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Prelude.hs
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Prelude.hs
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{-# LANGUAGE TypeOperators, ScopedTypeVariables #-}
-- |
-- Module : Data.Array.Accelerate.Prelude
-- Copyright : [2010..2011] Manuel M T Chakravarty, Ben Lever
-- License : BSD3
--
-- Maintainer : Manuel M T Chakravarty <chak@cse.unsw.edu.au>
-- Stability : experimental
-- Portability : non-portable (GHC extensions)
--
-- Standard functions that are not part of the core set (directly represented in the AST), but are
-- instead implemented in terms of the core set.
--
module Data.Array.Accelerate.Prelude (
-- * Zipping
zipWith3, zipWith4,
zip, zip3, zip4,
-- * Unzipping
unzip, unzip3, unzip4,
-- * Reductions
foldAll, fold1All,
-- ** Specialised folds
all, any, and, or, sum, product, minimum, maximum,
-- * Scans
prescanl, postscanl, prescanr, postscanr,
-- ** Segmented scans
scanlSeg, scanl'Seg, scanl1Seg, prescanlSeg, postscanlSeg,
scanrSeg, scanr'Seg, scanr1Seg, prescanrSeg, postscanrSeg,
-- * Shape manipulation
flatten,
-- * Enumeration and filling
fill, enumFromN, enumFromStepN,
-- * Working with predicates
-- ** Filtering
filter,
-- ** Scatter / Gather
scatter, scatterIf,
gather, gatherIf,
-- * Permutations
reverse, transpose,
-- * Extracting sub-vectors
init, tail, take, drop, slit
) where
-- avoid clashes with Prelude functions
--
import Data.Bits
import Data.Bool
import Prelude ((.), ($), (+), (-), (*), const, subtract, id)
import qualified Prelude as P
-- friends
import Data.Array.Accelerate.Array.Sugar hiding ((!), ignore, shape, size, index)
import Data.Array.Accelerate.Language
import Data.Array.Accelerate.Smart
import Data.Array.Accelerate.Type
-- Map-like composites
-- -------------------
-- | Zip three arrays with the given function
--
zipWith3 :: (Shape sh, Elt a, Elt b, Elt c, Elt d)
=> (Exp a -> Exp b -> Exp c -> Exp d)
-> Acc (Array sh a)
-> Acc (Array sh b)
-> Acc (Array sh c)
-> Acc (Array sh d)
zipWith3 f as bs cs
= map (\x -> let (a,b,c) = unlift x in f a b c)
$ zip3 as bs cs
-- | Zip four arrays with the given function
--
zipWith4 :: (Shape sh, Elt a, Elt b, Elt c, Elt d, Elt e)
=> (Exp a -> Exp b -> Exp c -> Exp d -> Exp e)
-> Acc (Array sh a)
-> Acc (Array sh b)
-> Acc (Array sh c)
-> Acc (Array sh d)
-> Acc (Array sh e)
zipWith4 f as bs cs ds
= map (\x -> let (a,b,c,d) = unlift x in f a b c d)
$ zip4 as bs cs ds
-- | Combine the elements of two arrays pairwise. The shape of the result is
-- the intersection of the two argument shapes.
--
zip :: (Shape sh, Elt a, Elt b)
=> Acc (Array sh a)
-> Acc (Array sh b)
-> Acc (Array sh (a, b))
zip = zipWith (curry lift)
-- | Take three arrays and return an array of triples, analogous to zip.
--
zip3 :: forall sh. forall a. forall b. forall c. (Shape sh, Elt a, Elt b, Elt c)
=> Acc (Array sh a)
-> Acc (Array sh b)
-> Acc (Array sh c)
-> Acc (Array sh (a, b, c))
zip3 as bs cs
= zipWith (\a bc -> let (b, c) = unlift bc :: (Exp b, Exp c) in lift (a, b, c)) as
$ zip bs cs
-- | Take four arrays and return an array of quadruples, analogous to zip.
