#!/usr/bin/env ruby
# frozen_string_literal: true

# sample-script.rb: summarise a small list of expenses by category.
# Free sample from 99helpers.com/file-types/rb
# Run:  ruby sample-script.rb                 (uses the built-in sample data)
#       ruby sample-script.rb expenses.txt    (lines of: date, category, amount, note)
#       ruby sample-script.rb --json
#
# 1. Shebang (line 1): on macOS and Linux, `chmod +x sample-script.rb` then
#    `./sample-script.rb` runs it with the first `ruby` on your PATH.
# 2. Magic comment (line 2): every string literal in this file is frozen
#    (read-only). It must sit in the first comment section of the file.

# 3. require loads a library. json ships with Ruby, so no Gemfile is needed.
require "json"

# 4. A module groups related constants and methods under one name.
module Expenses
  CURRENCY = "EUR"

  # 5. Data.define (Ruby 3.2+) creates a small immutable value class.
  Item = Data.define(:date, :category, :amount, :note)

  # 6. A squiggly heredoc: <<~ removes the common indentation.
  SAMPLE = <<~TEXT
    # date, category, amount, note
    2026-10-01, travel, 42.50, Train to Utrecht
    2026-10-02, food, 12.80, Lunch with Ada Example
    2026-10-03, office, 89.99, Desk lamp
    2026-10-05, food, 7.45, Coffee beans
    2026-10-06, travel, 18.00, Bike repair
    2026-10-07, food, twelve, A typo this script should skip
  TEXT

  # 7. Methods return their last expression. A do...end block can have its
  #    own rescue clause; filter_map drops the nils returned by `next`.
  def self.parse(text)
    text.each_line.with_index(1).filter_map do |line, number|
      next if line.strip.empty? || line.start_with?("#")

      date, category, amount, note = line.split(",", 4).map(&:strip)
      Item.new(date: date, category: category, amount: Float(amount), note: note)
    rescue ArgumentError, TypeError
      warn "line #{number}: skipped, can't read #{line.strip.inspect}"
      next
    end
  end

  # 8. Blocks and Enumerable: group, add up and sort in one chain.
  def self.totals(items)
    items.group_by(&:category)
         .transform_values { |list| list.sum(&:amount).round(2) }
         .sort_by { |_category, total| -total }
  end
end

# 9. String building: literals are frozen here, so +"" makes a mutable copy.
#    format() pads columns like printf in C.
def report(items)
  out = +""
  Expenses.totals(items).each do |category, total|
    out << format("%-8s %8.2f %s\n", category, total, Expenses::CURRENCY)
  end
  grand = items.sum(&:amount)
  out << format("%-8s %8.2f %s (%d items)\n", "total", grand, Expenses::CURRENCY, items.size)
end

# 10. Run only when this file is executed directly, not when another file
#     loads it with require_relative.
if __FILE__ == $PROGRAM_NAME
  json = ARGV.delete("--json")
  begin
    text = ARGV.empty? ? Expenses::SAMPLE : File.read(ARGV.first, encoding: "UTF-8")
  rescue Errno::ENOENT
    abort "error: #{ARGV.first} not found" # prints to stderr, exits with status 1
  end

  items = Expenses.parse(text)
  # 11. to_h on a Data object, then JSON.pretty_generate for --json.
  puts json ? JSON.pretty_generate(items.map(&:to_h)) : report(items)
end