# Shootout/Nsieve

A ShootoutEntry for the nsieve benchmark

diff program output N = 2 with this output file to check your program is correct before contributing.

Each program should count the prime numbers from 2 to M, using the same na�ve Sieve of Eratosthenes algorithm:

- create a sequence of M boolean flags
- for each index number
- if the flag value at that index is true
- set all the flag values at multiples of that index false
- increment the count

- if the flag value at that index is true

Calculate 3 prime counts, for M = 2N � 10000, 2N-1 � 10000, and 2N-2 � 10000.

The basic benchmark was described in "A High-Level Language Benchmark." BYTE, September 1981, p. 180, Jim Gilbreath.

Of course, there are more efficient implementations of the Sieve of Eratosthenes, and there are more efficient ways to sieve prime numbers, for example "Prime sieves using binary quadratic forms".

For more information see Eric W. Weisstein, "Sieve of Eratosthenes." From MathWorld PrimeCountingFunction--A Wolfram Web Resource.

## Benchmarks

Debian Linux/x86, N=10

|| Entry || Time || || Old || 1.961|| || New || 0.669||

## Current entry

Studying the Core shows the the mapM was preventing things from being unboxed properly. Big speedup. The extra `seqs` also help the unboxing. Should be around the fastest entry now.

*Is anyone else bothered by the usage of unsafeWrite? I agree that*
excellent speed is being gained, but this speed is gained at the cost
of functional-style coding. I'm a novice Haskeller, but using unsafeWrite
seems like an imperative hack to gain speed.* -- AlsonKemp*

*Response: unsafeWrite is just the same as a normal array update,*
without an extraneous bounds check (since it's already performed in the
outer loop). We're just avoiding redundant computation (as we do in all
entries for this problem). SPJ actually recommends unsafe* ops in
arrays, see the "loop performance bug" thread in the haskell mail
archives from 2005.

In summary, its not a hack -- the name just makes clear that you have to
do your own bounds check (Haskell is good at letting you know when there
are extra proof considerations for the programmer to take). Since this
program runs faster than GCC, then I think we should not be worried
about avoiding a redundant bounds check :)* -- DonStewart*

```
--
-- $Id: nsieve-ghc.code,v 1.27 2006/01/08 22:44:56 igouy-guest Exp $
-- Written by Einar Karttunen, optimised further by Don Stewart
--
import Data.Array.IO; import Data.Array.Base; import Data.Bits; import System; import Text.Printf
main = (\n -> mapM_ (sieve.(10000 *).shiftL 1) [n,n-1,n-2]) . read . head =<< getArgs
sieve m = do c <- newArray (2,m) True >>= \a -> loop a m 2 0
printf "Primes up to %8d %8d\n" (m::Int) (c::Int) :: IO ()
loop arr m n c | arr `seq` m `seq` n `seq` c `seq` False = undefined
loop arr m n c = if n == m then return c else do
el <- unsafeRead arr n
if el then let loop' j | j > m = loop arr m (n+1) (c + 1)
| otherwise = unsafeWrite arr j False >> loop' (j+n)
in loop' (n+n)
else loop (arr :: IOUArray Int Bool) m (n+1) c
```

## Entry 2

Shortest entry in any language

```
{-# OPTIONS -O2 -optc-O3 #-}
-- $Id: nsieve-ghc.code,v 1.27 2006/01/08 22:44:56 igouy-guest Exp $
-- Written by Einar Karttunen, shortened by Don Stewart
import Data.Array.IO; import Data.Array.Base; import Data.Bits; import System; import Text.Printf
loop arr m n c = if n == m then return c else do
el <- unsafeRead arr n
if el then do mapM_ (flip (unsafeWrite arr) False) (tail [n,n+n..m])
loop arr m (n+1) $! c + 1
else loop (arr :: IOUArray Int Bool) m (n+1) c
sieve m = do c <- newArray (2,m) True >>= \a -> loop a m 2 0
printf "Primes up to %8d %8d\n" (m::Int) (c::Int) :: IO ()
main = (\n -> mapM_ (sieve.(10000 *).shiftL 1) [n,n-1,n-2]) . read . head =<< getArgs
```

## Original entry

```
{-# OPTIONS -O2 -optc-O3 #-}
-- $Id: nsieve-ghc.code,v 1.27 2006/01/08 22:44:56 igouy-guest Exp $
-- written by Einar Karttunen
import Data.Array.IO
import Data.Array.Base
import Data.Bits (shiftL)
import System (getArgs)
loop :: IOUArray Int Bool -> Int -> Int -> Int -> IO Int
loop arr m n c | n == m = return c
| otherwise = do el <- unsafeRead arr n
if el then do mapM_ (\i -> unsafeWrite arr i False) (tail [n,n+n..m])
loop arr m (n+1) $! c+1
else do loop arr m (n+1) c
fmt width i = let is = show i in (take (width - length is) (repeat ' ')) ++ is
sieve n = do let m = (1 `shiftL` n) * 10000
arr <- newArray (2,m) True
c <- loop arr m 2 0
putStrLn ("Primes up to "++fmt 8 m++" "++fmt 8 c)
main = do n <- getArgs >>= readIO.head
sieve n >> sieve (n-1) >> sieve (n-2)
```