--
zip4 :: forall sh. forall a. forall b. forall c. forall d. (Shape sh, Elt a, Elt b, Elt c, Elt d)
=> Acc (Array sh a)
-> Acc (Array sh b)
-> Acc (Array sh c)
-> Acc (Array sh d)
-> Acc (Array sh (a, b, c, d))
zip4 as bs cs ds
= zipWith (\a bcd -> let (b, c, d) = unlift bcd :: (Exp b, Exp c, Exp d) in lift (a, b, c, d)) as
$ zip3 bs cs ds
-- | The converse of 'zip', but the shape of the two results is identical to the
-- shape of the argument.
--
unzip :: (Shape sh, Elt a, Elt b)
=> Acc (Array sh (a, b))
-> (Acc (Array sh a), Acc (Array sh b))
unzip arr = (map fst arr, map snd arr)
-- | Take an array of triples and return three arrays, analogous to unzip.
--
unzip3 :: (Shape sh, Elt a, Elt b, Elt c)
=> Acc (Array sh (a, b, c))
-> (Acc (Array sh a), Acc (Array sh b), Acc (Array sh c))
unzip3 abcs = (as, bs, cs)
where
(bs, cs) = unzip bcs
(as, bcs) = unzip $ map swizzle abcs
swizzle :: forall a b c. (Elt a, Elt b, Elt c)
=> Exp (a, b, c) -> Exp (a, (b, c))
swizzle abc = let (a, b, c) = unlift abc :: (Exp a, Exp b, Exp c)
bc = lift (b, c) :: Exp (b, c)
in lift (a, bc)
-- | Take an array of quadruples and return four arrays, analogous to unzip.
--
unzip4 :: (Shape sh, Elt a, Elt b, Elt c, Elt d)
=> Acc (Array sh (a, b, c, d))
-> (Acc (Array sh a), Acc (Array sh b), Acc (Array sh c), Acc (Array sh d))
unzip4 abcds = (as, bs, cs, ds)
where
(abs, cds) = unzip $ map swizzle abcds
(as, bs) = unzip abs
(cs, ds) = unzip cds
swizzle :: forall a b c d. (Elt a, Elt b, Elt c, Elt d)
=> Exp (a, b, c, d) -> Exp ((a, b), (c, d))
swizzle abcd = let (a, b, c, d) = unlift abcd :: (Exp a, Exp b, Exp c, Exp d)
ab = lift (a, b) :: Exp (a, b)
cd = lift (c, d) :: Exp (c, d)
in lift (ab, cd)
-- Reductions
-- ----------
-- | Reduction of an array of arbitrary rank to a single scalar value.
--
foldAll :: (Shape sh, Elt a)
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Array sh a)
-> Acc (Scalar a)
foldAll f e arr = fold f e (flatten arr)
-- | Variant of 'foldAll' that requires the reduced array to be non-empty and
-- doesn't need an default value.
--
fold1All :: (Shape sh, Elt a)
=> (Exp a -> Exp a -> Exp a)
-> Acc (Array sh a)
-> Acc (Scalar a)
fold1All f arr = fold1 f (flatten arr)
-- Specialised reductions
-- ----------------------
--
-- Leave the results of these as scalar arrays to make it clear that these are
-- array computations, and thus can not be nested.
