function readLongVarint() {
    // tends to deopt with local vars for octet etc.
    var bits = new LongBits(0, 0);
    var i = 0;
    if (this.len - this.pos > 4) { // fast route (lo)
        for (; i < 4; ++i) {
            // 1st..4th
            bits.lo = (bits.lo | (this.buf[this.pos] & 127) << i * 7) >>> 0;
            if (this.buf[this.pos++] < 128)
                return bits;
        }
        // 5th
        bits.lo = (bits.lo | (this.buf[this.pos] & 127) << 28) >>> 0;
        bits.hi = (bits.hi | (this.buf[this.pos] & 127) >>  4) >>> 0;
        if (this.buf[this.pos++] < 128)
            return bits;
        i = 0;
    } else {
        for (; i < 3; ++i) {
            /* istanbul ignore if */
            if (this.pos >= this.len)
                throw indexOutOfRange(this);
            // 1st..3th
            bits.lo = (bits.lo | (this.buf[this.pos] & 127) << i * 7) >>> 0;
            if (this.buf[this.pos++] < 128)
                return bits;
        }
        // 4th
        bits.lo = (bits.lo | (this.buf[this.pos++] & 127) << i * 7) >>> 0;
        return bits;
    }
    if (this.len - this.pos > 4) { // fast route (hi)
        for (; i < 5; ++i) {
            // 6th..10th
            bits.hi = (bits.hi | (this.buf[this.pos] & 127) << i * 7 + 3) >>> 0;
            if (this.buf[this.pos++] < 128)
                return bits;
        }
    } else {
        for (; i < 5; ++i) {
            /* istanbul ignore if */
            if (this.pos >= this.len)
                throw indexOutOfRange(this);
            // 6th..10th
            bits.hi = (bits.hi | (this.buf[this.pos] & 127) << i * 7 + 3) >>> 0;
            if (this.buf[this.pos++] < 128)
                return bits;
        }
    }
    /* istanbul ignore next */
    throw Error("invalid varint encoding");
}
Reads a zig-zag encoded varint as a signed 32 bit value.
@returns {number} Value read