// Lightweight GBK/GB18030 decoder helper
// Strategy:
// 1. Prefer native TextDecoder('gb18030'/'gbk') if available
// 2. Fallback to a reverse table built from utils/ecUnicodeToGBK.js (if present)
// 3. Final fallback: byte-by-byte ASCII/replace decoding

function toUint8Array(input) {
  if (!input) return new Uint8Array(0)
  if (input instanceof Uint8Array) return input
  if (input.buffer && input.buffer instanceof ArrayBuffer) return new Uint8Array(input.buffer)
  return new Uint8Array(input)
}

let reverseMap = null
function buildReverseFromTable(table) {
  try {
    const rev = Object.create(null)
    for (let i = 0; i < table.length; i++) {
      const v = table[i]
      if (v) rev[String(v).toUpperCase()] = Number(i)
    }
    return rev
  } catch (e) { return null }
}

function decodeWithTable(u8, table) {
  if (!table) return null
  if (!reverseMap) reverseMap = buildReverseFromTable(table)
  if (!reverseMap) return null
  let out = ''
  for (let i = 0; i < u8.length; i++) {
    const b = u8[i]
    if (b <= 0x7F) { out += String.fromCharCode(b); continue }
    const b2 = u8[i + 1]
    if (typeof b2 !== 'number') { out += '\uFFFD'; continue }
    const hex = b.toString(16).padStart(2, '0').toUpperCase() + b2.toString(16).padStart(2, '0').toUpperCase()
    const cp = reverseMap[hex]
    if (cp != null) {
      try { out += String.fromCodePoint(cp) } catch (e) { out += '\uFFFD' }
    } else {
      out += '\uFFFD'
    }
    i += 1
  }
  return out
}

function decode(u8in) {
  const u8 = toUint8Array(u8in)
  // 1) Native TextDecoder if environment supports gb18030/gbk
  try {
    if (typeof TextDecoder === 'function') {
      try { return new TextDecoder('gb18030').decode(u8) } catch (e) { /* ignore */ }
      try { return new TextDecoder('gbk').decode(u8) } catch (e) { /* ignore */ }
    }
  } catch (e) { /* ignore */ }

  // 2) Try a local Encoding lib if available (bundles may expose Encoding.convert)
  try {
    const enc = (typeof Encoding !== 'undefined') ? Encoding : (typeof window !== 'undefined' ? window.Encoding : null)
    if (enc && typeof enc.convert === 'function') {
      try { return enc.convert(u8, { to: 'UNICODE', from: 'GB18030', type: 'string' }) } catch (e) { /* ignore */ }
      try { return enc.convert(u8, { to: 'UTF8', from: 'GB18030', type: 'string' }) } catch (e) { /* ignore */ }
    }
  } catch (e) { /* ignore */ }

  // 3) Try building reverse map from project mapping utils (utils/ecUnicodeToGBK.js)
  try {
    const tableMod = require('../utils/ecUnicodeToGBK.js')
    const table = tableMod && (tableMod.table || tableMod.t || tableMod)
    if (table) {
      const s = decodeWithTable(u8, table)
      if (s && s.length) return s
    }
  } catch (e) { /* ignore */ }

  // 4) Final fallback: best-effort ASCII/replace pass
  try {
    let out = ''
    for (let i = 0; i < u8.length; i++) {
      const b = u8[i]
      if (b <= 0x7F) out += String.fromCharCode(b)
      else {
        const b2 = u8[i + 1]
        if (typeof b2 === 'number') { out += '\uFFFD'; i += 1 } else { out += '\uFFFD' }
      }
    }
    return out
  } catch (e) { return '' }
}

module.exports = { decode }