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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


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import           Data.List               (isPrefixOf)


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import           Hakyll.Core.Compiler
import           Hakyll.Core.Item


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-- | Compiler form of 'compressCss'
compressCssCompiler :: Compiler (Item String)
compressCssCompiler = fmap compressCss <$> getResourceString


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-- | Compress CSS to speed up your site.
compressCss :: String -> String
compressCss = compressSeparators . stripComments . compressWhitespace


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-- | Compresses certain forms of separators.
compressSeparators :: String -> String
compressSeparators [] = []
compressSeparators str
    | isConstant = head str : retainConstants compressSeparators (head str) (drop 1 str)
    | stripFirst  = compressSeparators (drop 1 str)
    | stripSecond = compressSeparators (head str : (drop 2 str))
    | otherwise   = head str : compressSeparators (drop 1 str)
  where
    isConstant  = or $ map (isOfPrefix str) ["\"", "'"]
    stripFirst  = or $ map (isOfPrefix str) ["  ", " {", " }", " :", ";;", ";}"]
    stripSecond = or $ map (isOfPrefix str) ["{ ", "} ", ": ", "; "]

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-- | Compresses all whitespace.
compressWhitespace :: String -> String
compressWhitespace [] = []
compressWhitespace str
    | isConstant = head str : retainConstants compressWhitespace (head str) (drop 1 str)
    | replaceOne = compressWhitespace (' ' : (drop 1 str))
    | replaceTwo = compressWhitespace (' ' : (drop 2 str))
    | otherwise = head str : compressWhitespace (drop 1 str)
  where
    isConstant = or $ map (isOfPrefix str) ["\"", "'"]
    replaceOne = or $ map (isOfPrefix str) ["\t", "\n", "\r"]
    replaceTwo = or $ map (isOfPrefix str) [" \t", " \n", " \r", "  "]

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-- | Function that strips CSS comments away.
stripComments :: String -> String
stripComments [] = []
stripComments str
    | isConstant = head str : retainConstants stripComments (head str) (drop 1 str)
    | isPrefixOf "/*" str = stripComments $ eatComments $ drop 2 str
    | otherwise = head str : stripComments (drop 1 str)
  where
    isConstant  = or $ map (isOfPrefix str) ["\"", "'"]
    eatComments str'
        | null str' = []
        | isPrefixOf "*/" str' = drop 2 str'
        | otherwise = eatComments $ drop 1 str'

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-- | Helper function to handle string constants correctly.
retainConstants :: (String -> String) -> Char -> String -> String
retainConstants f delim str
    | null str = []
    | isPrefixOf [delim] str = head str : f (drop 1 str)
    | otherwise = head str : retainConstants f delim (drop 1 str)

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-- | Helper function to determine whether a string is a substring.
isOfPrefix :: String -> String -> Bool
isOfPrefix = flip isPrefixOf