prog8/examples/primes.p8

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%import c64utils
%zeropage basicsafe
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main {
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ubyte[256] sieve
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ubyte candidate_prime = 2 ; is increased in the loop
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sub start() {
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memset(sieve, 256, false) ; clear the sieve, to reset starting situation on subsequent runs
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; calculate primes
c64scr.print("prime numbers up to 255:\n\n")
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ubyte amount=0
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while true {
ubyte prime = find_next_prime()
if prime==0
break
c64scr.print_ub(prime)
c64scr.print(", ")
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amount++
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}
c64.CHROUT('\n')
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c64scr.print("number of primes (expected 54): ")
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c64scr.print_ub(amount)
c64.CHROUT('\n')
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}
sub find_next_prime() -> ubyte {
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while sieve[candidate_prime] {
candidate_prime++
if candidate_prime==0
return 0 ; we wrapped; no more primes available in the sieve
}
; found next one, mark the multiples and return it.
sieve[candidate_prime] = true
uword multiple = candidate_prime
while multiple < len(sieve) {
sieve[lsb(multiple)] = true
multiple += candidate_prime
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}
return candidate_prime
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}
}