line-based iconv with fallback encoding

liconv.rb

#!/usr/bin/env ruby
# iconv line by line with fallback encoding
require 'optparse'

options = {
  from: Encoding::UTF_8,
  to: Encoding::UTF_8,
  fallback: Encoding::ISO_8859_1
}

def find_encoding(name)
  begin
    Encoding.find(name)
  rescue
    puts "Invalid encoding name: #{name}"
    exit 1
  end
end

OptionParser.new do |opts|
  opts.banner = 'Usage: liconv [-f ENCODING] [-b ENCODING] [-t ENCODING]'
  
  opts.on('-l', '--list', 'show list of encodings') do
    puts Encoding.list.map(&:names).map{|names| names.join(' ')}.sort_by(&:downcase)
    exit 0
  end
  
  opts.on('-fENCODING', '--from=ENCODING', 'input encoding') do |v|
    options[:from] = find_encoding(v)
  end
  
  opts.on('-tENCODING', '--to=ENCODING', 'output encoding') do |v|
    options[:to] = find_encoding(v)
  end
  
  opts.on('-bENCODING', '--fallback=ENCODING', 'fallback input encoding') do |v|
    options[:fallback] = find_encoding(v)
  end
  
  opts.on('-s', '--silent', 'suppress error messages') do |v|
    options[:silent] = v
  end
  
  opts.on('-h', '--help', 'show this help') do
    puts opts
    exit 0
  end
end.parse!

from, to, fallback = options.values_at(:from, :to, :fallback)

ln = 0
errors = []
while line = gets
  ln += 1
  if line.encoding == from and line.valid_encoding?
    puts line
  elsif fallback && line.force_encoding(fallback).valid_encoding?
    puts line.encode(to)
  else
    puts line.force_encoding(Encoding::ASCII).encode!(to, invalid: :replace)
    errors << ln
  end
end

unless options[:silent]
  errors.each { |lineno| STDERR.puts "input:#{lineno}: invalid encoding" }
end

exit errors.empty? ? 0 : 1