# Prime numbers miscellaneous

(Difference between revisions)

For a context to this, please see Prime numbers.

## 1 Implicit Heap

The following is an original implicit heap implementation for the sieve of Eratosthenes, kept here for historical record. The Prime_numbers#Tree Merging with Wheel section simplifies it, removing the `People a` structure altogether, and improves upon it by using a folding tree structure better adjusted for primes processing, and a wheel optimization.

See also the message threads Re: "no-coding" functional data structures via lazyness for more about how merging ordered lists amounts to creating an implicit heap and Re: Code and Perf. Data for Prime Finders for an explanation of the `People a` structure that makes it work.

```data People a = VIP a (People a) | Crowd [a]

mergeP :: Ord a => People a -> People a -> People a
mergeP (VIP x xt) ys                    = VIP x \$ mergeP xt ys
mergeP (Crowd xs) (Crowd ys)            = Crowd \$ merge  xs ys
mergeP xs@(Crowd (x:xt)) ys@(VIP y yt)  = case compare x y of
LT -> VIP x \$ mergeP (Crowd xt) ys
EQ -> VIP x \$ mergeP (Crowd xt) yt
GT -> VIP y \$ mergeP xs yt

merge :: Ord a => [a] -> [a] -> [a]
merge xs@(x:xt) ys@(y:yt) = case compare x y of
LT -> x : merge xt ys
EQ -> x : merge xt yt
GT -> y : merge xs yt

diff xs@(x:xt) ys@(y:yt) = case compare x y of
LT -> x : diff xt ys
EQ ->     diff xt yt
GT ->     diff xs yt

foldTree :: (a -> a -> a) -> [a] -> a
foldTree f ~(x:xs) = x `f` foldTree f (pairs xs)
where pairs ~(x: ~(y:ys)) = f x y : pairs ys

primes, nonprimes :: [Integer]
primes    = 2:3:diff [5,7..] nonprimes
nonprimes = serve . foldTree mergeP . map multiples \$ tail primes
where
multiples p = vip [p*p,p*p+2*p..]

vip (x:xs)       = VIP x \$ Crowd xs
serve (VIP x xs) = x:serve xs
serve (Crowd xs) = xs```

`nonprimes` effectively implements a heap, exploiting lazy evaluation.

## 2 Prime Wheels

The idea of only testing odd numbers can be extended further. For instance, it is a useful fact that every prime number other than 2 and 3 must be of the form 6k + 1 or 6k + 5. Thus, we only need to test these numbers:

```primes :: [Integer]
primes = 2:3:primes'
where
1:p:candidates = [6*k+r | k <- [0..], r <- [1,5]]
primes'        = p : filter isPrime candidates
isPrime n      = all (not . divides n)
\$ takeWhile (\p -> p*p <= n) primes'
divides n p    = n `mod` p == 0```
Here,
primes'
is the list of primes greater than 3 and
isPrime
does not test for divisibility by 2 or 3 because the
candidates
by construction don't have these numbers as factors. We also need to exclude 1 from the candidates and mark the next one as prime to start the recursion.

Such a scheme to generate candidate numbers first that avoid a given set of primes as divisors is called a prime wheel. Imagine that you had a wheel of circumference 6 to be rolled along the number line. With spikes positioned 1 and 5 units around the circumference, rolling the wheel will prick holes exactly in those positions on the line whose numbers are not divisible by 2 and 3.

A wheel can be represented by its circumference and the spiked positions.

`data Wheel = Wheel Integer [Integer]`

We prick out numbers by rolling the wheel.

`roll (Wheel n rs) = [n*k+r | k <- [0..], r <- rs]`

The smallest wheel is the unit wheel with one spike, it will prick out every number.

`w0 = Wheel 1 [1]`
We can create a larger wheel by rolling a smaller wheel of circumference
n
along a rim of circumference
p*n
while excluding spike positions at multiples of
p
.
```nextSize (Wheel n rs) p =
Wheel (p*n) [r' | k <- [0..(p-1)], r <- rs,
let r' = n*k+r, r' `mod` p /= 0]```
Combining both, we can make wheels that prick out numbers that avoid a given list
ds
of divisors.
`mkWheel ds = foldl nextSize w0 ds`

Now, we can generate prime numbers with a wheel that for instance avoids all multiples of 2, 3, 5 and 7.

```primes :: [Integer]
primes = small ++ large
where
1:p:candidates = roll \$ mkWheel small
small          = [2,3,5,7]
large          = p : filter isPrime candidates
isPrime n      = all (not . divides n)
\$ takeWhile (\p -> p*p <= n) large
divides n p    = n `mod` p == 0```

It's a pretty big wheel with a circumference of 210 and allows us to calculate the first 10000 primes in convenient time.

A fixed size wheel is fine, but how about adapting the wheel size while generating prime numbers? See Euler's Sieve, or the functional pearl titled Lazy wheel sieves and spirals of primes for more.