[project @ 1997-09-26 13:38:58 by simonm]

this file got lost somehow

ghc/tests/codeGen/should_run/cg034.hs

+import Ratio -- 1.3
+
+main = putStr (
+   shows tinyFloat  ( '\n'
+ : shows t_f	    ( '\n'
+ : shows hugeFloat  ( '\n'
+ : shows h_f	    ( '\n'
+ : shows tinyDouble ( '\n'
+ : shows t_d	    ( '\n'
+ : shows hugeDouble ( '\n'
+ : shows h_d	    ( '\n'
+ : shows x_f	    ( '\n'
+ : shows x_d	    ( '\n'
+ : shows y_f	    ( '\n'
+ : shows y_d	    ( "\n"
+ )))))))))))))
+  where
+    t_f :: Float
+    t_d :: Double
+    h_f :: Float
+    h_d :: Double
+    x_f :: Float
+    x_d :: Double
+    y_f :: Float
+    y_d :: Double
+    t_f = fromRationalX (toRational tinyFloat)
+    t_d = fromRationalX (toRational tinyDouble)
+    h_f = fromRationalX (toRational hugeFloat)
+    h_d = fromRationalX (toRational hugeDouble)
+    x_f = fromRationalX (1.82173691287639817263897126389712638972163e-300 :: Rational)
+    x_d = fromRationalX (1.82173691287639817263897126389712638972163e-300 :: Rational)
+    y_f = 1.82173691287639817263897126389712638972163e-300
+    y_d = 1.82173691287639817263897126389712638972163e-300
+
+--!! fromRational woes
+
+fromRationalX :: (RealFloat a) => Rational -> a
+fromRationalX r =
+	let 
+	    h = ceiling (huge `asTypeOf` x)
+	    b = toInteger (floatRadix x)
+	    x = fromRat 0 r
+	    fromRat e0 r' =
+		let d = denominator r'
+		    n = numerator r'
+	        in  if d > h then
+		       let e = integerLogBase b (d `div` h) + 1
+		       in  fromRat (e0-e) (n % (d `div` (b^e)))
+		    else if abs n > h then
+		       let e = integerLogBase b (abs n `div` h) + 1
+		       in  fromRat (e0+e) ((n `div` (b^e)) % d)
+		    else
+		       scaleFloat e0 (rationalToRealFloat {-fromRational-} r')
+	in  x
+
+{-
+fromRationalX r =
+  rationalToRealFloat r
+{- Hmmm...
+	let 
+	    h = ceiling (huge `asTypeOf` x)
+	    b = toInteger (floatRadix x)
+	    x = fromRat 0 r
+
+	    fromRat e0 r' =
+{--}		trace (shows e0 ('/' : shows r' ('/' : shows h "\n"))) (
+		let d = denominator r'
+		    n = numerator r'
+	        in  if d > h then
+		       let e = integerLogBase b (d `div` h) + 1
+		       in  fromRat (e0-e) (n % (d `div` (b^e)))
+		    else if abs n > h then
+		       let e = integerLogBase b (abs n `div` h) + 1
+		       in  fromRat (e0+e) ((n `div` (b^e)) % d)
+		    else
+		       scaleFloat e0 (rationalToRealFloat r')
+		       -- now that we know things are in-bounds,
+		       -- we use the "old" Prelude code.
+{--}		)
+	in  x
+-}
+-}
+
+-- Compute the discrete 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
+
+
+------------
+
+-- Compute smallest and largest floating point values.
+tiny :: (RealFloat a) => a
+tiny =
+	let (l, _) = floatRange x
+	    x = encodeFloat 1 (l-1)
+	in  x
+
+huge :: (RealFloat a) => a
+huge =
+	let (_, u) = floatRange x
+	    d = floatDigits x
+	    x = encodeFloat (floatRadix x ^ d - 1) (u - d)
+	in  x
+
+tinyDouble = tiny :: Double
+tinyFloat  = tiny :: Float
+hugeDouble = huge :: Double
+hugeFloat  = huge :: Float
+
+{-
+[In response to a request by simonpj, Joe Fasel writes:]
+
+A quite reasonable request!  This code was added to the Prelude just
+before the 1.2 release, when Lennart, working with an early version
+of hbi, noticed that (read . show) was not the identity for
+floating-point numbers.	 (There was a one-bit error about half the time.)
+The original version of the conversion function was in fact simply
+a floating-point divide, as you suggest above.	The new version is,
+I grant you, somewhat denser.
+
+How's this?
+
+--Joe
+-}
+
+
+rationalToRealFloat :: (RealFloat a) => Rational -> a
+
+rationalToRealFloat x	=  x'
+	where x'    = f e
+
+--		If the exponent of the nearest floating-point number to x 
+--		is e, then the significand is the integer nearest xb^(-e),
+--		where b is the floating-point radix.  We start with a good
+--		guess for e, and if it is correct, the exponent of the
+--		floating-point number we construct will again be e.  If
+--		not, one more iteration is needed.
+
+	      f e   = if e' == e then y else f e'
+		      where y	   = encodeFloat (round (x * (1%b)^^e)) e
+			    (_,e') = decodeFloat y
+	      b	    = floatRadix x'
+
+--		We obtain a trial exponent by doing a floating-point
+--		division of x's numerator by its denominator.  The
+--		result of this division may not itself be the ultimate
+--		result, because of an accumulation of three rounding
+--		errors.
+
+	      (s,e) = decodeFloat (fromInteger (numerator x) `asTypeOf` x'
+					/ fromInteger (denominator x))
+