-- | Check if all elements satisfy a predicate
--
all :: (Shape sh, Elt e)
=> (Exp e -> Exp Bool)
-> Acc (Array sh e)
-> Acc (Scalar Bool)
all f = and . map f
-- | Check if any element satisfies the predicate
--
any :: (Shape sh, Elt e)
=> (Exp e -> Exp Bool)
-> Acc (Array sh e)
-> Acc (Scalar Bool)
any f = or . map f
-- | Check if all elements are 'True'
--
and :: Shape sh
=> Acc (Array sh Bool)
-> Acc (Scalar Bool)
and = foldAll (&&*) (constant True)
-- | Check if any element is 'True'
--
or :: Shape sh
=> Acc (Array sh Bool)
-> Acc (Scalar Bool)
or = foldAll (||*) (constant False)
-- | Compute the sum of elements
--
sum :: (Shape sh, Elt e, IsNum e)
=> Acc (Array sh e)
-> Acc (Scalar e)
sum = foldAll (+) 0
-- | Compute the product of the elements
--
product :: (Shape sh, Elt e, IsNum e)
=> Acc (Array sh e)
-> Acc (Scalar e)
product = foldAll (*) 1
-- | Yield the minimum element of an array. The array must not be empty.
--
minimum :: (Shape sh, Elt e, IsScalar e)
=> Acc (Array sh e)
-> Acc (Scalar e)
minimum = fold1All min
-- | Yield the maximum element of an array. The array must not be empty.
--
maximum :: (Shape sh, Elt e, IsScalar e)
=> Acc (Array sh e)
-> Acc (Scalar e)
maximum = fold1All max
-- Composite scans
-- ---------------
-- |Left-to-right prescan (aka exclusive scan). As for 'scan', the first argument must be an
-- /associative/ function. Denotationally, we have
--
-- > prescanl f e = Prelude.fst . scanl' f e
--
prescanl :: Elt a
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Vector a)
prescanl f e = P.fst . scanl' f e
-- |Left-to-right postscan, a variant of 'scanl1' with an initial value. Denotationally, we have
--
-- > postscanl f e = map (e `f`) . scanl1 f
--
postscanl :: Elt a
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Vector a)
postscanl f e = map (e `f`) . scanl1 f
-- |Right-to-left prescan (aka exclusive scan). As for 'scan', the first argument must be an
-- /associative/ function. Denotationally, we have
--
-- > prescanr f e = Prelude.fst . scanr' f e
--
prescanr :: Elt a
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Vector a)
prescanr f e = P.fst . scanr' f e
-- |Right-to-left postscan, a variant of 'scanr1' with an initial value. Denotationally, we have
--
-- > postscanr f e = map (e `f`) . scanr1 f
--
postscanr :: Elt a
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Vector a)
postscanr f e = map (`f` e) . scanr1 f
-- Segmented scans
-- ---------------
-- |Segmented version of 'scanl'
--
scanlSeg :: (Elt a, Elt i, IsIntegral i)
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Segments i)
-> Acc (Vector a)
scanlSeg f z vec seg = scanl1Seg f vec' seg'
where
-- Segmented exclusive scan is implemented by first injecting the seed
-- element at the head of each segment, and then performing a segmented
-- inclusive scan.
--
-- This is done by creating a creating a vector entirely of the seed
-- element, and overlaying the input data in all places other than at the
-- start of a segment.
--
seg' = map (+1) seg
vec' = permute const
(fill (index1 $ size vec + size seg) z)
(\ix -> index1' $ unindex1' ix + inc ! ix)
vec
-- Each element in the segments must be shifted to the right one additional
-- place for each successive segment, to make room for the seed element.
-- Here, we make use of the fact that the vector returned by 'mkHeadFlags'
-- contains non-unit entries, which indicate zero length segments.
--
flags = mkHeadFlags seg
inc = scanl1 (+) flags
-- |Segmented version of 'scanl''
--
-- The first element of the resulting tuple is a vector of scanned values. The
-- second element is a vector of segment scan totals and has the same size as
-- the segment vector.
--
scanl'Seg :: forall a i. (Elt a, Elt i, IsIntegral i)
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Segments i)
-> Acc (Vector a, Vector a)
scanl'Seg f z vec seg = result
where
-- Returned the result combined, so that the sub-calculations are shared
-- should the user require both results.
--
result = lift (body, sums)
-- Segmented scan' is implemented by deconstructing a segmented exclusive
-- scan, to separate the final value and scan body.
