Talk:Prime numbers
From HaskellWiki
(move comment to the correct year (d'oh).) 

(24 intermediate revisions by 3 users not shown) 
Latest revision as of 10:21, 2 February 2013
Contents 
[edit] 1 2007
Here's an interesting question: will the program go faster if we replace all thoseOn one hand, a composite integer cannot possess a factor greater than its square root.
On the other hand, since the list we're looking through contains all possible prime numbers, we are guaranteed to find a factor or an exact match eventually, so do we need theThrowing this over to somebody with a bigger brain than me...
MathematicalOrchid 16:41, 5 February 2007 (UTC)
a composite can indeed have factors greater than its square root, and indeed most do. what you mean is that a composite will definitely have at least one factor smallerequal than its square root.
why not useLOL! That is indeed what I meant.
It turns out my comment above is correct  theMathematicalOrchid 10:17, 6 February 2007 (UTC)
[edit] 2 2009
The section Simple Prime Sieve II is not a sieve in the same sense that the first one is. It really implements a primality test as a filter.
A more "sievelike" version of the simple sieve which exploits the fact that we need not check for primes larger than the square root would be
primes = sieve [2..]
where sieve (p:xs) = p : sieve [x  x<xs, (x< p*p)  (x `mod` p /= 0)]
However, this runs even slower than the original!
Kapil Hari Paranjape 06:51, 4 February 2009 (UTC)
I want to thank Leon P. Smith for showing me the idea of producing the spans of odds directly, for version IV. I had a combination of span and infinite odds list, as in span (< p*p) [3,5..] etc. That sped it up some 20% more, when GHCcompiled.
The markandcomb version that I put under Simple Sieve of Eratosthenes seems to me very "faithful" to the original (IYKWIM). Strangely it shows exactly same asymptotic behavior when GHCcompiled (tested inside GHCi) as IV. Does this prove that priority queuebased code is better than the original? :)
BTW "unzip" is somehow screwed up inside "haskell" block, I don't know how to fix that.
 not anymore WillNess 13:39, 10 February 2011 (UTC)
I've also added the postponedfilters version to the first sieve code to show that the squares optimization does matter and gives huge efficiency advantage just by itself. The odds only trick gives it a dozen or two percent improvement, but it's nothing compared to this 20x massive speedup!
Written in listcomprehension style, it's
primes = 2: 3: sieve (tail primes) [5,7..] where
sieve (p:ps) xs
= h ++ sieve ps [xx<t, x `rem` p /= 0]
where (h,(_:t))=span (< p*p) xs
Which BTW is faster than the IV version itself, when interpreted in GHCi. So what are we comparing here, code versions or Haskell implementations??
WillNess 10:46, 15 November 2009 (UTC)
I've added the code for Euler's sieve which is just the postponed filters with minimal modification, substituting (t `minus` multiples p)
for (filter (nodivs p) t)
.
 as it later turned out it was not a Euler sieve, but rather an approximation. WillNess 13:27, 10 February 2011 (UTC)
Now it is obvious that (...(((s  a)  b)  c)  ...)
is the same as (s  (a + b + c + ...))
and this is the next code, the "merged multiples" variation of Euler's sieve.
It is very much like the streamlined and further optimized famous Richard Bird's code (appearing in Melissa O'Neill's JFP article), which copyright status is unknown to me, so I couldn't reference it in the main article body. The code as written in the article has the wrong clause order in merge
.
 when using
compare
, clause order doesn't matter. WillNess 15:32, 26 January 2010 (UTC)
I've also changed the span patternbinding to the more correct, lazy pattern, (h,~(_:t))
.
WillNess 17:10, 5 December 2009 (UTC)
New treefolding merge is inspired by apfelmus's VIP code from Implicit Heap; but it uses a different structure, better at primes multiples generation: instead of his 1+(2+(4+(8+...))) it's (2+4)+( (4+8) + ( (8+16) + ...)). The reason I put my version here is to show the natural progression of development from the postponed filters to Euler's sieve to merged multiples to treefoldmerged multiples. I.e. it's not some adhoc invention; it's logical. It is also stepbystep.
I estimate the total cost of producing primes multiples as Sum (1/p)*d, where d is the leaf's depth, i.e. the amount of merge nodes its produced prime must pass on its way up to the top. The values for cost function correspond very well with the actual time performance of the respective algorithms: it's better by 10%12% and the performance boost is 10%12% too.
