{-# LANGUAGE OverloadedStrings #-}
module Network.URI.Cache(shouldCacheHTTP, cacheHTTP, readCacheHTTP) where
import Network.HTTP.Client
import Network.HTTP.Types.Status
import Network.HTTP.Types.Header

-- For escaping filepaths, since I already have this dependency
import Network.URI (escapeURIString, isUnescapedInURIComponent, URI, uriToString)
import Data.Time.Clock
import Data.Time.Format

import Data.ByteString as Strict
import Data.ByteString.Char8 as C
import Data.ByteString.Lazy as Lazy
import System.IO as IO
import System.FilePath
import System.Directory

import Data.Maybe
import Data.Char (isSpace)
import Data.List as L
import Control.Monad (forM)
import Text.Read (readMaybe)

strip = C.dropWhile isSpace -- FIXME Upgrade bytestring dependency for a real strip function.

httpCacheDirective :: Response b -> Strict.ByteString -> Maybe Strict.ByteString
httpCacheDirective response key | Just header <- lookup hCacheControl $ responseHeaders response =
        let directives = Prelude.map strip $ C.split ',' header
        in if key `Prelude.elem` directives
            then Just ""
            else listToMaybe $ mapMaybe (C.stripPrefix $ C.snoc key '=') directives
    | otherwise = Nothing

shouldCacheHTTP :: Response b -> Bool
-- IETF RFC7234 Section 3
shouldCacheHTTP response = -- Assume GET
    statusCode (responseStatus response) `Prelude.elem` [200, 201, 404] && -- Supported response code
        isNothing (httpCacheDirective response "no-store") -- Honor no-store
        -- This is a private cache, don't check for Cache-Control: private
        -- Also, I'll cache anything for supported response codes, regardless of explicit expiry times.

uriToString' uri = uriToString id uri ""
parseHTTPTime str | ',' `L.elem` str = parseTimeM True defaultTimeLocale rfc822DateFormat str
parseHTTPTime str = parseTimeM True defaultTimeLocale "%_d %b %Y %H:%M:%S %Z" str
secondsFromNow i = do
    now <- getCurrentTime
    -- This ugliness required because regex depends on outdated version of time.
    return $ addUTCTime (fromRational $ toRational $ secondsToDiffTime i) now

computeExpires :: Response a -> IO UTCTime
computeExpires resp
  | Just header <- lookup hExpires $ responseHeaders resp,
        Just time <- parseHTTPTime $ C.unpack header = return time
  | Just pragma <- httpCacheDirective resp "max-age",
        Just seconds <- readMaybe $ C.unpack pragma = secondsFromNow seconds
  | otherwise = secondsFromNow (60*60*24) -- One day

cacheHTTP :: URI -> Response Lazy.ByteString -> IO ()
cacheHTTP uri resp | shouldCacheHTTP resp = do
    expires <- computeExpires resp
    writeKV (uriToString' uri) ([
            ("mime", C.unpack $ fromMaybe "application/octet-stream" $ 
                    lookup "content-type" $ responseHeaders resp),
            ("expires", show expires)
        ], responseBody resp)
cacheHTTP _ _ = return ()

readCacheHTTP :: URI -> IO (Maybe (String, Lazy.ByteString))
readCacheHTTP uri = do
    (headers, body) <- readKV (uriToString' uri)
    let mime = fromMaybe "application/octet-stream" $ lookup "mime" headers
    case readMaybe =<< lookup "expires" headers of
        Nothing -> return Nothing
        Just expiry -> do
            now <- getCurrentTime
            if expiry <= now then return $ Just (mime, body)
            else do
                -- TODO validate via a HEAD request
                return Nothing

------
--- Key-value storage
------

readKV :: String -> IO ([(String, String)], Lazy.ByteString)
writeKV :: String -> ([(String, String)], Lazy.ByteString) -> IO ()
openKV :: String -> IO.IOMode -> (Handle -> IO r) -> IO r

openKV key mode act = do
    dir <- getXdgDirectory XdgCache "nz.geek.adrian.hurl"
    IO.withFile (dir </> "http" </> escapeURIString isUnescapedInURIComponent key) mode act

readKV key = openKV key ReadMode parseHeaders
  where
    parseHeaders h = do
        line <- IO.hGetLine h
        case L.break isSpace $ strip' line of
            ("", "") -> do
                body <- Lazy.hGetContents h
                return ([], body)
            (key, value) -> do
                (headers, body) <- parseHeaders h
                return ((key, strip' value):headers, body)
    strip' = L.dropWhile isSpace . L.dropWhileEnd isSpace

writeKV key (headers, body) = openKV key WriteMode $ \h -> do
    forM headers $ \(key, value) -> do
        IO.hPutStrLn h (key++' ':value)
    IO.hPutStrLn h ""
    Lazy.hPut h body