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+-- | This module exposes a function which can relativize URL's on a webpage.
+--
+-- This means that one can deploy the resulting site on
+-- @http:\/\/example.com\/@, but also on @http:\/\/example.com\/some-folder\/@
+-- without having to change anything (simply copy over the files).
+--
+-- To use it, you should use absolute URL's from the site root everywhere. For
+-- example, use
+--
+-- > <img src="/images/lolcat.png" alt="Funny zomgroflcopter" />
+--
+-- in a blogpost. When running this through the relativize URL's module, this
+-- will result in (suppose your blogpost is located at @\/posts\/foo.html@:
+--
+-- > <img src="../images/lolcat.png" alt="Funny zomgroflcopter" />
+--
+module Hakyll.Web.RelativizeUrls
+    ( relativizeUrlsCompiler
+    , relativizeUrls
+    ) where
+
+import Prelude hiding (id)
+import Control.Category (id)
+import Control.Arrow ((&&&), (>>^))
+import Data.List (isPrefixOf)
+import qualified Data.Set as S
+
+import Text.HTML.TagSoup
+
+import Hakyll.Core.Compiler
+import Hakyll.Web.Page
+import Hakyll.Web.Util.Url
+
+-- | Compiler form of 'compressCss' which automatically picks the right root
+-- path
+--
+relativizeUrlsCompiler :: Compiler (Page String) (Page String)
+relativizeUrlsCompiler = getRoute &&& id >>^ uncurry relativize
+  where
+    relativize Nothing = id
+    relativize (Just r) = fmap (relativizeUrls $ toSiteRoot r)
+
+-- | Relativize URL's in HTML
+--
+relativizeUrls :: String  -- ^ Path to the site root
+               -> String  -- ^ HTML to relativize
+               -> String  -- ^ Resulting HTML
+relativizeUrls root = renderTags . map relativizeUrls' . parseTags
+  where
+    relativizeUrls' (TagOpen s a) = TagOpen s $ map (relativizeUrlsAttrs root) a
+    relativizeUrls' x = x
+
+-- | Relativize URL's in attributes
+--
+relativizeUrlsAttrs :: String            -- ^ Path to the site root
+                    -> Attribute String  -- ^ Attribute to relativize
+                    -> Attribute String  -- ^ Resulting attribute
+relativizeUrlsAttrs root (key, value)
+    | key `S.member` urls && "/" `isPrefixOf` value = (key, root ++ value)
+    | otherwise = (key, value)
+  where
+    urls = S.fromList ["src", "href"]