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我正在玩一些Data Parallel Haskell代码,发现自己需要prefix sum。但是,我没有看到dph package前缀总和中的任何基本运算符。数据并行Haskell前缀总和
我推出我自己的,但由于我是新来的DPH,我不知道这是否是正确采取并行的优势:
{-# LANGUAGE ParallelArrays #-}
{-# OPTIONS_GHC -fvectorise #-}
module PrefixSum (scanP) where
import Data.Array.Parallel (lengthP, indexedP, mapP, zipWithP, concatP, filterP, singletonP, sliceP, (+:+), (!:))
import Data.Array.Parallel.Prelude.Int ((<=), (-), (==), Int, mod)
-- hide prelude
import qualified Prelude
-- assuming zipWithP (a -> b -> c) given
-- [:a:] of length n and
-- [:b:] of length m, n /= m
-- will return
-- [:c:] of length min n m
scanP :: (a -> a -> a) -> [:a:] -> [:a:]
scanP f xs = if lengthP xs <= 1
then xs
else head +:+ tail
where -- [: x_0, x_2, ..., x_2n :]
evens = mapP snd . filterP (even . fst) $ indexedP xs
-- [: x_1, x_3 ... :]
odds = mapP snd . filterP (odd . fst) $ indexedP xs
lenEvens = lengthP evens
lenOdds = lengthP odds
-- calculate the prefix sums [:w:] of the pair sums [:z:]
psums = scanP f $ zipWithP f evens odds
-- calculate the total prefix sums as
-- [: x_0, w_0, f w_0 x_2, w_1, f w_1 x_4, ...,
head = singletonP (evens !: 0)
body = concatP . zipWithP (\p e -> [: p, f p e :]) psums $ sliceP 1 lenOdds evens
-- ending at either
-- ... w_{n-1}, f w_{n-1} x_2n :]
-- or
-- ... w_{n-1}, f w_{n-1} x_2n, w_n :]
-- depending on whether the length of [:x:] is 2n+1 or 2n+2
tail = if lenEvens == lenOdds then body +:+ singletonP (psums !: (lenEvens - 1)) else body
-- reimplement some of Prelude so it can be vectorised
f $ x = f x
infixr 0 $
(.) f g y = f (g y)
snd (a,b) = b
fst (a,b) = a
even n = n `mod` 2 == 0
odd n = n `mod` 2 == 1
嗯,它可以并行吗?看起来像一个漂亮的连续想法,但也许我错过了一些东西。 – luqui 2012-01-31 15:16:02
@luqui:大小为“n”的数组的并行前缀和算法需要执行“O(log n)”循环的并行计算。有两个阶段。在前进阶段,给定'{a_i |我在[0,2n-1]}'你计算出{a_2i + a_ {2i + 1} | i \ in [0,n-1]}'使用'n/2'并行加法。在后退阶段,给定'{\ sum_0^{2i + 1} a_j |我在[0,n-1]}'和'{a_i |我在[0,2n-1]}'中计算了'{\ sum_0^i a_j |我在[0,2n-1}'中使用了'n/2'并行加法。 – rampion 2012-01-31 16:24:40
@luqui:自然,这只适用于关联性'+',因为存在内在的假设,即(a_0 + a_1)+(a_2 + a_3)==((a_0 + a_1)+ a_2)+ a_3' – rampion 2012-01-31 16:27:10