-----------------------------------------------------------------------------
-- Standard Library: Numeric operations
--
-- Suitable for use with Hugs 98
-----------------------------------------------------------------------------

module Hugs.Numeric
	( fromRat 	-- :: (RealFloat a) => Rational -> a

	, showEFloat	-- :: (RealFloat a) => Maybe Int -> a -> ShowS
	, showFFloat	-- :: (RealFloat a) => Maybe Int -> a -> ShowS
	, showGFloat	-- :: (RealFloat a) => Maybe Int -> a -> ShowS
	, showFloat	-- :: (RealFloat a) => a -> ShowS

	, floatToDigits -- :: (RealFloat a) => Integer -> a -> ([Int], Int)
	) where

import Data.Char   ( intToDigit )
import Data.Ratio  ( (%), numerator, denominator )
import Hugs.Array  ( (!), Array, array )

-- This converts a rational to a floating.  This should be used in the
-- Fractional instances of Float and Double.

fromRat :: (RealFloat a) => Rational -> a
fromRat x 
 | x == 0    = encodeFloat 0 0    -- Handle exceptional cases
 | x < 0     = -fromRat' (-x)     -- first.
 | otherwise = fromRat' x

-- Conversion process:
-- Scale the rational number by the RealFloat base until
-- it lies in the range of the mantissa (as used by decodeFloat/encodeFloat).
-- Then round the rational to an Integer and encode it with the exponent
-- that we got from the scaling.
-- To speed up the scaling process we compute the log2 of the number to get
-- a first guess of the exponent.
fromRat' :: (RealFloat a) => Rational -> a
fromRat' x = r
  where b = floatRadix r
        p = floatDigits r
        (minExp0, _) = floatRange r
        minExp = minExp0 - p            -- the real minimum exponent
        xMin = toRational (expt b (p-1))
        xMax = toRational (expt b p)
        p0 = (integerLogBase b (numerator x) -
              integerLogBase b (denominator x) - p) `max` minExp
        f = if p0 < 0 then 1 % expt b (-p0) else expt b p0 % 1
        (x', p') = scaleRat (toRational b) minExp xMin xMax p0 (x / f)
        r = encodeFloat (round x') p'

-- Scale x until xMin <= x < xMax, or p (the exponent) <= minExp.
scaleRat :: Rational -> Int -> Rational -> Rational -> 
             Int -> Rational -> (Rational, Int)
scaleRat b minExp xMin xMax p x
 | p <= minExp = (x,p)
 | x >= xMax   = scaleRat b minExp xMin xMax (p+1) (x/b)
 | x <  xMin   = scaleRat b minExp xMin xMax (p-1) (x*b)
 | otherwise   = (x, p)

-- Exponentiation with a cache for the most common numbers.
minExpt = 0::Int
maxExpt = 1100::Int
expt :: Integer -> Int -> Integer
expt base n =
    if base == 2 && n >= minExpt && n <= maxExpt then
        expts!n
    else
        base^n

expts :: Array Int Integer
expts = array (minExpt,maxExpt) [(n,2^n) | n <- [minExpt .. maxExpt]]

-- Compute the (floor of the) log of i in base b.
-- Simplest way would be just divide i by b until it's smaller then b,
-- but that would be very slow!  We are just slightly more clever.
integerLogBase :: Integer -> Integer -> Int
integerLogBase b i =
     if i < b then
        0
     else
        -- Try squaring the base first to cut down the number of divisions.
        let l = 2 * integerLogBase (b*b) i
            doDiv :: Integer -> Int -> Int
            doDiv i l = if i < b then l else doDiv (i `div` b) (l+1)
        in  doDiv (i `div` (b^l)) l

-- Misc utilities to show integers and floats

showEFloat     :: (RealFloat a) => Maybe Int -> a -> ShowS
showFFloat     :: (RealFloat a) => Maybe Int -> a -> ShowS
showGFloat     :: (RealFloat a) => Maybe Int -> a -> ShowS
showFloat      :: (RealFloat a) => a -> ShowS

showEFloat d x =  showString (formatRealFloat FFExponent d x)
showFFloat d x =  showString (formatRealFloat FFFixed d x)
showGFloat d x =  showString (formatRealFloat FFGeneric d x)
showFloat      =  showGFloat Nothing 

-- These are the format types.  This type is not exported.

data FFFormat = FFExponent | FFFixed | FFGeneric

formatRealFloat :: (RealFloat a) => FFFormat -> Maybe Int -> a -> String
formatRealFloat fmt decs x 
  | isNaN      x = "NaN"
  | isInfinite x = if x < 0 then "-Infinity" else "Infinity"
  | x < 0 || isNegativeZero x = '-' : doFmt fmt (floatToDigits (toInteger base) (-x))
  | otherwise    = doFmt fmt (floatToDigits (toInteger base) x)
  where base = 10