I will also add this code further improved with the Wheel optimization here. That one beats the PQbased code from Melissa ONeill's ZIP file by a constant margin of around 20%, its asymptotic behaviour *exactly* the same.
 that was with the incomplete code which only rolled the wheel on numbers supply, and not on multiples. It had e.g. [11*11,11*13,11*15,11*17...] but of course 11*15 could've been eliminated in advance too (and 11*25, 11*35, 11*45, 11*49, etc...). Fixing that made it run twice faster than before. WillNess 08:33, 29 December 2009 (UTC)
 these tests were most probably wrong, either on GHCi or without using the O2 switch WillNess 13:27, 10 February 2011 (UTC)
I measure local asymptotics by taking a logBase of run time ratio in base of a problem size ratio. I've settled on testing code performance as interpreted, inside GHCi. Running a compiled code feels more like testing a compiler itself. Too many times I saw two operationally equivalent expressions running in wildly different times. It can't be anything else other than the compiler's quirks, and we're not interested in those, here. :)
Apparently, arrays are very fast. :) (using accumArray as seen in Thorkil Naur's Haskell cafe post, but still without the Wheel).
WillNess 14:47, 25 December 2009 (UTC)
[edit] 3 2010
NEW! A *major* improvement to treefold merge by Daniel Fischer's reimplementing spMerge into a properly tailrecursive thingy, fixing a space leak there. :) WillNess 06:56, 9 January 2010 (UTC)
 AND his other idea: making `tfold' strict  which really brings down the memory consumption. The only caveat: use at least 6 primes to bootstrap the treefolding. At each tree expansion it needs additional 3*2^n, n=1,... primes, but is producing PI( (PRIMES !! SUM(i=1..n)(3*2^i)) ^ 2) which is way more than that. WillNess 10:02, 25 January 2010 (UTC)
 I've also inlined spMerge completely into mergeSP itself (now called unionSP) along the lines of Leon P. Smith's Data.OrdList.mergeAll implementation. Looks yet simpler that way. Haven't tested it though. WillNess 23:21, 28 February 2010 (UTC)
 changed Data.OrdList to Data.List.Ordered as per the new version of dataordlist package. WillNess 07:45, 16 March 2010 (UTC)
Euler's Sieve initial oneliner code is due to Daniel Fischer. WillNess 16:35, 19 January 2010 (UTC)
Here's new streamlined code for immutable arrays:
primes = 2: 3: sieve [] (tail primes) 3 where sieve fs (p:ps) x = [i*2 + x  (i,e) < assocs a, e] ++ sieve fs' ps (p*p) where q = (p*px)`div`2 fs' = (p,0) : [(s, rem (yq) s)  (s,y) < fs] a = accumArray (\ b c > False) True (1,q1) [(i,())  (s,y) < fs, i < [y+s,y+s+s..q]]
It is twice faster, but more obscure; so I thought I'd keep the previous version on the main page for didactic reasons. WillNess 08:43, 18 July 2010 (UTC)
 I've added it now to the main page and restyled the treefold code a bit. The test entries on Ideone.com are here. WillNess 11:00, 6 August 2010 (UTC)
 ST Array code also becomes much faster (3x for 1 mlnth prime in fact) when translated into working on odds only, like the immutable array version  but its memory footprint is also large. WillNess 10:34, 13 August 2010 (UTC)
[edit] 4 2011
Augmenting the latest Treefold Merged Multiples, with Wheel version to work with VIPs does nothing except slow it down a little bit. Lazy pattern in join/tfold also starts causing space leak then, so primes' becomes necessary to be more defined upfront to prevent a loop when the tilde is removed. WillNess 18:31, 6 February 2011 (UTC)
Compared with Melissa O’Neill’s considerably more complex priority queuebased code it runs (compiled by ghc 6.10.1 with O2 switch) about 1.6x slower at producing 1 million primes, and about 1.7x slower at 10 to 20 million, with empirical complexity of n^1.23 vs ONeill's n^1.20; both versions having same low and nearconstant memory consumption. Ideone.com uses (ghc6.8.2) and limits run time to 15s; there the ratio is 1.16x .. 1.25x for generating 1 .. 6 million primes, and the empirical complexities are n^1.24 vs ONeill's n^1.20. WillNess 07:55, 13 February 2011 (UTC)
The primesFrom
function in Section 2.1.2.3 does not compile. Specifically, the sieve
function is defined to take in a list as its first argument, but is passed a number instead : (length h)
.
Gphilip 15:43, 16 February 2011 (UTC)
 thanks for catching this  I've tweaked the main function too many times and forgot to change the call appropriately. The arguments' order is now in sync. WillNess 00:53, 7 March 2011 (UTC)
[edit] 5 2012
[edit] 6 2013
Ideone.com now apparently works with ghc7.4.1. Test entry http://ideone.com/n3XzZY shows TMWE works OK, within nearly constant memory.