feat: Reimplement the OMEMOMessage protobuf schema

lib/src/protobuf/protobuf.dart

@@ -0,0 +1,60 @@
+/// Masks the 7 LSB
+const lsb7Mask = 0x7F;
+
+/// Constant for setting the MSB
+const msb = 1 << 7;
+
+class VarintDecode {
+
+  const VarintDecode(this.n, this.length);
+  final int n;
+  final int length;
+}
+
+/// Decode a Varint that begins at [input]'s index [offset].
+VarintDecode decodeVarint(List<int> input, int offset) {
+  // The return value
+  var n = 0;
+  // The byte offset counter
+  var i = 0;
+
+  // Iterate until the MSB of the byte is 0
+  while (true) {
+    // Mask only the 7 LSB and "move" them accordingly
+    n += (input[offset + i] & lsb7Mask) << (7 * i);
+
+    // Break if we reached the end
+    if (input[offset + i] & 1 << 7 == 0) {
+      break;
+    }
+    i++;
+  }
+
+  return VarintDecode(n, i + 1);
+}
+
+// Encodes the integer [i] into a Varint.
+List<int> encodeVarint(int i) {
+  assert(i >= 0, "Two's complement is not implemented");
+  final ret = List<int>.empty(growable: true);
+
+  var j = 0;
+  while (true) {
+    // The 7 LSB of the byte we're creating
+    final x = (i & (lsb7Mask << j * 7)) >> j * 7;
+    // The next bits
+    final next = i & (lsb7Mask << (j + 1) * 7);
+
+    if (next == 0) {
+      // If we were to shift further, we only get zero, so we're at the end
+      ret.add(x);
+      break;
+    } else {
+      // We still have at least one bit more to go, so set the MSB to 1
+      ret.add(x + msb);
+      j++;
+    }
+  }
+
+  return ret;
+}