scope byteArrayFromBase128Utf8

static int #byteFromArray(byte[] data, int i) {
  ret i >= data.length ? 0 : ubyteToInt(data[i]);
}

static byte[] byteArrayFromBase128Utf8(byte[] data) {
  int n = l(data);
  bool anomaly = (n & 7) == 1 && (data[n-1] & 1) != 0;
  int n2 = anomaly ? n-1 : n;
  int nOut = toInt(n2*7L/8);
  byte[] out = new byte[nOut];
  int iIn = 0, iOut = 0;
  while (iIn < n2) {
    // take all 7 bits of byte 0 + top bit of byte 1
    out[iOut++] = (byte) (data[iIn] << (8-1) | ubyteToInt(data[iIn+1]) >> (8-1);
    if (++iIn >= n2) break;
    // take bottom bit of byte 0 + top 6 bits of byte 1
    out[iOut] = (byte) (0x7F & (byteFromArray(data, iIn) << (7-1) | byteFromArray(data, iIn+1) >> (8-6));
    if (++iOut >= nOut) break;
    // take bottom 2 bits of byte 1 + top 5 bits of byte 2
    out[iOut] = (byte) (0x7F & (byteFromArray(data, iIn+1) << (7-2) | byteFromArray(data, iIn+2) >> (8-5));
    if (++iOut >= nOut) break;
    // take bottom 3 bits of byte 2 + top 4 bits of byte 3
    out[iOut] = (byte) (0x7F & (byteFromArray(data, iIn+2) << (7-3) | byteFromArray(data, iIn+3) >> (8-4));
    if (++iOut >= nOut) break;
    // take bottom 4 bits of byte 3 + top 3 bits of byte 4
    out[iOut] = (byte) (0x7F & (byteFromArray(data, iIn+3) << (7-4) | byteFromArray(data, iIn+4) >> (8-3));
    if (++iOut >= nOut) break;
    // take bottom 5 bits of byte 4 + top 2 bits of byte 5
    out[iOut] = (byte) (0x7F & (byteFromArray(data, iIn+4) << (7-5) | byteFromArray(data, iIn+5) >> (8-2));
    if (++iOut >= nOut) break;
    // take bottom 6 bits of byte 5 + top bit of byte 6
    out[iOut] = (byte) (0x7F & (byteFromArray(data, iIn+5) << (7-6) | byteFromArray(data, iIn+6) >> (8-1));
    if (++iOut >= nOut) break;
    // take bottom 7 bits of byte 6
    out[iOut] = (byte) (0x7F & byteFromArray(data, iIn+6));
    if (++iOut >= nOut) break;
    iIn += 7;
  }
  ret out;
}

Began life as a copy of #1035231