Data commit

Task/Additive-primes/Crystal/additive-primes.crystal

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+# Fast/simple way to generate primes for small values.
+# Uses P3 Prime Generator (PG) and its Prime Generator Sequence (PGS).
+
+def prime?(n) # P3 Prime Generator primality test
+  return false unless (n | 1 == 3 if n < 5) || (n % 6) | 4 == 5
+  sqrt_n = Math.isqrt(n)  # For Crystal < 1.2.0 use Math.sqrt(n).to_i
+  pc = typeof(n).new(5)
+  while pc <= sqrt_n
+    return false if n % pc == 0 || n % (pc + 2) == 0
+    pc += 6
+  end
+  true
+end
+
+def additive_primes(n)
+  primes = [2, 3]
+  pc, inc = 5, 2
+  while pc < n
+    primes << pc if prime?(pc) && prime?(pc.digits.sum)
+    pc += inc; inc ^= 0b110  # generate P3 sequence: 5 7 11 13 17 19 ...
+  end
+  primes # list of additive primes <= n
+end
+
+nn = 500
+addprimes = additive_primes(nn)
+maxdigits = addprimes.last.digits.size
+addprimes.each_with_index { |n, idx| printf "%*d ", maxdigits, n; print "\n" if idx % 10 == 9 } # more efficient
+#addprimes.each_with_index { |n, idx| print "%#{maxdigits}d " % n; print "\n" if idx % 10 == 9} # alternatively
+puts "\n#{addprimes.size} additive primes below #{nn}."
+
+puts
+
+nn = 5000
+addprimes = additive_primes(nn)
+maxdigits = addprimes.last.digits.size
+addprimes.each_with_index { |n, idx| printf "%*d ", maxdigits, n; print "\n" if idx % 10 == 9 } # more efficient
+puts "\n#{addprimes.size} additive primes below #{nn}."