--
-- TLM: Segmented scans, and this version in particular, expend a lot of
-- effort scanning flag arrays. On inspection it appears that several
-- of these operations are duplicated, but this will not be picked up
-- by sharing _observation_. Perhaps a global CSE-style pass would be
-- beneficial.
--
vec' = scanlSeg f z vec seg
-- Extract the final reduction value for each segment, which is at the last
-- index of each segment.
--
seg' = map (+1) seg
tails = zipWith (+) seg . P.fst $ scanl' (+) 0 seg'
sums = backpermute (shape seg) (\ix -> index1' $ tails ! ix) vec'
-- Slice out the body of each segment.
--
-- Build a head-flags representation based on the original segment
-- descriptor. This contains the target length of each of the body segments,
-- which is one fewer element than the actual bodies stored in vec'. Thus,
-- the flags align with the last element of each body section, and when
-- scanned, this element will be incremented over.
--
offset = scanl1 (+) seg
inc = scanl1 (+)
$ permute (+) (fill (index1 $ size vec + 1) 0)
(\ix -> index1' $ offset ! ix)
(fill (shape seg) (1 :: Exp i))
body = backpermute (shape vec)
(\ix -> index1' $ unindex1' ix + inc ! ix)
vec'
-- |Segmented version of 'scanl1'.
--
scanl1Seg :: (Elt a, Elt i, IsIntegral i)
=> (Exp a -> Exp a -> Exp a)
-> Acc (Vector a)
-> Acc (Segments i)
-> Acc (Vector a)
scanl1Seg f vec seg
= P.snd
. unzip
. scanl1 (segmented f)
$ zip (mkHeadFlags seg) vec
-- |Segmented version of 'prescanl'.
--
prescanlSeg :: (Elt a, Elt i, IsIntegral i)
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Segments i)
-> Acc (Vector a)
prescanlSeg f e vec seg
= P.fst
. unatup2
$ scanl'Seg f e vec seg
-- |Segmented version of 'postscanl'.
--
postscanlSeg :: (Elt a, Elt i, IsIntegral i)
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Segments i)
-> Acc (Vector a)
postscanlSeg f e vec seg
= map (f e)
$ scanl1Seg f vec seg
-- |Segmented version of 'scanr'.
--
scanrSeg :: (Elt a, Elt i, IsIntegral i)
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Segments i)
-> Acc (Vector a)
scanrSeg f z vec seg = scanr1Seg f vec' seg'
where
-- Using technique described for 'scanlSeg', where we intersperse the array
-- with the seed element at the start of each segment, and then perform an
-- inclusive segmented scan.
--
inc = scanl1 (+) (mkHeadFlags seg)
seg' = map (+1) seg
vec' = permute const
(fill (index1 $ size vec + size seg) z)
(\ix -> index1' $ unindex1' ix + inc ! ix - 1)
vec
-- | Segmented version of 'scanr''.
--
scanr'Seg :: forall a i. (Elt a, Elt i, IsIntegral i)
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Segments i)
-> Acc (Vector a, Vector a)
scanr'Seg f z vec seg = result
where
-- Using technique described for scanl'Seg
--
result = lift (body, sums)
vec' = scanrSeg f z vec seg
-- reduction values
seg' = map (+1) seg
heads = P.fst $ scanl' (+) 0 seg'
sums = backpermute (shape seg) (\ix -> index1' $ heads ! ix) vec'
-- body segments
inc = scanl1 (+) $ mkHeadFlags seg
body = backpermute (shape vec)
(\ix -> index1' $ unindex1' ix + inc ! ix)
vec'
-- |Segmented version of 'scanr1'.
--
scanr1Seg :: (Elt a, Elt i, IsIntegral i)
=> (Exp a -> Exp a -> Exp a)
-> Acc (Vector a)
-> Acc (Segments i)
-> Acc (Vector a)
scanr1Seg f vec seg
= P.snd
. unzip
. scanr1 (segmented f)
$ zip (mkTailFlags seg) vec
-- |Segmented version of 'prescanr'.
