61 lines
1.8 KiB
Haskell
61 lines
1.8 KiB
Haskell
{- html detection
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-
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- Copyright 2017-2021 Joey Hess <id@joeyh.name>
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-
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- License: BSD-2-clause
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-}
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module Utility.HtmlDetect (
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isHtml,
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isHtmlBs,
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isHtmlFile,
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htmlPrefixLength,
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) where
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import Text.HTML.TagSoup
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import System.IO
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import Data.Char
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import qualified Data.ByteString.Lazy as B
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import qualified Data.ByteString.Lazy.Char8 as B8
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-- | Detect if a String is a html document.
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--
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-- The document many not be valid, or may be truncated, and will
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-- still be detected as html, as long as it starts with a
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-- "<html>" or "<!DOCTYPE html>" tag.
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--
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-- Html fragments like "<p>this</p>" are not detected as being html,
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-- although some browsers may chose to render them as html.
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isHtml :: String -> Bool
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isHtml = evaluate . canonicalizeTags . parseTags . take htmlPrefixLength
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where
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evaluate (TagOpen "!DOCTYPE" ((t, _):_):_) = map toLower t == "html"
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evaluate (TagOpen "html" _:_) = True
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-- Allow some leading whitespace before the tag.
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evaluate (TagText t:rest)
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| all isSpace t = evaluate rest
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| otherwise = False
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-- It would be pretty weird to have a html comment before the html
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-- tag, but easy to allow for.
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evaluate (TagComment _:rest) = evaluate rest
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evaluate _ = False
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-- | Detect if a ByteString is a html document.
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isHtmlBs :: B.ByteString -> Bool
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-- The encoding of the ByteString is not known, but isHtml only
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-- looks for ascii strings.
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isHtmlBs = isHtml . B8.unpack
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-- | Check if the file is html.
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--
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-- It would be equivalent to use isHtml <$> readFile file,
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-- but since that would not read all of the file, the handle
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-- would remain open until it got garbage collected sometime later.
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isHtmlFile :: FilePath -> IO Bool
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isHtmlFile file = withFile file ReadMode $ \h ->
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isHtmlBs <$> B.hGet h htmlPrefixLength
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-- | How much of the beginning of a html document is needed to detect it.
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-- (conservatively)
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htmlPrefixLength :: Int
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htmlPrefixLength = 8192
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