Data commit

Task/Twin-primes/Nim/twin-primes.nim

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+import math, strformat, strutils
+
+const N = 1_000_000_000
+
+proc sieve(n: Positive): seq[bool] =
+  ## Build and fill a sieve of Erathosthenes.
+  result.setLen(n + 1)  # Default to false which means prime.
+  result[0] = true
+  result[1] = true
+  for n in countup(3, sqrt(N.toFloat).int, 2):
+    if not result[n]:
+      for k in countup(n * n, N, 2 * n):
+        result[k] = true
+
+let composite = sieve(N)
+
+proc findTwins(composite: openArray[bool]) =
+  var
+    lim = 10
+    count = 1     # Start with 3, 5 which is a special case.
+    n = 7         # First prime congruent to 1 modulo 3.
+  while true:
+    if not composite[n] and not composite[n - 2]: inc count
+    inc n, 6      # Next odd number congruent to 1 modulo 3.
+    if n > lim:
+      echo &"There are {insertSep($count)} pairs of twin primes under {insertSep($lim)}."
+      lim *= 10
+      if lim > N: break
+
+composite.findTwins()