--
prescanrSeg :: (Elt a, Elt i, IsIntegral i)
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Segments i)
-> Acc (Vector a)
prescanrSeg f e vec seg
= P.fst
. unatup2
$ scanr'Seg f e vec seg
-- |Segmented version of 'postscanr'.
--
postscanrSeg :: (Elt a, Elt i, IsIntegral i)
=> (Exp a -> Exp a -> Exp a)
-> Exp a
-> Acc (Vector a)
-> Acc (Segments i)
-> Acc (Vector a)
postscanrSeg f e vec seg
= map (f e)
$ scanr1Seg f vec seg
-- Segmented scan helpers
-- ----------------------
-- |Compute head flags vector from segment vector for left-scans.
--
-- The vector will be full of zeros in the body of a segment, and non-zero
-- otherwise. The "flag" value, if greater than one, indicates that several
-- empty segments are represented by this single flag entry. This is additional
-- data is used by exclusive segmented scan.
--
mkHeadFlags :: (Elt i, IsIntegral i) => Acc (Segments i) -> Acc (Segments i)
mkHeadFlags seg
= init
$ permute (+) zeros (\ix -> index1' (offset ! ix)) ones
where
(offset, len) = scanl' (+) 0 seg
zeros = fill (index1' $ the len + 1) 0
ones = fill (index1 $ size offset) 1
-- |Compute tail flags vector from segment vector for right-scans. That is, the
-- flag is placed at the last place in each segment.
--
mkTailFlags :: (Elt i, IsIntegral i) => Acc (Segments i) -> Acc (Segments i)
mkTailFlags seg
= init
$ permute (+) zeros (\ix -> index1' (the len - 1 - offset ! ix)) ones
where
(offset, len) = scanr' (+) 0 seg
zeros = fill (index1' $ the len + 1) 0
ones = fill (index1 $ size offset) 1
-- |Construct a segmented version of a function from a non-segmented version.
-- The segmented apply operates on a head-flag value tuple, and follows the
-- procedure of Sengupta et. al.
--
segmented :: (Elt e, Elt i, IsIntegral i)
=> (Exp e -> Exp e -> Exp e)
-> Exp (i, e) -> Exp (i, e) -> Exp (i, e)
segmented f a b =
let (aF, aV) = unlift a
(bF, bV) = unlift b
in
lift (aF .|. bF, bF /=* 0 ? (bV, f aV bV))
-- |Index construction and destruction generalised to integral types.
--
-- We generalise the segment descriptor to integral types because some
-- architectures, such as GPUs, have poor performance for 64-bit types. So,
-- there is a tension between performance and requiring 64-bit indices for some
-- applications, and we would not like to restrict ourselves to either one.
--
-- As we don't yet support non-Int dimensions in shapes, we will need to convert
-- back to concrete Int. However, don't put these generalised forms into the
-- base library, because it results in too many ambiguity errors.
--
index1' :: (Elt i, IsIntegral i) => Exp i -> Exp DIM1
index1' i = lift (Z :. fromIntegral i)
unindex1' :: (Elt i, IsIntegral i) => Exp DIM1 -> Exp i
unindex1' ix = let Z :. i = unlift ix in fromIntegral i
-- Reshaping of arrays
-- -------------------
-- | Flattens a given array of arbitrary dimension.
--
flatten :: (Shape ix, Elt a) => Acc (Array ix a) -> Acc (Vector a)
flatten a = reshape (index1 $ size a) a
-- Enumeration and filling
-- -----------------------
-- | Create an array where all elements are the same value.
--
fill :: (Shape sh, Elt e) => Exp sh -> Exp e -> Acc (Array sh e)
fill sh c = generate sh (const c)
-- | Create an array of the given shape containing the values x, x+1, etc (in
-- row-major order).
