{- Copyright (C) 2011 Dr. Alistair Ward This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. -} {- | [@AUTHOR@] Dr. Alistair Ward [@DESCRIPTION@] * Describes a bounded set of, typically integral, quantities. * Operations have been defined, on the list of /consecutive/ quantities delimited by these endpoints. * The point is that if the list is composed from /consecutive/ quantities, the intermediate values can be inferred, rather than physically represented. [@CAVEATS@] * The API was driven top-down by its caller's requirements, rather than a bottom-up attempt to provide a complete interface. consequently there may be omissions from the view point of future callers. * Thought similar to the mathematical concept of an /interval/, the latter technically relates to /real/ numbers; <http://en.wikipedia.org/wiki/Interval_%28mathematics%29>. * No account has been made for /semi-closed/ or /open/ intervals. -} module Factory.Data.Interval( -- * Types -- ** Type-synonyms Interval, -- * Constants closedUnitInterval, mkBounded, -- * Functions -- divideAndConquer, elem', -- getLength, normalise, product', shift, splitAt', toList, -- ** Accessors getMinBound, getMaxBound, -- ** Constructors precisely, -- ** Predicates isReversed ) where import Control.Arrow((***), (&&&)) import qualified Control.Parallel.Strategies import qualified Data.Monoid import qualified Data.Ratio import qualified Data.Tuple import qualified ToolShed.Data.Pair -- | Defines a closed (inclusive) interval of consecutive values. type Interval endPoint = (endPoint, endPoint) -- | Accessor. {-# INLINE getMinBound #-} getMinBound :: Interval endPoint -> endPoint getMinBound = fst -- | Accessor. {-# INLINE getMaxBound #-} getMaxBound :: Interval endPoint -> endPoint getMaxBound = snd -- | Construct the /unsigned closed unit-interval/; <http://en.wikipedia.org/wiki/Unit_interval>. closedUnitInterval :: Num n => Interval n closedUnitInterval = (0, 1) -- | Construct an /interval/ from a bounded type. mkBounded :: Bounded endPoint => Interval endPoint mkBounded = (minBound, maxBound) -- | Construct an /interval/ from a single value. precisely :: endPoint -> Interval endPoint precisely = id &&& id -- | Shift of both /end-points/ of the /interval/ by the specified amount. shift :: Num endPoint => endPoint -- ^ The magnitude of the require shift. -> Interval endPoint -- ^ The interval to be shifted. -> Interval endPoint shift i = ToolShed.Data.Pair.mirror (+ i) -- | True if the specified value is within the inclusive bounds of the /interval/. elem' :: Ord endPoint => endPoint -> Interval endPoint -> Bool elem' x = uncurry (&&) . ((<= x) *** (x <=)) -- | True if 'getMinBound' exceeds 'getMaxBound' extent. isReversed :: Ord endPoint => Interval endPoint -> Bool isReversed = uncurry (>) -- | Swap the /end-points/ where they were originally reversed, but otherwise do nothing. normalise :: Ord endPoint => Interval endPoint -> Interval endPoint normalise b | isReversed b = Data.Tuple.swap b | otherwise = b -- | Bisect the /interval/ at the specified /end-point/; which should be between the two existing /end-points/. splitAt' :: ( Enum endPoint, Num endPoint, Ord endPoint, Show endPoint ) => endPoint -> Interval endPoint -> (Interval endPoint, Interval endPoint) splitAt' i interval@(l, r) | any ($ i) [(< l), (>= r)] = error $ "Factory.Data.Interval.splitAt':\tunsuitable index=" ++ show i ++ " for interval=" ++ show interval ++ "." | otherwise = ((l, i), (succ i, r)) {- | * The distance between the endpoints, which for 'Integral' quantities is the same as the number of items in closed interval; though the latter concept would return type 'Int'. * CAVEAT: the implementation accounts for the potential fence-post error, for closed intervals of integers, but this results in the opposite error when used with /Fractional/ quantities. So, though most of the module merely requires 'Enum', this function is further restricted to 'Integral'. -} {-# INLINE getLength #-} getLength :: Integral endPoint => Interval endPoint -> endPoint getLength (l, r) = succ r - l {- | * Converts 'Interval' to a list by enumerating the values. * CAVEAT: produces rather odd results for 'Fractional' types, but no stranger than considering such types Enumerable in the first place. -} {-# INLINE toList #-} toList :: Enum endPoint => Interval endPoint -> [endPoint] toList = uncurry enumFromTo -- CAVEAT: in this eta-reduced form, it'll only be inlined when called without arguments. {- | * Reduces 'Interval' to a single integral value encapsulated in a 'Data.Monoid.Monoid', using a /divide-and-conquer/ strategy, bisecting the /interval/ and recursively evaluating each part; <http://en.wikipedia.org/wiki/Divide_and_conquer_algorithm>. * By choosing a 'ratio' other than @(1 % 2)@, the bisection can be made asymmetrical. The specified ratio represents the length of the left-hand portion over the original list-length; eg. @(1 % 3)@ results in the first part, half the length of the second. * This process of recursive bisection, is terminated beneath the specified minimum length, after which the 'Interval' are expanded into the corresponding list, and the /monoid/'s binary operator is directly /folded/ over it. * One can view this as a <http://en.wikipedia.org/wiki/Hylomorphism_%28computer_science%29>, in which 'Interval' is exploded into a binary tree-structure (each leaf of which contains a list of up to 'minLength' integers, and each node of which contains an associative binary operator), and then collapsed to a scalar, by application of the operators. -} divideAndConquer :: (Data.Monoid.Monoid monoid, Integral i, Show i) => (i -> monoid) -- ^ The monoid's constructor. -> Data.Ratio.Ratio i -- ^ The ratio of the original span, at which to bisect the 'Interval'. -> i -- ^ For efficiency, the /interval/ will not be bisected, when it's length has been reduced to this value. -> Interval i -> monoid -- ^ The resulting scalar. divideAndConquer monoidConstructor ratio minLength | any ($ ratio) [ (< 0), (>= 1) ] = error $ "Factory.Data.Interval.divideAndConquer:\tunsuitable ratio='" ++ show ratio ++ "'." | minLength < 1 = error $ "Factory.Data.Interval.divideAndConquer:\tunsuitable minLength=" ++ show minLength ++ "." | otherwise = slave where slave interval@(l, r) | getLength interval <= minLength = Data.Monoid.mconcat . map monoidConstructor $ toList interval -- Fold the monoid's binary operator over the delimited list. | otherwise = uncurry Data.Monoid.mappend . Control.Parallel.Strategies.withStrategy ( Control.Parallel.Strategies.parTuple2 Control.Parallel.Strategies.rseq Control.Parallel.Strategies.rseq ) . ToolShed.Data.Pair.mirror slave $ splitAt' ( l + (r - l) * Data.Ratio.numerator ratio `div` Data.Ratio.denominator ratio -- Use the ratio to generate the split-index. ) interval -- Apply the monoid's binary operator to the two operands resulting from bisection. {- | * Multiplies the consecutive sequence of integers within 'Interval'. * Since the result can be large, 'divideAndConquer' is used to form operands of a similar order of magnitude, thus improving the efficiency of the big-number multiplication. -} product' :: (Integral i, Show i) => Data.Ratio.Ratio i -- ^ The ratio at which to bisect the 'Interval'. -> i -- ^ For efficiency, the /interval/ will not be bisected, when it's length has been reduced to this value. -> Interval i -> i -- ^ The resulting product. product' ratio minLength interval | elem' 0 interval = 0 | otherwise = Data.Monoid.getProduct $ divideAndConquer Data.Monoid.Product ratio minLength interval