Iterate slice by reference (not by value)

[ysbaddaden]

Iterating a Slice(T) with #each leads to iterating the slice by value. This is usually fine when T is a primitive (e.g. UInt8) or a Reference to a class instance, but when T is a struct it leads to getting a copy. Sometimes this is what we want, but at other times we'd like to avoid the copy and/or need to mutate the struct in-place.

This stems from the nature of Ruby where everything's supposed to be a Reference or a primitive, not a value that is passed by copy (except for primitives).

To solve this, we currently need to get the raw pointer and loop/increment it manually, but the intent ain't immediately explicit, in addition to be ugly Crystal:

slice.size.times do |i|
  ptr = slice.to_unsafe + i
  # ...
end

We could at least abstract a method, for example #each_ref(&) or #each_reference(&). We'd still have to deal with a Pointer(T), but at least the iteration part would be much more readable:

struct Slice(T)
  def each_ref(&)
    size.times { yield to_unsafe + i }
  end
end

slice.each_ref do |ptr|
  # ...
end