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------------------------------------------------------------------------------
--- Library with some functions for reading and converting numeric tokens.
--
--- @author Michael Hanus, Frank Huch, Bjoern Peemoeller
--- @version November 2016
--- @category general
------------------------------------------------------------------------------

module ReadNumeric
  ( readInt, readNat, readHex, readOct, readBin
  ) where

import Char (digitToInt, isBinDigit, isOctDigit, isDigit, isHexDigit, isSpace)

--- Read a (possibly negative) integer as a first token in a string.
--- The string might contain leadings blanks and the integer is read
--- up to the first non-digit.
--- If the string does not start with an integer token, `Nothing` is returned,
--- otherwise the result is `Just (v, s)`, where `v` is the value of the integer
--- and `s` is the remaing string without the integer token.
readInt :: String -> Maybe (Int, String)
readInt str = case dropWhile isSpace str of
  []       -> Nothing
  '-':str1 -> maybe Nothing (\ (val,rstr) -> Just (-val,rstr)) (readNat str1)
  str1     -> readNat str1

--- Read a natural number as a first token in a string.
--- The string might contain leadings blanks and the number is read
--- up to the first non-digit.
--- If the string does not start with a natural number token,
--- `Nothing` is returned,
--- otherwise the result is `Just (v, s)` where `v` is the value of the number
--- and s is the remaing string without the number token.
readNat :: String -> Maybe (Int, String)
readNat str =
  readNumPrefix (dropWhile isSpace str) Nothing 10 isDigit digitToInt

--- Read a hexadecimal number as a first token in a string.
--- The string might contain leadings blanks and the number is read
--- up to the first non-hexadecimal digit.
--- If the string does not start with a hexadecimal number token,
--- `Nothing` is returned,
--- otherwise the result is `Just (v, s)` where `v` is the value of the number
--- and s is the remaing string without the number token.
readHex :: String -> Maybe (Int, String)
readHex l =
  readNumPrefix (dropWhile isSpace l) Nothing 16 isHexDigit digitToInt

--- Read an octal number as a first token in a string.
--- The string might contain leadings blanks and the number is read
--- up to the first non-octal digit.
--- If the string does not start with an octal number token,
--- `Nothing` is returned,
--- otherwise the result is `Just (v, s)` where `v` is the value of the number
--- and s is the remaing string without the number token.
readOct :: String -> Maybe (Int, String)
readOct l =
  readNumPrefix (dropWhile isSpace l) Nothing 8 isOctDigit digitToInt

--- Read a binary number as a first token in a string.
--- The string might contain leadings blanks and the number is read
--- up to the first non-binary digit.
--- If the string does not start with a binary number token,
--- `Nothing` is returned,
--- otherwise the result is `Just (v, s)` where `v` is the value of the number
--- and s is the remaing string without the number token.
readBin :: String -> Maybe (Int, String)
readBin l =
  readNumPrefix (dropWhile isSpace l) Nothing 2 isBinDigit digitToInt

--- Read an integral number prefix where the value of an already read number
--- prefix is provided as the second argument.
--- The third argument is the base, the fourth argument
--- is a predicate to distinguish valid digits, and the fifth argument converts
--- valid digits into integer values.
readNumPrefix :: String -> Maybe Int -> Int -> (Char -> Bool) -> (Char -> Int)
              -> Maybe (Int, String)
readNumPrefix []     Nothing  _    _       _       = Nothing
readNumPrefix []     (Just n) _    _       _       = Just (n,"")
readNumPrefix (c:cs) (Just n) base isdigit valueof
  | isdigit c = readNumPrefix cs (Just (base*n+valueof c)) base isdigit valueof
  | otherwise = Just (n,c:cs)
readNumPrefix (c:cs) Nothing base isdigit valueof
  | isdigit c = readNumPrefix cs (Just (valueof c)) base isdigit valueof
  | otherwise = Nothing