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--------------------------------------------------------------------------------
-- | Module used for CSS compression. The compression is currently in a simple
-- state, but would typically reduce the number of bytes by about 25%.
module Hakyll.Web.CompressCss
( compressCssCompiler
, compressCss
) where
--------------------------------------------------------------------------------
import Data.Char (isSpace)
import Data.List (dropWhileEnd, isPrefixOf)
--------------------------------------------------------------------------------
import Hakyll.Core.Compiler
import Hakyll.Core.Item
import Hakyll.Core.Util.String
--------------------------------------------------------------------------------
-- | Compiler form of 'compressCss'
compressCssCompiler :: Compiler (Item String)
compressCssCompiler = fmap compressCss <$> getResourceString
--------------------------------------------------------------------------------
-- | Compress CSS to speed up your site.
compressCss :: String -> String
compressCss = withoutStrings (handleCalcExpressions compressSeparators . compressWhitespace)
. dropWhileEnd isSpace
. dropWhile isSpace
. stripComments
--------------------------------------------------------------------------------
-- | Compresses certain forms of separators.
compressSeparators :: String -> String
compressSeparators =
replaceAll "; *}" (const "}") .
replaceAll ";+" (const ";") .
replaceAll " *[{};,>+~!] *" (take 1 . dropWhile isSpace) .
replaceAll ": *" (take 1) -- not destroying pseudo selectors (#323)
-- | Uses `compressCalcExpression` on all parenthesised calc expressions
-- and applies `transform` to all parts outside of them
handleCalcExpressions :: (String -> String) -> String -> String
handleCalcExpressions transform = top transform
where
top f "" = f ""
top f str | "calc(" `isPrefixOf` str = f "calc" ++ nested 0 compressCalcExpression (drop 4 str)
top f (x:xs) = top (f . (x:)) xs
-- when called with depth=0, the first character must be a '('
nested :: Int -> (String -> String) -> String -> String
nested _ f "" = f "" -- shouldn't happen, mismatched nesting
nested depth f str | "calc(" `isPrefixOf` str = nested depth f (drop 4 str)
nested 1 f (')':xs) = f ")" ++ top transform xs
nested depth f (x:xs) = nested (case x of
'(' -> depth + 1
')' -> depth - 1 -- assert: depth > 1
_ -> depth
) (f . (x:)) xs
-- | does not remove whitespace around + and -, which is important in calc() expressions
compressCalcExpression :: String -> String
compressCalcExpression =
replaceAll " *[*/] *| *\\)|\\( *" (take 1 . dropWhile isSpace)
--------------------------------------------------------------------------------
-- | Compresses all whitespace.
compressWhitespace :: String -> String
compressWhitespace = replaceAll "[ \t\n\r]+" (const " ")
--------------------------------------------------------------------------------
-- | Function that strips CSS comments away (outside of strings).
stripComments :: String -> String
stripComments "" = ""
stripComments ('/':'*':str) = stripComments $ eatComment str
stripComments (x:xs) | x `elem` "\"'" = retainString x xs stripComments
| otherwise = x : stripComments xs
eatComment :: String -> String
eatComment "" = ""
eatComment ('*':'/':str) = str
eatComment (_:str) = eatComment str
--------------------------------------------------------------------------------
-- | Helper functions to handle string tokens correctly.
-- TODO: handle backslash escapes
withoutStrings :: (String -> String) -> String -> String
withoutStrings f str = case span (`notElem` "\"'") str of
(text, "") -> f text
(text, d:rest) -> f text ++ retainString d rest (withoutStrings f)
retainString :: Char -> String -> (String -> String) -> String
retainString delim str cont = case span (/= delim) str of
(val, "") -> delim : val
(val, _:rest) -> delim : val ++ delim : cont rest
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