csv2ctb.rb

require './ctb'
require 'csv'

## New CTB Instance, write empty file
ctb = CTB.new("Organisation", "ID_123", "A11111111")
ctb_output_file = "ctb-#{Time.now.strftime("%Y%m%d")}.txt"
File.write(ctb_output_file, '');

# Define input files
svb_input_file = 'svb.txt'

# Loop through row in the SVB file
CSV.foreach(svb_input_file) do |record|
  if record[0] == "R"
    # Write CTB entry from CSV data
    File.write(ctb_output_file, ctb.entry(
      # record[6],
      record[3],
      record[4],
      record[7],
      record[1],
      record[2]
    ), mode: 'a');
  end
end

ctb.rb

class CTB
  RECORD_LENGTH = 85
  SEPARATOR = '#'
  TERMINATOR = "\r\n"

  def initialize (company_name, company_id, company_account, date = Time.now.strftime("%Y%m%d"))
    @company_name = company_name
    @company_id = company_id
    @company_account = company_account
    @date = date
  end

  def entry (
    receiver_bank_code,
    receiver_account,
    amount,
    username,
    receiver_name
  )
    r = Array.new(RECORD_LENGTH)
    r[2] = "US" # Must contain "US"
    r[3] = "ACH" # Must contain "ACH"
    r[4] = @date # Value date
    r[10] = format('%.2f', amount.to_f) # Must have explicit decimal - 8.2 format
    r[12] = @company_account # DEBIT_ACCOUNT - 8 digits
    r[23] = r[4] # Processing date
    r[25] = "" # Transaction Reference Number
    r[30] = "PPD" # PPD or CCD
    r[31] = "Payment" # Entry Description (10)
    r[32] = username # (15)
    r[33] = "Y" # "Y" if payment amount = 0
    r[35] = @company_name # Originating Company Name
    r[36] = @company_id # Originating Company ID
    r[44] = receiver_account # Beneficiary Account or other id (17x)
    r[45] = receiver_name # Beneficiary Name (30x)
    r[51] = receiver_bank_code # Beneficiary Bank Routing Code

    # remove element [0] because these fields are 1-indexed
    r.shift
    r.join(SEPARATOR) + TERMINATOR
  end
end

ctb_spec.rb

require_relative 'ctb'

RSpec.describe CTB do
  before(:each) do
    @date = "19991215"
    @ctb = CTB.new("ABC Corp", "123456789", "98765432", @date)
  end

  describe "#entry" do
    it "generates a correct CTB line for a given set of inputs" do
      receiver_bank_code = "12345678"
      receiver_account = "9876543210"
      amount = 100.0
      username = "johndoe"
      receiver_name = "Jane Doe"

      expected_line = "#US#ACH##{@date}######100.00##98765432############{@date}#######PPD#Payment##{username}#Y##ABC Corp#123456789########9876543210#Jane Doe######12345678#################################\r\n"

      expect(@ctb.entry(receiver_bank_code, receiver_account, amount, username, receiver_name)).to eq(expected_line)
    end
  end
end