Ruby: Don't read a file into memory all at once. Read it n lines at a time. (spoiler: Use enumerators!)

ruby_read_file_in_chunks.rb

# (still working on the comments / descriptions for this)

# When you don't want to read an entire file into memory at once, you can use Enumerators to read it in 1 part at a time.
#
# An Enumerator is really like someone that keeps track of a _where you are_ in a collection of things,
# and knows how to do a few things related to the current location, like:
#    - give me one thing at a time, a.k.a.  ".each"
#    - go to the next thing, a.k.a.  ".next"
#    - get the next thing, but don't actually move your location forward to it, a.k.a. ".peek"
#    - tell me how many things there are, a.k.a. ".size"
#    - go back to the start, a.k.a. ".rewind"
#   and other cool stuff.

# There is a very cool subclass of Enumerator that will _only work with a certain number of things at a time._
# It is class Lazy < Enumerator  ( in the enumerator.rb file)
#
# Since it only works with a certain number things at a time from your collection,
# it means you can _read from an infinite collection!_  Yes!  it's true!
#
#
# Among the methods it has are:
#
#

    # return the number of matches in the file.
    # Don't read the entire file into memory; read it only (num_lines_per_batch) lines at a time
    #  @url https://stackoverflow.com/questions/2962134/ruby-read-file-in-batches/41069373#41069373
    def  num_matches_in_file(fname, match_regexp)

      num_lines_per_batch   = 5000

      num_matched = 0
      File.open(fname, "r") do |f|

        # use an enumerator to read just (num_lines_per_batch) lines at a time
        f.lazy.each_slice(num_lines_per_batch) do |lines|

          num_matched += lines.select{ | line | line.match(match_regexp)}.count

        end

      end

      num_matched

    end