Minor Description
Long values are used for many different areas of the spec, and in particular a 'zero' value is used often. for example:
a string is encoded as a long followed by that many bytes of UTF-8 encoded character data.
Arrays are encoded as a series of blocks. Each block consists of a long count value, followed by that many array items. A block with count zero indicates the end of the array. Each item is encoded per the array’s item schema.
Maps are encoded as a series of blocks. Each block consists of a long count value, followed by that many key/value pairs. A block with count zero indicates the end of the map. Each item is encoded per the map’s value schema.
Because of this, long values actually tend to be pretty small on average, and so can often fit within the first byte of the variable-length array. Therefore, the first byte should be prioritized.
For the first byte, if the high-order bit is set, then not only does it mean there are more bytes to follow, but that the signed value of the byte will be negative. Therefore, the inverse is that for a positive number (>=0), then there are not more bytes to follow.
Check the first byte, and if it is positive, exit early, if it is zero, return zero.