        doFmt fmt (is, e) =
            let ds = map intToDigit is
            in  case fmt of
                FFGeneric -> 
                    doFmt (if e < 0 || e > 7 then FFExponent else FFFixed)
                          (is, e)
                FFExponent ->
                    case decs of
                    Nothing ->
                        case ds of
			 []    -> "0.0e0"
                         [d]   -> d : ".0e" ++ show (e-1)
                         d:ds  -> d : '.' : ds ++ 'e':show (e-1)
                    Just dec ->
                        let dec' = max dec 1 in
                        case is of
                         [] -> '0':'.':take dec' (repeat '0') ++ "e0"
                         _ ->
                          let (ei, is') = roundTo base (dec'+1) is
                              d:ds = map intToDigit
                                         (if ei > 0 then init is' else is')
                          in d:'.':ds  ++ "e" ++ show (e-1+ei)
                FFFixed ->
                  case decs of
                    Nothing 
		     | e > 0 -> take e (ds ++ repeat '0')
		     	        ++ '.' : mk0 (drop e ds)
		     | otherwise -> '0' : '.' : mk0 (replicate (-e) '0' ++ ds)
                    Just dec ->
                        let dec' = max dec 0 in
                        if e >= 0 then
                            let (ei, is') = roundTo base (dec' + e) is
                                (ls, rs) = splitAt (e+ei) (map intToDigit is')
                            in  mk0 ls ++ mkdot0 rs
                        else
                            let (ei, is') = roundTo base dec'
                                              (replicate (-e) 0 ++ is)
                                d : ds = map intToDigit
                                            (if ei > 0 then is' else 0:is')
                            in  d : mkdot0 ds
		  where
		    mk0 "" = "0"     -- Used to ensure we print 34.0, not 34.
	       	    mk0 s  = s       -- and 0.34 not .34
    
    		    mkdot0 "" = ""   -- Used to ensure we print 34, not 34.
		    mkdot0 s  = '.' : s

roundTo :: Int -> Int -> [Int] -> (Int, [Int])
roundTo base d is = case f d is of
                v@(0, is) -> v
                (1, is)   -> (1, 1 : is)
  where b2 = base `div` 2
        f n [] = (0, replicate n 0)
        f 0 (i:_) = (if i >= b2 then 1 else 0, [])
        f d (i:is) = 
            let (c, ds) = f (d-1) is
                i' = c + i
            in  if i' == base then (1, 0:ds) else (0, i':ds)

--
-- Based on "Printing Floating-Point Numbers Quickly and Accurately"
-- by R.G. Burger and R. K. Dybvig, in PLDI 96.
-- This version uses a much slower logarithm estimator.  It should be improved.

-- This function returns a list of digits (Ints in [0..base-1]) and an
-- exponent.

floatToDigits :: (RealFloat a) => Integer -> a -> ([Int], Int)

floatToDigits _ 0 = ([0], 0)
floatToDigits base x =
    let (f0, e0) = decodeFloat x
        (minExp0, _) = floatRange x
        p = floatDigits x
        b = floatRadix x
        minExp = minExp0 - p            -- the real minimum exponent
        -- Haskell requires that f be adjusted so denormalized numbers
        -- will have an impossibly low exponent.  Adjust for this.
	f :: Integer
	e :: Int
        (f, e) = let n = minExp - e0
                 in  if n > 0 then (f0 `div` (b^n), e0+n) else (f0, e0)

        (r, s, mUp, mDn) =
           if e >= 0 then
               let be = b^e in
               if f == b^(p-1) then
                   (f*be*b*2, 2*b, be*b, b)
               else
                   (f*be*2, 2, be, be)
           else
               if e > minExp && f == b^(p-1) then
                   (f*b*2, b^(-e+1)*2, b, 1)
               else
                   (f*2, b^(-e)*2, 1, 1)
        k = 
            let k0 =
                    if b==2 && base==10 then
                        -- logBase 10 2 is slightly bigger than 3/10 so
                        -- the following will err on the low side.  Ignoring
                        -- the fraction will make it err even more.
                        -- Haskell promises that p-1 <= logBase b f < p.
                        (p - 1 + e0) * 3 `div` 10
                    else
                        ceiling ((log (fromInteger (f+1)) + 
                                 fromIntegral e * log (fromInteger b)) / 
                                  log (fromInteger base))
                fixup n =
                    if n >= 0 then
                        if r + mUp <= expt base n * s then n else fixup (n+1)
                    else
                        if expt base (-n) * (r + mUp) <= s then n
                                                           else fixup (n+1)
            in  fixup k0

        gen ds rn sN mUpN mDnN =
            let (dn, rn') = (rn * base) `divMod` sN
                mUpN' = mUpN * base
                mDnN' = mDnN * base
            in  case (rn' < mDnN', rn' + mUpN' > sN) of
                (True,  False) -> dn : ds
                (False, True)  -> dn+1 : ds
                (True,  True)  -> if rn' * 2 < sN then dn : ds else dn+1 : ds
                (False, False) -> gen (dn:ds) rn' sN mUpN' mDnN'
        rds =
            if k >= 0 then
                gen [] r (s * expt base k) mUp mDn
            else
                let bk = expt base (-k)
                in  gen [] (r * bk) s (mUp * bk) (mDn * bk)
    in  (map fromIntegral (reverse rds), k)
