module Function (filter) where

import Prelude hiding (scanl, map, filter, length)
import Data.Array.Accelerate

-- stolen from examples/simple/Filter.hs
--
filter :: Elem a
       => (Exp a -> Exp Bool)
       -> Acc (Vector a)
       -> Acc (Vector a)
filter p arr
  = let flags               = map (boolToInt . p) arr
        (targetIdx, length) = scanl (+) 0 flags
        arr'                = backpermute (length!(constant ())) id arr
    in
    permute const arr' (\ix -> flags!ix ==* 0 ? (ignore, 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.