AEMO Checksum Ruby Code

I can't write Ruby code, but I found this ruby code to calculate checksum for AEMO NMI

def checksum
  summation = 0
  @nmi.reverse.split(//).each_index do |i|
    value = nmi[nmi.length - i - 1].ord
    value *= 2 if i.even?
    value = value.to_s.split(//).map(&:to_i).reduce(:+)
    summation += value
  end
  checksum = (10 - (summation % 10)) % 10
  checksum
end

Could someone please help me to explain what does this line mean?

value = value.to_s.split(//).map(&:to_i).reduce(:+)

I try to convert the code above to VBA for excel.

For an input of "4103738516" will give you 8 "4102030716" ==> 2 "QFFF0000LV" ==> 7

in page 40 of this document has JavaScript code to calculate it but I can't understand the code.

https://www.aemo.com.au/-/media/Files/PDF/0610-0008-pdf.pdf

Thank you

Solution

Below code should make you understand that statement better:

# Say
value = 82478923    # any random number
value.to_s          # => "82478923"

# `split(//)` is similar to `split` and `split('')`, they split a string after each character to generate an array of characters.
ary = value.to_s.split(//)      # => ["8", "2", "4", "7", "8", "9", "2", "3"]

ary.map(&:to_i)     # => [8, 2, 4, 7, 8, 9, 2, 3]

# `inject(:+)` will iterate the array and calculate sum of all numbers
ary.map(&:to_i).inject(:+)      # => 43