20, warn: 30, error: 40 }

/** Rotate at 2 MB, keep 3 old files (~8 MB worst case for a dev tool's log). */
const MAX_BYTES = 2 * 1024 * 1024
const KEEP = 3

interface LoggerState {
  file: string | null
  dir: string | null
  min: number
  mirror: boolean
  /** Byte size tracked in-process so the common path avoids a stat() per line. */
  size: number
}

const state: LoggerState = { file: null, dir: null, min: LEVELS.debug, mirror: false, size: 0 }

/** Absolute path of the active log file, or null before initLogger(). */
export function getLogPath(): string | null {
  return state.file
}

export function getLogDir(): string | null {
  return state.dir
}

/**
 * Point the logger at `dir` (created if needed). Safe to call once per process.
 * `mirror` also echoes to the console, which is useful in `npm run dev` where a
 * terminal IS attached. `level` gates the floor (default: everything).
 */
export function initLogger(opts: { dir: string; mirror?: boolean; level?: LogLevel }): void {
  state.dir = opts.dir
  state.file = join(opts.dir, 'helder.log')
  state.mirror = !!opts.mirror
  state.min = LEVELS[opts.level ?? 'debug']
  try {
    mkdirSync(opts.dir, { recursive: true })
    state.size = statSync(state.file).size
  } catch {
    // Missing file is the normal first-run case (size stays 0). A genuinely
    // unwritable dir surfaces on the first write() instead, which no-ops.
    state.size = 0
  }
}

/**
 * `helder.log` → `helder.1.log` → … → dropped after KEEP. Called when the live
 * file crosses MAX_BYTES. Several project processes share one file and could in
 * principle rotate at the same moment; the renames are best-effort and a lost
 * race costs at most some log lines, never a crash — hence the blanket catch.
 */
function rotate(): void {
  const dir = state.dir
  const file = state.file
  if (!dir || !file) return
  try {
    const oldest = join(dir, `helder.${KEEP}.log`)
    try { unlinkSync(oldest) } catch { /* wasn't there */ }
    for (let i = KEEP - 1; i >= 1; i--) {
      try { renameSync(join(dir, `helder.${i}.log`), join(dir, `helder.${i + 1}.log`)) } catch { /* gap in the chain */ }
    }
    renameSync(file, join(dir, 'helder.1.log'))
    state.size = 0
  } catch { /* another process rotated first; keep appending */ }
}

/** JSON that can't throw on cycles/BigInt — a logger must never be the crash. */
function safeJson(value: unknown): string {
  const seen = new WeakSet<object>()
  try {
    return JSON.stringify(value, (_k, v) => {
      if (typeof v === 'bigint') return `${v}n`
      if (typeof v === 'function') return `[Function ${v.name || 'anonymous'}]`
      if (typeof v === 'object' && v !== null) {
        if (seen.has(v as object)) return '[Circular]'
        seen.add(v as object)
      }
      return v
    }) ?? String(value)
  } catch {
    return '[unserializable]'
  }
}

/**
 * Normalise anything thrown into a loggable shape. Non-Errors get stringified
 * (people throw strings), and `cause` is followed so wrapped errors keep their
 * root cause — usually the line that actually explains the failure.
 */
export function formatErr(e: unknown): { message: string; stack?: string; cause?: string } {
  if (e instanceof Error) {
    const out: { message: string; stack?: string; cause?: string } = { message: e.message }
    if (e.stack) out.stack = e.stack
    if (e.cause !== undefined) out.cause = e.cause instanceof Error ? (e.cause.stack || e.cause.message) : safeJson(e.cause)
    return out
  }
  return { message: typeof e === 'string' ? e : safeJson(e) }
}

/**
 * One log line: ISO ts · level · pid · scope · message · context JSON.
 *
 * Continuation lines are indented, never bare: a message can carry newlines of
 * its own (Electron's console warnings do, and so does any stack passed as the
 * message), and an unindented second line is indistinguishable from a new entry
 * to both a human skimming the file and to `grep`.
 */
export function formatLine(level: LogLevel, scope: string, msg: string, ctx: unknown, pid: number, now: string): string {
  const head = `${now} ${level.toUpperCase().padEnd(5)} ${String(pid).padStart(5)} ${scope.padEnd(9)} ${indent(msg)}`
  if (ctx === undefined) return head + '\n'
  return head + ' ' + indent(safeJson(ctx)) + '\n'
}

function indent(s: string): string {
  return s.replace(/\r?\n/g, '\n    ')
}

export function log(level: LogLevel, scope: string, msg: string, ctx?: unknown): void {
  if (LEVELS[level] < state.min) return
  const line = formatLine(level, scope, msg, ctx, process.pid, new Date().toISOString())

  if (state.mirror) {
    const fn = level === 'error' ? console.error : level === 'warn' ? console.warn : console.log
    fn(line.trimEnd())
  }

  const file = state.file
  if (!file) return
  if (state.size + line.length > MAX_BYTES) rotate()
  try {
    // Sync + O_APPEND: the write lands before an imminent crash can eat it, and
    // concurrent appends from sibling project processes don't interleave.
    appendFileSync(file, line, { encoding: 'utf8' })
    state.size += Buffer.byteLength(line)
  } catch { /* disk full / no permission — never let logging break the app */ }
}

export const logger = {
  debug: (scope: string, msg: string, ctx?: unknown): void => log('debug', scope, msg, ctx),
  info: (scope: string, msg: string, ctx?: unknown): void => log('info', scope, msg, ctx),
  warn: (scope: string, msg: string, ctx?: unknown): void => log('warn', scope, msg, ctx),
  error: (scope: string, msg: string, err?: unknown, ctx?: Record<string, unknown>): void =>
    log('error', scope, msg, err === undefined ? ctx : { ...ctx, err: formatErr(err) }),
}

/** Reset for tests. Not used by the app. */
export function _resetLogger(): void {
  state.file = null; state.dir = null; state.min = LEVELS.debug; state.mirror = false; state.size = 0
}