Feature #21796: unpack variant that returns the final offset

• Status: Open
• Assignee: -
• Target version: -

[nobu (Nobuyoshi Nakada)]

mame (Yusuke Endoh) wrote in #note-4:

It's a shame unpack doesn't tell you how many bytes it read. You'd probably want a unpack variant that returns the final offset too, or a specifier that returns the current offset (like o?).

bytes = "\x01\x02\x03"
offset = 0
leb128_value1, offset = bytes.unpack("Ro", offset: offset) #=> 1
leb128_value2, offset = bytes.unpack("Ro", offset: offset) #=> 2
leb128_value3, offset = bytes.unpack("Ro", offset: offset) #=> 3

mame (Yusuke Endoh) wrote in #note-6:

You could tell how many bytes you read based on the size of the leb128_value returned.

That apparoach is unreliable because LEB128 is redundant. For example, both "\x03" and "\x83\x00" are valid LEB128 encodings of the value 3.
See the note of the section Values - Integers, in the Wasm spec.
https://webassembly.github.io/spec/core/binary/values.html#integers

[byroot (Jean Boussier)]

• Description updated (diff)

[byroot (Jean Boussier)]

• Related to Feature #21785: Add signed and unsigned LEB128 support to pack / unpack added

[byroot (Jean Boussier)]

It would be useful indeed, but I'm not sure a new method is the best way?

I think the simplest would be a new keyword parameter:

offset, *values = bytes.unpack("Ro", offset: offset, return_offset:true)

Another possibility would be to add an unpack like method to StringScanner, for the case where you want to iteratively deserialize a binary string.