URI::Generic#merge_path

# File tmp/rubies/ruby-3.0.5/lib/uri/generic.rb, line 990
def merge_path(base, rel)

  # RFC2396, Section 5.2, 5)
  # RFC2396, Section 5.2, 6)
  base_path = split_path(base)
  rel_path  = split_path(rel)

  # RFC2396, Section 5.2, 6), a)
  base_path << '' if base_path.last == '..'
  while i = base_path.index('..')
    base_path.slice!(i - 1, 2)
  end

  if (first = rel_path.first) and first.empty?
    base_path.clear
    rel_path.shift
  end

  # RFC2396, Section 5.2, 6), c)
  # RFC2396, Section 5.2, 6), d)
  rel_path.push('') if rel_path.last == '.' || rel_path.last == '..'
  rel_path.delete('.')

  # RFC2396, Section 5.2, 6), e)
  tmp = []
  rel_path.each do |x|
    if x == '..' &&
        !(tmp.empty? || tmp.last == '..')
      tmp.pop
    else
      tmp << x
    end
  end

  add_trailer_slash = !tmp.empty?
  if base_path.empty?
    base_path = [''] # keep '/' for root directory
  elsif add_trailer_slash
    base_path.pop
  end
  while x = tmp.shift
    if x == '..'
      # RFC2396, Section 4
      # a .. or . in an absolute path has no special meaning
      base_path.pop if base_path.size > 1
    else
      # if x == '..'
      #   valid absolute (but abnormal) path "/../..."
      # else
      #   valid absolute path
      # end
      base_path << x
      tmp.each {|t| base_path << t}
      add_trailer_slash = false
      break
    end
  end
  base_path.push('') if add_trailer_slash

  return base_path.join('/')
end