--
enumFromN :: (Shape sh, Elt e, IsNum e) => Exp sh -> Exp e -> Acc (Array sh e)
enumFromN sh x = enumFromStepN sh x 1
-- | Create an array of the given shape containing the values @x@, @x+y@,
-- @x+y+y@ etc. (in row-major order).
--
enumFromStepN :: (Shape sh, Elt e, IsNum e)
=> Exp sh
-> Exp e -- ^ x: start
-> Exp e -- ^ y: step
-> Acc (Array sh e)
enumFromStepN sh x y
= reshape sh
$ generate (index1 $ shapeSize sh)
(\ix -> (fromIntegral (unindex1 ix :: Exp Int) * y) + x)
-- Filtering
-- ---------
-- | Drop elements that do not satisfy the predicate
--
filter :: Elt a
=> (Exp a -> Exp Bool)
-> Acc (Vector a)
-> Acc (Vector a)
filter p arr
= let flags = map (boolToInt . p) arr
(targetIdx, len) = scanl' (+) 0 flags
arr' = backpermute (index1 $ the len) id arr
in
permute const arr' (\ix -> flags!ix ==* 0 ? (ignore, index1 $ targetIdx!ix)) arr
-- FIXME: This is abusing 'permute' in that the first two arguments are
-- only justified because we know the permutation function will
-- write to each location in the target exactly once.
-- Instead, we should have a primitive that directly encodes the
-- compaction pattern of the permutation function.
-- Gather operations
-- -----------------
-- | Copy elements from source array to destination array according to a map. This
-- is a backpermute operation where a 'map' vector encodes the ouput to input
-- index mapping.
--
-- For example:
--
-- > input = [1, 9, 6, 4, 4, 2, 0, 1, 2]
-- > map = [1, 3, 7, 2, 5, 3]
-- >
-- > output = [9, 4, 1, 6, 2, 4]
--
gather :: (Elt e)
=> Acc (Vector Int) -- ^map
-> Acc (Vector e) -- ^input
-> Acc (Vector e) -- ^output
gather mapV inputV = backpermute (shape mapV) bpF inputV
where
bpF ix = lift (Z :. (mapV ! ix))
-- | Conditionally copy elements from source array to destination array according
-- to a map. This is a backpermute opereation where a 'map' vector encdes the
-- output to input index mapping. In addition, there is a 'mask' vector, and an
-- associated predication function, that specifies whether an element will be
-- copied. If not copied, the output array assumes the default vector's value.
--
-- For example:
--
-- > default = [6, 6, 6, 6, 6, 6]
-- > map = [1, 3, 7, 2, 5, 3]
-- > mask = [3, 4, 9, 2, 7, 5]
-- > pred = (> 4)
-- > input = [1, 9, 6, 4, 4, 2, 0, 1, 2]
-- >
-- > output = [6, 6, 1, 6, 2, 4]
--
gatherIf :: (Elt e, Elt e')
=> Acc (Vector Int) -- ^map
-> Acc (Vector e) -- ^mask
-> (Exp e -> Exp Bool) -- ^predicate
-> Acc (Vector e') -- ^default
-> Acc (Vector e') -- ^input
-> Acc (Vector e') -- ^output
gatherIf mapV maskV pred defaultV inputV = zipWith zwF predV gatheredV
where
zwF p g = p ? (unlift g)
gatheredV = zip (gather mapV inputV) defaultV
predV = map pred maskV
-- Scatter operations
-- ------------------
-- | Copy elements from source array to destination array according to a map. This
-- is a forward-permute operation where a 'map' vector encodes an input to output
-- index mapping. Output elements for indices that are not mapped assume the
-- default vector's value.
--
-- For example:
--
-- > default = [0, 0, 0, 0, 0, 0, 0, 0, 0]
-- > map = [1, 3, 7, 2, 5, 8]
-- > input = [1, 9, 6, 4, 4, 2, 5]
-- >
-- > output = [0, 1, 4, 9, 0, 4, 0, 6, 2]
--
-- Note if the same index appears in the map more than once, the result is
-- undefined. The map vector cannot be larger than the input vector.
