Consecutive primes with ascending or descending differences: Difference between revisions

=={{header|Pari/GP}}==

Code is pretty reasonable, runs in ~70 ms at 1,000,000. Running under PARI (with gp2c translation) could take advantage of the diff structure of the prime table directly for small cases and avoid substantial overhead, gaining at least a factor of 2 in performance.

<lang parigp>showPrecPrimes(p, n)=

{

my(v=vector(n));

v[n]=p;

forstep(i=n-1,1,-1,

v[i]=precprime(v[i+1]-1)

);

for(i=1,n, print1(v[i]" "));

}

list(lim)=

{

my(p=3,asc,dec,ar,dr,arAt=3,drAt=3,last=2);

forprime(q=5,lim,

my(g=q-p);

if(g<last,

asc=0;

if(desc++>dr,

dr=desc;

drAt=q

)

,g>last,

desc=0;

if(asc++>ar,

ar=asc;

arAt=q

)

,

asc=desc=0

);

p=q;

last=g

);

print("Descending differences:");

showPrecPrimes(drAt, dr+2);

print("\nAscending differences:");

showPrecPrimes(arAt, ar+2);

}

list(10^6)</lang>

{{out}}

<pre>Descending differences:

322171 322193 322213 322229 322237 322243 322247 322249

Ascending differences:

128981 128983 128987 128993 129001 129011 129023 129037</pre>