mitems with start

[juancarlospaco]

Working on improving the random.sampleBuffer, I made this change:

proc sampleBuffer*[T](r: var Rand, buffer: var openArray[T]; alphabet: set[T]; start = 0.Natural) =
  # for c in buffer.mitems: c = sample(r, alphabet)              # OLD code.
  for i in start .. buffer.len: buffer[i] = sample(r, alphabet)  # See "start".

I need a mitems but that takes a start: Natural argument, but no such overload exists,
would be very nice because mitems tends to be used for in-place good performance loops!

https://nim-lang.github.io/Nim/iterators.html#mitems.i%2Carray%5BIX%2CT%5D

What if I have a gigabyte sized iterable but only want to iterate only the last 9 items ?.

• Add an overload to mitems that takes 1 extra argument start: Natural.
• Or a range 0..9 being start..end, but with the start at least is Ok.

The code of mitems is not massively complex, should be easy to implement.

iterator mitems*[T](a: var openArray[T]): var T {.inline.} =
  var i = 0
  while i < len(a):
    yield a[i]
    inc(i)

Not only performance, but consistency and correctness of the code,
switching mitems with for i in start..end and back again is just wrong.

[timotheecour]

but openArray can be sliced (modulo some backend specific bugs, see nim-lang#15952); assuming nim-lang#15952) is fixed, doesnt' a.toOpenArray(start, end) subsume your proposal?

[juancarlospaco]

toOpenArray is iterator and {.inline.} ?. 🤔

[timotheecour]

instead of:

proc sampleBuffer*[T](r: var Rand, buffer: var openArray[T]; alphabet: set[T]; start = 0.Natural) = ...
sampleBuffer(r, buf, alpha, start = 2)

use:

proc sampleBuffer*[T](r: var Rand, buffer: var openArray[T]; alphabet: set[T]) = ...
sampleBuffer(r, buf.toOpenArray(2, buf.high), alpha)

[juancarlospaco]

Then this bug is kinda fixed.

• But would be nice to document that, on the mitems documentation.

[timotheecour]

But would be nice to document that, on the mitems documentation.

not sure it belongs to mitems documentation, toOpenarray likewise can be used in other contexts (eg items and plenty other places). But we should definitely fix nim-lang#15952