--
scatter :: (Elt e)
=> Acc (Vector Int) -- ^map
-> Acc (Vector e) -- ^default
-> Acc (Vector e) -- ^input
-> Acc (Vector e) -- ^output
scatter mapV defaultV inputV = permute (const) defaultV pF inputV
where
pF ix = lift (Z :. (mapV ! ix))
-- | Conditionally copy elements from source array to destination array according
-- to a map. This is a forward-permute operation where a 'map' vector encodes an
-- input to output index mapping. In addition, there is a 'mask' vector, and an
-- associated predicate function, that specifies whether an elements will be
-- copied. If not copied, the output array assumes the default vector's value.
--
-- For example:
--
-- > default = [0, 0, 0, 0, 0, 0, 0, 0, 0]
-- > map = [1, 3, 7, 2, 5, 8]
-- > mask = [3, 4, 9, 2, 7, 5]
-- > pred = (> 4)
-- > input = [1, 9, 6, 4, 4, 2]
-- >
-- > output = [0, 0, 0, 0, 0, 4, 0, 6, 2]
--
-- Note if the same index appears in the map more than once, the result is
-- undefined. The map and input vector must be of the same length.
--
scatterIf :: (Elt e, Elt e')
=> Acc (Vector Int) -- ^map
-> Acc (Vector e) -- ^mask
-> (Exp e -> Exp Bool) -- ^predicate
-> Acc (Vector e') -- ^default
-> Acc (Vector e') -- ^input
-> Acc (Vector e') -- ^output
scatterIf mapV maskV pred defaultV inputV = permute const defaultV pF inputV
where
pF ix = (pred (maskV ! ix)) ? (lift (Z :. (mapV ! ix)), ignore)
-- Permutations
-- ------------
-- | Reverse the elements of a vector.
--
reverse :: Elt e => Acc (Vector e) -> Acc (Vector e)
reverse xs =
let len = unindex1 (shape xs)
pf i = len - i - 1
in backpermute (shape xs) (ilift1 pf) xs
-- | Transpose the rows and columns of a matrix.
--
transpose :: Elt e => Acc (Array DIM2 e) -> Acc (Array DIM2 e)
transpose mat =
let swap = lift1 $ \(Z:.x:.y) -> Z:.y:.x :: Z:.Exp Int:.Exp Int
in backpermute (swap $ shape mat) swap mat
-- Extracting sub-vectors
-- ----------------------
-- | Yield the first @n@ elements of the input vector. The vector must contain
-- no more than @n@ elements.
--
take :: Elt e => Exp Int -> Acc (Vector e) -> Acc (Vector e)
take n = backpermute (index1 n) id
-- | Yield all but the first @n@ elements of the input vector. The vector must
-- contain no more than @n@ elements.
--
drop :: Elt e => Exp Int -> Acc (Vector e) -> Acc (Vector e)
drop n arr = backpermute (ilift1 (\x -> x - n) $ shape arr) (ilift1 (+ n)) arr
-- | Yield all but the last element of the input vector. The vector must not be
-- empty.
--
init :: Elt e => Acc (Vector e) -> Acc (Vector e)
init arr = take ((unindex1 $ shape arr) - 1) arr
-- | Yield all but the first element of the input vector. The vector must not be
-- empty.
--
tail :: Elt e => Acc (Vector e) -> Acc (Vector e)
tail = drop 1
-- | Yield a slit (slice) from the vector. The vector must contain at least
-- @i + n@ elements. Denotationally, we have:
--
-- > slit i n = take n . drop i
--
slit :: Elt e
=> Exp Int
-> Exp Int
-> Acc (Vector e)
-> Acc (Vector e)
slit i n = backpermute (index1 n) (ilift1 (+ i))