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Task/Unbias-a-random-generator/Nim/unbias-a-random-generator.nim

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+import math, strutils
+randomize()
+
+template newSeqWith(len: int, init: expr): expr =
+  var result {.gensym.} = newSeq[type(init)](len)
+  for i in 0 .. <len:
+    result[i] = init
+  result
+
+proc randN(n): (proc: range[0..1]) =
+  result = proc(): range[0..1] =
+    if random(n) == 0: 1 else: 0
+
+proc unbiased(biased): range[0..1] =
+  var (this, that) = (biased(), biased())
+  while this == that:
+    this = biased()
+    that = biased()
+  return this
+
+for n in 3..6:
+  var biased = randN(n)
+  var v = newSeqWith(1_000_000, biased())
+  var cnt0, cnt1 = 0
+  for x in v:
+    if x == 0: inc cnt0
+    else:      inc cnt1
+  echo "Biased(",n,")  = count1=",cnt1,", count0=",cnt0,", percent=",
+       formatFloat(100 * float(cnt1)/float(cnt1+cnt0), ffDecimal, 3)
+
+  v = newSeqWith(1_000_000, unbiased(biased))
+  cnt0 = 0
+  cnt1 = 0
+  for x in v:
+    if x == 0: inc cnt0
+    else:      inc cnt1
+  echo "  Unbiased = count1=",cnt1,", count0=",cnt0,", percent=",
+       formatFloat(100 * float(cnt1)/float(cnt1+cnt0), ffDecimal, 3)