Vec/Str need an efficient way to shift() multi-items

[liigo]

to avoid copy/alloc/realloc memory.

in C or unsafe code, you just need to offset the data pointer and len-=n.

[thestinger]

The ring buffer type offers O(1) indexing, O(1) pop from both ends and O(1) amortized append to both ends so I don't think there should be many uses of a vector this way.

The vector type is as good as it's going to get without it becoming another data structure. It just makes sure there's reserved space (much like push) and then shifts it over via our equivalent to memmove.

[liigo]

shift() is very bad in performance, and it removes only one item at a time. if you shift() multiple times, ...

[thestinger]

Ah, so you want a method to shift more than one at once. It might be useful, although I still think we should be encouraging use of ring buffers :P.

[huonw]

This could be remove_many(&mut self, starting_index: uint, number_to_remove: uint), and then shifting 10 would be .remove_many(0, 10) (I personally think this is rare enough, and should generally be discouraged in favour of ring buffers, that we don't need a sugary .shift_many(10)).

[lifthrasiir]

@huonw More generally, how about splice(&mut self, start: uint, end: uint, replacements: &[T]) and splice_move(&mut self, start: uint, end: uint, replacements: Vec<T>)? It is the most general form of array manipulation, so having splice methods and nothing else seems to be sufficient.

[liigo]

in the PR #12976, I can't find an efficient/safe way to remove the utf-8 BOM header from source string, to avoid copy/alloc/realloc-ing memory.

[steveklabnik]

Today, changes like this would require an RFC, so if you're interested in proposing these methods, you should write one up. :)

[bluss]

Vec::drain now provides this! As soon as a snapshot includes it, we can solve the fixme in the code with this issue's tag using String::drain.