# I took your philosophy of method chaining to its logic conclusion,
# removing the local variable `sum` from the `valid?` method.
# I was able to remove both instances of .map { |char| char.to_i }
# by doing the the integer conversion in the subsequent step, saving
# both time and space. I also reversed the conditional statement in
# `maybe_double`, since I think it makes it clearer when it doubles.

def valid?(card_number)
  card_number                                           # => "4929735477250543"
    .reverse                                            # => "3450527745379294"
    .chars                                              # => ["3", "4", "5", "0", "5", "2", "7", "7", "4", "5", "3", "7", "9", "2", "9", "4"]
    .map.with_index { |n, i| maybe_double(n.to_i, i) }  # => [3, 8, 5, 0, 5, 4, 7, 14, 4, 10, 3, 14, 9, 4, 9, 8]
    .inject { |s, n| s + sum_digits(n) }                # => 80
    .modulo(10)                                         # => 0
    .zero?                                              # => true
end

def sum_digits(number)
  number                              # => 3, 8, 5, 0, 5, 4, 7, 14, 4, 10, 3, 14, 9, 4, 9, 8
    .to_s                             # => "3", "8", "5", "0", "5", "4", "7", "14", "4", "10", "3", "14", "9", "4", "9", "8"
    .chars                            # => ["3"], ["8"], ["5"], ["0"], ["5"], ["4"], ["7"], ["1", "4"], ["4"], ["1", "0"], ["3"], ["1", "4"], ["9"], ["4"], ["9"], ["8"]
    .inject(0) { |s, n| s + n.to_i }  # => 3, 8, 5, 0, 5, 4, 7, 5, 4, 1, 3, 5, 9, 4, 9, 8
end

def maybe_double(number, index)
  if index.odd?
    number * 2
  else
    number
  end
end

# # valid
# ['79927398713', '5541808923795240', '4024007136512380', '6011797668867828']
#   .map { |num| valid? num } # => [true, true, true, true]
# 
# # invalid
# ['5541801923795240', '4024007106512380', '6011797668868728']
#   .map { |num| valid? num } # => [false, false, false]

# process the example
if valid? "4929735477250543"
  puts "The number is valid!"
else
  puts "The number is invalid!"
end

# >> The number is valid!