Added skipping support

[m-wichmann]

This is another attempt to add skipping support (see also #5). This implementation is based on code from CWPack and does not use recursion.

Before this is merged, two things should be considered:

1. obj_skip_count is currently a uint32_t and could overflow (e.g. while reading malicious data). Extending the type to uint64_t would help with the problem (at least a bit), but I didn't really want to change it, since I use cmp on an embedded device. Maybe instead, we could check for overflow and exit with an error.
2. Skipping of strings/bin/ext currently reads one byte at a time, since there is no buffer available. This could be replaced with a skip/seek function pointer. Instead a buffer of configurable size could be added (see #3).

[antoinealb]

Nice to see that you added the error checking!

For your first point, you could define obj_skip_count as size_t: http://stackoverflow.com/questions/2550774/what-is-size-t-in-c : size_t is a type guaranteed to hold any array index.

[m-wichmann]

For your first point, you could define obj_skip_count as size_t

I thought about that, but size_t is only guaranteed to be at least 16 bit. Although I don't know of any platform that uses less than 32 bits, it might be a problem for some obscure architectures or compilers. MsgPack allows for arrays and maps to contain up to 2^32 entries (2^33 if you count the keys/values in a map separately). And nested arrays/maps also increment the obj_skip_count. So a single array with 2^32 entries, containing an array with 20 elements will result in an overflow, although it should be valid data. I'm not sure if such a case has to be handled correctly (rather unrealistic data) or if it could be used maliciously.

[antoinealb]

It is also guaranteed to be able to represent any sizeof() result, therefore it will be at least 32 bit on 32 bit machines and 64 bits on 64 bits machines.

[camgunz]

Hey, thank you so much :) :) :+1:

I'll try and dedicate some time to this tomorrow, sorry I've been slacking.

[camgunz]

I've started working on a different approach to skipping, functions starting here. Let me know what you think.

[camgunz]

Closing this; different implementation has been merged into master.