index.js

'use strict'

let table

function initGbkTable() {
  // https://en.wikipedia.org/wiki/GBK_(character_encoding)#Encoding
  const ranges = [
    [0xA1, 0xA9,  0xA1, 0xFE],
    [0xB0, 0xF7,  0xA1, 0xFE],
    [0x81, 0xA0,  0x40, 0xFE],
    [0xAA, 0xFE,  0x40, 0xA0],
    [0xA8, 0xA9,  0x40, 0xA0],
    [0xAA, 0xAF,  0xA1, 0xFE],
    [0xF8, 0xFE,  0xA1, 0xFE],
    [0xA1, 0xA7,  0x40, 0xA0],
  ]
  const codes = new Uint16Array(23940)
  let i = 0

  for (const [b1Begin, b1End, b2Begin, b2End] of ranges) {
    for (let b2 = b2Begin; b2 <= b2End; b2++) {
      if (b2 !== 0x7F) {
        for (let b1 = b1Begin; b1 <= b1End; b1++) {
          codes[i++] = b2 << 8 | b1
        }
      }
    }
  }
  table = new Uint16Array(65536)
  table.fill(0xFFFF)

  const str = new TextDecoder('gbk').decode(codes)
  for (let i = 0; i < str.length; i++) {
    table[str.charCodeAt(i)] = codes[i]
  }
}


const NodeJsBufAlloc = typeof Buffer === 'function' && Buffer.allocUnsafe

const defaultOnAlloc = NodeJsBufAlloc
  ? (len) => NodeJsBufAlloc(len)
  : (len) => new Uint8Array(len)

const defaultOnError = () => 63   // '?'


export default function(str, opt = {}) {
  if (!table) {
    initGbkTable()
  }
  const onAlloc = opt.onAlloc || defaultOnAlloc
  const onError = opt.onError || defaultOnError

  const buf = onAlloc(str.length * 2)
  let n = 0

  for (let i = 0; i < str.length; i++) {
    const code = str.charCodeAt(i)
    if (code < 0x80) {
      buf[n++] = code
      continue
    }
    const gbk = table[code]

    if (gbk !== 0xFFFF) {
      buf[n++] = gbk
      buf[n++] = gbk >> 8
    } else if (code === 8364) {
      // 8364 == '€'.charCodeAt(0)
      // Code Page 936 has a single-byte euro sign at 0x80
      buf[n++] = 0x80
    } else {
      const ret = onError(i, str)
      if (ret === -1) {
        break
      }
      if (ret > 0xFF) {
        buf[n++] = ret
        buf[n++] = ret >> 8
      } else {
        buf[n++] = ret
      }
    }
  }
  return buf.subarray(0, n)
}