writeVarint method

void writeVarint(int value)

Writes value as a varint; a negative one as the ten bytes of its 64-bit two's complement, as protobuf encodes int32 and int64.

Exact on the web: it never shifts past 32 bits.

Implementation

void writeVarint(int value) {
  if (value >= 0) {
    var v = value;
    while (v >= 0x80) {
      _out.addByte((v & 0x7F) | 0x80);
      v = v ~/ 0x80;
    }
    _out.addByte(v & 0x7F);
    return;
  }
  // The unsigned 64-bit two's complement, as 32-bit halves.
  final magnitude = -value;
  final magLo = magnitude & 0xFFFFFFFF;
  final magHi = (magnitude ~/ 0x100000000) & 0xFFFFFFFF;
  final invLo = (~magLo) & 0xFFFFFFFF;
  final invHi = (~magHi) & 0xFFFFFFFF;
  var lo = (invLo + 1) & 0xFFFFFFFF;
  final carry = (invLo + 1) > 0xFFFFFFFF ? 1 : 0;
  var hi = (invHi + carry) & 0xFFFFFFFF;
  // Nine groups of seven bits, each step moving hi's low bits into lo.
  for (var i = 0; i < 9; i++) {
    _out.addByte((lo & 0x7F) | 0x80);
    final carryBits = (hi & 0x7F) * 0x2000000; // hi[0..6] -> lo[25..31]
    lo = ((lo ~/ 0x80) + carryBits) & 0xFFFFFFFF;
    hi = hi ~/ 0x80;
  }
  _out.addByte(lo & 0x7F);
}