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+{- data size display and parsing
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Utility.DataUnits (
+	dataUnits,
+	storageUnits,
+	memoryUnits,
+	bandwidthUnits,
+	oldSchoolUnits,
+
+	roughSize,
+	compareSizes,
+	readSize
+) where
+
+import Data.List
+import Data.Char
+
+type ByteSize = Integer
+type Name = String
+type Abbrev = String
+data Unit = Unit ByteSize Abbrev Name
+	deriving (Ord, Show, Eq)
+
+{- And now a rant: 
+ -
+ - In the beginning, we had powers of two, and they were good.
+ -
+ - Disk drive manufacturers noticed that some powers of two were
+ - sorta close to some powers of ten, and that rounding down to the nearest
+ - power of ten allowed them to advertise their drives were bigger. This
+ - was sorta annoying.
+ -
+ - Then drives got big. Really, really big. This was good.
+ -
+ - Except that the small rounding error perpretrated by the drive
+ - manufacturers suffered the fate of a small error, and became a large
+ - error. This was bad.
+ -
+ - So, a committee was formed. And it arrived at a committee-like decision,
+ - which satisfied noone, confused everyone, and made the world an uglier
+ - place. As with all committees, this was meh.
+ -
+ - And the drive manufacturers happily continued selling drives that are
+ - increasingly smaller than you'd expect, if you don't count on your
+ - fingers. But that are increasingly too big for anyone to much notice.
+ - This caused me to need git-annex.
+ -
+ - Thus, I use units here that I loathe. Because if I didn't, people would
+ - be confused that their drives seem the wrong size, and other people would
+ - complain at me for not being standards compliant. And we call this
+ - progress?
+ -}
+
+dataUnits :: [Unit]
+dataUnits = storageUnits ++ memoryUnits
+
+{- Storage units are (stupidly) powers of ten. -}
+storageUnits :: [Unit]
+storageUnits =
+	[ Unit (p 8) "YB" "yottabyte"
+	, Unit (p 7) "ZB" "zettabyte"
+	, Unit (p 6) "EB" "exabyte"
+	, Unit (p 5) "PB" "petabyte"
+	, Unit (p 4) "TB" "terabyte"
+	, Unit (p 3) "GB" "gigabyte"
+	, Unit (p 2) "MB" "megabyte"
+	, Unit (p 1) "kB" "kilobyte" -- weird capitalization thanks to committe
+	, Unit (p 0) "B" "byte"
+	]
+	where
+		p :: Integer -> Integer
+		p n = 1000^n
+
+{- Memory units are (stupidly named) powers of 2. -}
+memoryUnits :: [Unit]
+memoryUnits =
+	[ Unit (p 8) "YiB" "yobibyte"
+	, Unit (p 7) "ZiB" "zebibyte"
+	, Unit (p 6) "EiB" "exbibyte"
+	, Unit (p 5) "PiB" "pebibyte"
+	, Unit (p 4) "TiB" "tebibyte"
+	, Unit (p 3) "GiB" "gigabyte"
+	, Unit (p 2) "MiB" "mebibyte"
+	, Unit (p 1) "KiB" "kibibyte"
+	, Unit (p 0) "B" "byte"
+	]
+	where
+		p :: Integer -> Integer
+		p n = 2^(n*10)
+
+{- Bandwidth units are only measured in bits if you're some crazy telco. -}
+bandwidthUnits :: [Unit]
+bandwidthUnits = error "stop trying to rip people off"
+
+{- Do you yearn for the days when men were men and megabytes were megabytes? -}
+oldSchoolUnits :: [Unit]
+oldSchoolUnits = map mingle $ zip storageUnits memoryUnits
+	where
+		mingle (Unit _ a n, Unit s' _ _) = Unit s' a n
+
+{- approximate display of a particular number of bytes -}
+roughSize :: [Unit] -> Bool -> ByteSize -> String
+roughSize units abbrev i
+	| i < 0 = "-" ++ findUnit units' (negate i)
+	| otherwise = findUnit units' i
+	where
+		units' = reverse $ sort units -- largest first
+
+		findUnit (u@(Unit s _ _):us) i'
+			| i' >= s = showUnit i' u
+			| otherwise = findUnit us i'
+		findUnit [] i' = showUnit i' (last units') -- bytes
+
+		showUnit i' (Unit s a n) = let num = chop i' s in
+			show num ++ " " ++
+			(if abbrev then a else plural num n)
+
+		chop :: Integer -> Integer -> Integer
+		chop i' d = round $ (fromInteger i' :: Double) / fromInteger d
+
+		plural n u
+			| n == 1 = u
+			| otherwise = u ++ "s"
+
+{- displays comparison of two sizes -}
+compareSizes :: [Unit] -> Bool -> ByteSize -> ByteSize -> String
+compareSizes units abbrev old new
+	| old > new = roughSize units abbrev (old - new) ++ " smaller"
+	| old < new = roughSize units abbrev (new - old) ++ " larger"
+	| otherwise = "same"
+
+{- Parses strings like "10 kilobytes" or "0.5tb". -}
+readSize :: [Unit] -> String -> Maybe ByteSize
+readSize units input
+	| null parsednum = Nothing
+	| null parsedunit = Nothing
+	| otherwise = Just $ round $ number * (fromIntegral multiplier)
+	where
+		(number, rest) = head parsednum
+		multiplier = head $ parsedunit
+		unitname = takeWhile isAlpha $ dropWhile isSpace rest
+
+		parsednum = reads input :: [(Double, String)]
+		parsedunit = lookupUnit units unitname
+
+		lookupUnit _ [] = [1] -- no unit given, assume bytes
+		lookupUnit [] _ = []
+		lookupUnit (Unit s a n:us) v
+			| a ~~ v || n ~~ v = [s]
+			| plural n ~~ v || a ~~ byteabbrev v = [s]
+			| otherwise = lookupUnit us v
+		
+		a ~~ b = map toLower a == map toLower b
+		
+		plural n = n ++ "s"
+		byteabbrev a = a ++ "b"