gzip/lib/utils/strings.js

// Quick check if we can use fast array to bin string conversion
//
// - apply(Array) can fail on Android 2.2
// - apply(Uint8Array) can fail on iOS 5.1 Safari
//
let STR_APPLY_UIA_OK = true

try {
  String.fromCharCode.apply(null, new Uint8Array(1))
} catch (__) {
  STR_APPLY_UIA_OK = false
}

// Table with utf8 lengths (calculated by first byte of sequence)
// Note, that 5 & 6-byte values and some 4-byte values can not be represented in JS,
// because max possible codepoint is 0x10ffff
const _utf8len = new Uint8Array(256)
for (let q = 0; q < 256; q++) {
  _utf8len[q] =
    q >= 252 ? 6 : q >= 248 ? 5 : q >= 240 ? 4 : q >= 224 ? 3 : q >= 192 ? 2 : 1
}
_utf8len[254] = _utf8len[254] = 1 // Invalid sequence start

// convert string to array (typed, when possible)
export const string2buf = str => {
  if (typeof TextEncoder === 'function' && TextEncoder.prototype.encode) {
    return new TextEncoder().encode(str)
  }

  let buf,
    c,
    c2,
    m_pos,
    i,
    str_len = str.length,
    buf_len = 0

  // count binary size
  for (m_pos = 0; m_pos < str_len; m_pos++) {
    c = str.charCodeAt(m_pos)
    if ((c & 0xfc00) === 0xd800 && m_pos + 1 < str_len) {
      c2 = str.charCodeAt(m_pos + 1)
      if ((c2 & 0xfc00) === 0xdc00) {
        c = 0x10000 + ((c - 0xd800) << 10) + (c2 - 0xdc00)
        m_pos++
      }
    }
    buf_len += c < 0x80 ? 1 : c < 0x800 ? 2 : c < 0x10000 ? 3 : 4
  }

  // allocate buffer
  buf = new Uint8Array(buf_len)

  // convert
  for (i = 0, m_pos = 0; i < buf_len; m_pos++) {
    c = str.charCodeAt(m_pos)
    if ((c & 0xfc00) === 0xd800 && m_pos + 1 < str_len) {
      c2 = str.charCodeAt(m_pos + 1)
      if ((c2 & 0xfc00) === 0xdc00) {
        c = 0x10000 + ((c - 0xd800) << 10) + (c2 - 0xdc00)
        m_pos++
      }
    }
    if (c < 0x80) {
      /* one byte */
      buf[i++] = c
    } else if (c < 0x800) {
      /* two bytes */
      buf[i++] = 0xc0 | (c >>> 6)
      buf[i++] = 0x80 | (c & 0x3f)
    } else if (c < 0x10000) {
      /* three bytes */
      buf[i++] = 0xe0 | (c >>> 12)
      buf[i++] = 0x80 | ((c >>> 6) & 0x3f)
      buf[i++] = 0x80 | (c & 0x3f)
    } else {
      /* four bytes */
      buf[i++] = 0xf0 | (c >>> 18)
      buf[i++] = 0x80 | ((c >>> 12) & 0x3f)
      buf[i++] = 0x80 | ((c >>> 6) & 0x3f)
      buf[i++] = 0x80 | (c & 0x3f)
    }
  }

  return buf
}

// Helper
const buf2binstring = (buf, len) => {
  // On Chrome, the arguments in a function call that are allowed is `65534`.
  // If the length of the buffer is smaller than that, we can use this optimization,
  // otherwise we will take a slower path.
  if (len < 65534) {
    if (buf.subarray && STR_APPLY_UIA_OK) {
      return String.fromCharCode.apply(
        null,
        buf.length === len ? buf : buf.subarray(0, len)
      )
    }
  }

  let result = ''
  for (let i = 0; i < len; i++) {
    result += String.fromCharCode(buf[i])
  }
  return result
}

// convert array to string
export const buf2string = (buf, max) => {
  const len = max || buf.length

  if (typeof TextDecoder === 'function' && TextDecoder.prototype.decode) {
    return new TextDecoder().decode(buf.subarray(0, max))
  }

  let i, out

  // Reserve max possible length (2 words per char)
  // NB: by unknown reasons, Array is significantly faster for
  //     String.fromCharCode.apply than Uint16Array.
  const utf16buf = new Array(len * 2)

  for (out = 0, i = 0; i < len; ) {
    let c = buf[i++]
    // quick process ascii
    if (c < 0x80) {
      utf16buf[out++] = c
      continue
    }

    let c_len = _utf8len[c]
    // skip 5 & 6 byte codes
    if (c_len > 4) {
      utf16buf[out++] = 0xfffd
      i += c_len - 1
      continue
    }

    // apply mask on first byte
    c &= c_len === 2 ? 0x1f : c_len === 3 ? 0x0f : 0x07
    // join the rest
    while (c_len > 1 && i < len) {
      c = (c << 6) | (buf[i++] & 0x3f)
      c_len--
    }

    // terminated by end of string?
    if (c_len > 1) {
      utf16buf[out++] = 0xfffd
      continue
    }

    if (c < 0x10000) {
      utf16buf[out++] = c
    } else {
      c -= 0x10000
      utf16buf[out++] = 0xd800 | ((c >> 10) & 0x3ff)
      utf16buf[out++] = 0xdc00 | (c & 0x3ff)
    }
  }

  return buf2binstring(utf16buf, out)
}

// Calculate max possible position in utf8 buffer,
// that will not break sequence. If that's not possible
// - (very small limits) return max size as is.
//
// buf[] - utf8 bytes array
// max   - length limit (mandatory);
export const utf8border = (buf, max) => {
  max = max || buf.length
  if (max > buf.length) {
    max = buf.length
  }

  // go back from last position, until start of sequence found
  let pos = max - 1
  while (pos >= 0 && (buf[pos] & 0xc0) === 0x80) {
    pos--
  }

  // Very small and broken sequence,
  // return max, because we should return something anyway.
  if (pos < 0) {
    return max
  }

  // If we came to start of buffer - that means buffer is too small,
  // return max too.
  if (pos === 0) {
    return max
  }

  return pos + _utf8len[buf[pos]] > max ? pos : max
}