mail_header.rb

# typed: true
# Ruby 2.6 (Function Composition, filter)

require 'sorbet-runtime'

class MailHeader
    extend T::Sig

    TEST_FILE_PATH = '/tmp/__mail_header_test_file__'

    sig {void}
    def self.make_test_file()
        if FileTest.file?(TEST_FILE_PATH)
            return
        end

        text = <<-EOS
Received: from mail2.example.jp (mail2 [127.0.0.1])
 by mail2.example.jp (Postfix) with SMTP id 8EA8999C0C2;
 Tue, 23 Aug 2011 11:06:24 +0900 (JST)
Received: from mail1.example.jp (unknown [172.16.0.1])
        by mail2.example.jp (Postfix) with SMTP;
        Tue, 23 Aug 2011 11:06:24 +0900 (JST)
Received: from ([192.168.1.1]) (envelope sender: <test01@example.com>)
        by mail1.example.jp with Active!Hunter esmtp server;
        Tue, 23 Aug 2011 11:06:18 +0900
From: Taro Yamada <taro@example.com>
To: Jiro Tanaka <tanaka@example.jp>, Saburo Watanabe <watanabe@example.jp>,
	Hanako Takahashi <takahashi@example.jp>
Date: Mon, 22 Aug 2011 19:06:14 -0700
Subject: Hello
Message-ID: <CA79398B.2383D%yamada@example.com>
In-Reply-To: <20110804105830.000013F2.0681@example.jp>
Content-Type: text/plain
MIME-Version: 1.0
Empty:

Hi Taro
This is a test message.

END
        EOS

        File.write(TEST_FILE_PATH, text.gsub(/\n/, "\r\n"))
    end

    sig {params(text: String).returns(T::Array[String])}
    def self.split_header_body(text)
        res = text.split("\r\n\r\n", 2)
        if res.length != 2 then raise ArgumentError end
        res
    end

    sig {params(header: String).returns(String)}
    def self.unfold_header(header)
        header.gsub(/\r\n[ \t]+/, ' ')
    end

    sig {params(line: String).returns(T.nilable(T::Array[T.nilable(String)]))}
    def self.split_field(line)
        m = line.match(/(\S+): *(.+)?/)
        m.nil? ? nil : [m[1], m[2].nil? ? nil : T.must(m[2]).rstrip]
    end

    sig {params(header: String).returns(T::Hash[String, T::Array[T.nilable(String)]])}
    def self.parse_header(header)
        if header.empty? then raise ArgumentError end

        header.lines.map(&:chomp)
            .map(&method(:split_field))
            .filter{|a| !a.nil?}
            .group_by{|fn,_| fn}
            .map{|k,v| [k.downcase, v.map{|h| h[1]}]}.to_h
    end

    sig {void}
    def self.main()
        make_test_file()

        text = File.read(TEST_FILE_PATH)
        header, _ = split_header_body(text)
        puts (method(:unfold_header) >> method(:parse_header)).call(T.must(header))
    end
end

if __FILE__ == $PROGRAM_NAME
    MailHeader.main()
end