enum Writable(
    Stdout,
    File(path:Path),
)
    func write(self:Writable, text:Text, newline:Bool=no)
        when self is Stdout
            say(text, newline)
            return
        is File(path)
            _ := path.append(text)
            if newline
                _ := path.append("\n")
            return

struct Option(option: Text, value: Text)
    func parse(line: Text, expected: Text -> Option?)
        val : Text = ""
        if not line.starts_with(expected, &val)
            return none

        if val == ""
            return none

        foundequals : Bool = no
        cursor := 1
        while yes
            if val[cursor] == " "
                cursor += 1
                continue

            if not foundequals and val[cursor] == "="
                foundequals = yes
                cursor += 1
                continue

            if foundequals and cursor <= val.length
                return Option(expected, val.slice(cursor))

            return none
        return none

fmjconfpath := ".fmjconf"
fmj_extension := "fmj"

description := "description"
title := "title"
link := "link"
default_author := "default_author"
author := "author"
category := "category"
pub_date := "pub_date"

default_author_value : Text? = none

func write_feed_header(out: Writable, config: Text?)
    out.write("<?xml version=\"1.0\" encoding=\"UTF-8\" ?><rss version=\"2.0\"><channel>")

    found_title : Bool = no
    found_link : Bool = no
    found_description : Bool = no
    found_default_author : Bool = no
    if config
        for line in config.lines()
            opt : Option? = none
            opt = Option.parse(line, title)
            if not found_title and opt
                out.write("<title>$(opt!.value)</title>")
                found_title = yes
                continue
            opt = Option.parse(line, link)
            if  not found_link and opt
                out.write("<link>$(opt!.value)</link>")
                found_link = yes
                continue
            opt = Option.parse(line, description)
            if not found_description and opt
                out.write("<description>$(opt!.value)</description>")
                found_description = yes
                continue
            opt = Option.parse(line, default_author)
            if not found_default_author and opt
                default_author_value = opt!.value
                found_default_author = yes
                continue

    if not found_title
        out.write("<title>fmj generated feed</title>")

    if not found_link
        out.write("<link>http://example.com</link>")

    if not found_description
        out.write("<description>an rss feed generated by fmj</description>")

    out.write("<generator>fmj</generator>")

func write_feed_footer(out : Writable)
    out.write("</channel></rss>")

func write_feed_item(out : Writable, path : Path)
    if not (path.extension() == fmj_extension)
        return
    lines := path.read()!.lines()
    has_requirements : Bool = no
    for line in lines
        if Option.parse(line, title) or Option.parse(line, description)
            has_requirements = yes
            break
    if not has_requirements
        return

    found_title : Bool = no
    found_description : Bool = no
    found_link : Bool = no
    found_pub_date : Bool = no
    found_author : Bool = no

    out.write("<item>")
    for line in lines
        opt : Option? = none
        opt = Option.parse(line, title)
        if not found_title and opt
            out.write("<title>$(opt!.value)</title>")
            found_title = yes
            continue
        opt = Option.parse(line, link)
        if  not found_link and opt
            out.write("<link>$(opt!.value)</link>")
            found_link = yes
            continue
        opt = Option.parse(line, description)
        if not found_description and opt
            out.write("<description>$(opt!.value)</description>")
            found_description = yes
            continue
        opt = Option.parse(line, author)
        if not found_author and opt
            out.write("<author>$(opt!.value)</description>")
            found_author = yes
            continue
        opt = Option.parse(line, pub_date)
        if not found_pub_date and opt
            out.write("<pubDate>$(opt!.value)</pubDate>")
            found_pub_date = yes
            continue
        opt = Option.parse(line, category)
        if opt
            out.write("<category>$(opt.value)</category>")
            continue
    if not found_author and default_author_value
        out.write("<author>$(default_author_value)</author>")
    out.write("</item>")

func main(dir: Path, output: Text = "rss.xml")
    usingStdout : Bool = no
    outfile : Path? = none
    if output == "-"
        usingStdout = yes
    else
        outfile = Path.from_text(output)

    if not usingStdout and outfile!.exists()
        _ := outfile!.remove()

    out : Writable = if usingStdout
        Writable.Stdout
    else
        Writable.File(outfile!)

    config := dir.child(fmjconfpath).read()

    out.write_feed_header(config)

    for path in dir.children()
        out.write_feed_item(path)

    out.write_feed_footer()