Copyright | (c) 2011 Daniel Fischer |
---|---|
License | MIT |
Maintainer | Daniel Fischer <daniel.is.fischer@googlemail.com> |
Safe Haskell | None |
Language | Haskell2010 |
Prime generation using a sieve. Currently, an enhanced sieve of Eratosthenes is used, switching to an Atkin sieve is planned (if I get around to implementing it and it's not slower).
The sieve used is segmented, with a chunk size chosen to give good (enough)
cache locality while still getting something substantial done per chunk.
However, that means we must store data for primes up to the square root of
where sieving is done, thus sieving primes up to n
requires
O(sqrt n/log n)
space.
Synopsis
- primes :: Integral a => [Prime a]
- sieveFrom :: Integer -> [Prime Integer]
- data PrimeSieve
- primeSieve :: Integer -> PrimeSieve
- psieveList :: [PrimeSieve]
- psieveFrom :: Integer -> [PrimeSieve]
- primeList :: forall a. Integral a => PrimeSieve -> [Prime a]
Limitations
There are three factors limiting the range of these sieves.
- Memory
- Overflow
- The internal representation of the state
An Eratosthenes type sieve needs to store the primes up to the square root of
the currently sieved region, thus requires O(sqrt n/log n)
space.We store 16
bytes
of information per prime, thus a Gigabyte of memory takes you to about 1.6*10^18
.
The log
doesn't change much in that range, so as a first approximation, doubling
the storage increases the sieve range by a factor of four.
On a 64-bit system, this is (currently) the only limitation to be concerned with, but
with more than four Terabyte of memory, the fact that the internal representation
currently limits the sieve range to about 6.8*10^25
could become relevant.
Overflow in array indexing doesn't become a concern before memory and internal
representation would allow to sieve past 10^37
.
On a 32-bit system, the internal representation imposes no additional limits,
but overflow has to be reckoned with. On the one hand, the fact that arrays are
Int
-indexed restricts the size of the prime store, on the other hand, overflow
in calculating the indices to cross off multiples is possible before running out
of memory. The former limits the upper bound of the monolithic primeSieve
to
shortly above 8*10^9
, the latter limits the range of the segmented sieves to
about 1.7*10^18
.
Sieves and lists
primes :: Integral a => [Prime a] Source #
Ascending list of primes.
>>>
take 10 primes
[Prime 2,Prime 3,Prime 5,Prime 7,Prime 11,Prime 13,Prime 17,Prime 19,Prime 23,Prime 29]
primes
is a polymorphic list, so the results of computations are not retained in memory.
Make it monomorphic to take advantages of memoization. Compare
>>>
:set +s
>>>
primes !! 1000000 :: Prime Int
Prime 15485867 (5.32 secs, 6,945,267,496 bytes)>>>
primes !! 1000000 :: Prime Int
Prime 15485867 (5.19 secs, 6,945,267,496 bytes)
against
>>>
let primes' = primes :: [Prime Int]
>>>
primes' !! 1000000 :: Prime Int
Prime 15485867 (5.29 secs, 6,945,269,856 bytes)>>>
primes' !! 1000000 :: Prime Int
Prime 15485867 (0.02 secs, 336,232 bytes)
sieveFrom :: Integer -> [Prime Integer] Source #
creates the list of primes not less than sieveFrom
nn
.
data PrimeSieve Source #
Compact store of primality flags.
primeSieve :: Integer -> PrimeSieve Source #
Sieve primes up to (and including) a bound (or 7, if bound is smaller). For small enough bounds, this is more efficient than using the segmented sieve.
Since arrays are Int
-indexed, overflow occurs when the sieve
size comes near
, that corresponds to an
upper bound near maxBound
:: Int
15/8*
. On maxBound
32
-bit systems, that
is often within memory limits, so don't give bounds larger than
8*10^9
there.
psieveList :: [PrimeSieve] Source #
List of primes in the form of a list of PrimeSieve
s, more compact than
primes
, thus it may be better to use
than keeping the list of primes alive during the entire run.psieveList
>>= primeList
psieveFrom :: Integer -> [PrimeSieve] Source #
creates the list of psieveFrom
nPrimeSieve
s starting roughly
at n
. Due to the organisation of the sieve, the list may contain
a few primes less than n
.
This form uses less memory than [
, hence it may be preferable
to use this if it is to be reused.Integer
]
primeList :: forall a. Integral a => PrimeSieve -> [Prime a] Source #
Generate a list of primes for consumption from a
PrimeSieve
.