48 lines
1.6 KiB
Haskell
48 lines
1.6 KiB
Haskell
import Data.Char
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import Data.Map
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charToInt :: Char -> Int
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charToInt c = ord c - ord '0'
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-- Given a string, decode a single character from the string.
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-- Return the decoded char and the remaining undecoded string.
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decodeChar :: String -> (Char,String)
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decodeChar ('7':'9':r:rs) = (r,rs)
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decodeChar ('7':r:rs) = ("PQUVWXYZ. " !! charToInt r, rs)
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decodeChar ('3':r:rs) = ("ABCDFGIJKN" !! charToInt r, rs)
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decodeChar (r:rs) = ("HOL MES RT" !! charToInt r, rs)
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-- Decode an entire string.
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decode :: String -> String
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decode [] = []
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decode st = let (c, s) = decodeChar st in c:decode s
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-- Given a string, decode a single character from the string.
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-- Return the decoded char and the part of the encoded string
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-- used to encode that character.
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revEnc :: String -> (Char, String)
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revEnc enc = let (dec, rm) = decodeChar enc in (dec, take (length enc - length rm) enc)
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ds :: String
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ds = ['0'..'9']
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-- Decode all 1000 possible encodings of three digits and
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-- use results to construct map used to encode.
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encodeMap :: Map Char String
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encodeMap = fromList [ revEnc [d2,d1,d0] | d2 <- ds, d1 <- ds, d0 <- ds ]
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-- Encode a single char using encoding map.
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encodeChar :: Char -> String
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encodeChar c = findWithDefault "" c encodeMap
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-- Encode an entire string.
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encode :: String -> String
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encode st = concatMap encodeChar $ fmap toUpper st
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-- Test by encoding, decoding, printing results.
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main = let orig = "One night-it was on the twentieth of March, 1888-I was returning"
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enc = encode orig
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dec = decode enc
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in mapM_ putStrLn [ "Original: " ++ orig
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, "Encoded: " ++ enc
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, "Decoded: " ++ dec ]
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