--
-- | Fetch URL page titles of HTML links.
--
module Plugin.Url (theModule) where

import Plugin

PLUGIN Url

instance Module UrlModule Bool where
    moduleHelp  _ "url-title"     = "url-title <url>. Fetch the page title."
    moduleCmds  _                 = ["url-title"]
    modulePrivs _                 = ["url-on", "url-off"]
    moduleDefState _              = return True -- url on

    process_    _ "url-title" url = fetchTitle url
    process_    _ "url-on"    _   = writeMS True  >> return ["Url enabled"]
    process_    _ "url-off"   _   = writeMS False >> return ["Url disabled"]

    contextual  _ _ _ text        = do 
      alive <- readMS
      if alive && (not $ areSubstringsOf ignoredStrings text)
        then case containsUrl text of
               Nothing  -> return []
               Just url -> fetchTitle url
        else return []

------------------------------------------------------------------------

-- | Fetch the title of the specified URL.
fetchTitle :: String -> LB [String]
fetchTitle url = do
    title <- io $ urlPageTitle url (proxy config)
    return $ maybe [] return title

-- | List of strings that, if present in a contextual message, will
-- prevent the looking up of titles.  This list can be used to stop 
-- responses to lisppaste for example.  Another important use is to
-- another lambdabot looking up a url title that contains another 
-- url in it (infinite loop).  Ideally, this list could be added to 
-- by an admin via a privileged command (TODO).
ignoredStrings :: [String]
ignoredStrings = 
    ["paste",                -- Ignore lisppaste, rafb.net
     "cpp.sourcforge.net",   -- C++ paste bin
     "HaskellIrcPastePage",  -- Ignore paste page
     "title of that page",   -- Ignore others like the old me
     urlTitlePrompt]         -- Ignore others like me

-- | Suffixes that should be stripped off when identifying URLs in
-- contextual messages.  These strings may be punctuation in the
-- current sentence vs part of a URL.  Included here is the NUL
-- character as well.
ignoredUrlSuffixes :: [String]
ignoredUrlSuffixes = [".", ",", ";", ")", "\"", "\1"]

-- | Searches a string for an embeddded URL and returns it.
containsUrl :: String -> Maybe String
containsUrl text = do
    (_,kind,rest,_) <- matchRegexAll begreg text
    let url = takeWhile (/=' ') rest
    return $ stripSuffixes ignoredUrlSuffixes $ kind ++ url
    where
      begreg = mkRegexWithOpts "https?://"  True False

-- | Utility function to remove potential suffixes from a string.
-- Note, once a suffix is found, it is stripped and returned, no other
-- suffixes are searched for at that point.
stripSuffixes :: [String] -> String -> String
stripSuffixes []   str   = str
stripSuffixes (s:ss) str
    | isSuffixOf s str   = take (length str - length s) $ str
    | otherwise          = stripSuffixes ss str


-- | Utility function to check of any of the Strings in the specified
-- list are substrings of the String.
areSubstringsOf :: [String] -> String -> Bool 
areSubstringsOf = flip (any . flip isSubstringOf)
    where
      isSubstringOf s str = any (isPrefixOf s) (tails str)