percentage library

Utility/Percentage.hs

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+{- percentages
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Utility.Percentage (
+	Percentage,
+	percentage,
+	showPercentage
+) where
+
+import Data.Ratio
+
+newtype Percentage = Percentage (Ratio Integer)
+
+instance Show Percentage where
+	show = showPercentage 0
+
+{- Normally the big number comes first. But 110% is allowed if desired. :) -}
+percentage :: Integer -> Integer -> Percentage
+percentage 0 _ = Percentage 0
+percentage full have = Percentage $ have * 100 % full
+
+{- Pretty-print a Percentage, with a specified level of precision. -}
+showPercentage :: Int -> Percentage -> String
+showPercentage precision (Percentage p)
+	| precision == 0 || remainder == 0 = go $ show int
+	| otherwise = go $ show int ++ "." ++ strip0s (show remainder)
+	where
+		go v = v ++ "%"
+		int :: Integer
+		(int, frac) = properFraction (fromRational p)
+		remainder = floor (frac * multiplier) :: Integer
+		strip0s = reverse . dropWhile (== '0') . reverse
+		multiplier :: Float
+		multiplier = 10 ** (fromIntegral precision)