rewrite bytes2hex to be more efficient

[StefanKarpinski]

I was being really lazy when I wrote this one-liner:

bytes2hex(arr::Vector{UInt8}) = join([hex(i,2) for i in arr])

It's a good intro issue to rewrite this to pre-allocate a byte buffer, go through and fill it with hex characters appropriately and then return that buffer wrapped in a string object.

[ScottPJones]

Would this be better as a method of hex? Currently, there is no method hex(arr::Vector{UInt8})

[StefanKarpinski]

Perhaps, but there's a nice symmetry between bytes2hex and hex2bytes.

[StefanKarpinski]

While whoever does this is at it, adding tests for hex2bytes and bytes2hex would be good.

[ScottPJones]

Come to think of it, num2hex could also maybe be better as hex(val::AbstractFloat,pad).
About the symmetry, it might be OK to leave those as aliases to hex, but it doesn't seem very julian to have hex, bytes2hex, and num2hex (with some overlap), and not have hex be able to handle all cases.

[mason-bially]

Sort of ties into #14340 doesn't it? About creating a well factored set of names.

[tkelman]

function names with 2 in them do often seem like artifacts of languages that avoid dealing with types

[StefanKarpinski]

I agree about num2hex, but I'm unconvinced about making this a method of hex – what does hex mean then? One natural option would be to vectorize it, in which case it would create an array of strings that are the hex representation of each number. But I agree that it would be nice to rejigger all of this functionality in some way so that we can express all of these things more generally.

[nalimilan]

map(hex, arr) exists for when you want to create an array of strings. hex(::AbstractArray{UInt8}) should rather return a single string, replacing bytes2hex.

[StefanKarpinski]

Ok, fine, but what does hex(::AbstractArray{UInt16}) or hex(::AbstractArray{Int}) do? This is the burden of generic functions – it's not enough to say what one method should do, we have to figure out what the function means.

[StefanKarpinski]

One possibility is to say that hex{T<:Unsigned}(::AbstractArray{T}) interprets the array of unsigned values as representing a number as a sequence of digits where each unsigned integer is a digit. The number is then printed as a hexadecimal string. With that meaning, hex(::Vector{UInt8}) would almost do what the current bytes2hex does – the main difference being padding.

[nalimilan]

I can't really say what definition makes the most sense, I don't see what are the uses of this function.