From cade17a9a65f04cb80692bbf2ef143fe88880381 Mon Sep 17 00:00:00 2001
From: Joseph Montanaro <jfmonty2@gmail.com>
Date: Sun, 18 Jul 2021 22:07:37 -0700
Subject: [PATCH] initial implementation

---
 fastrand.nim | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)
 create mode 100644 fastrand.nim

diff --git a/fastrand.nim b/fastrand.nim
new file mode 100644
index 0000000..9480411
--- /dev/null
+++ b/fastrand.nim
@@ -0,0 +1,52 @@
+import random, math
+import times, std/monotimes, strformat, strutils
+
+
+proc formatNum(n: SomeNumber): string =
+  let s = $(n.round)
+  let t = s[0 .. s.len - 3]
+  var count = 1
+  for i in countdown(t.high, 0):
+    result.insert($t[i], 0)
+    if count mod 3 == 0 and i != 0:
+      result.insert(",", 0)
+    count += 1
+
+
+proc formatRate(n: Natural, d: Duration): string =
+  result = formatNum(1_000_000'f64 * n.float64 / d.inMicroseconds.float64)
+
+
+proc fastRand(r: var Rand, x: Natural): uint64 =
+  # return ((r.next shr 32) * x.uint64) shr 32
+  let x = x.uint64
+  if x <= (uint64.high shl 32):
+    return ((r.next shr 32) * x.uint64) shr 32
+  else:
+    return r.next mod x.uint64
+
+
+proc testFastRand(num = 1_000_000_000): Duration =
+  var r = initRand(rand(int64))
+  let start = getMonoTime()
+  for i in 1 .. num:
+    discard r.fastRand(5)
+  result = getMonoTime() - start
+  # echo "fastrand execution rate: ",  1000 * num / dur.inMilliseconds.int, " generated per second."
+
+
+proc testStdRand(num = 1_000_000_000): Duration =
+  var r = initRand(rand(int64))
+  let start = getMonoTime()
+  for i in 1 .. num:
+    discard r.rand(4)
+  result = getMonoTime() - start
+  # echo "std rand execution rate: ", 1000 * num / dur.inMilliseconds.int, " generated per second."
+
+
+randomize()
+let runs = 100_000_000
+let fr = testFastRand(runs)
+echo "fastrand execution rate: ", formatNum(1_000_000 * runs / fr.inMicroseconds.int)
+let sr = testStdRand(runs)
+echo "standard execution rate: ", formatNum(1_000_000 * runs / sr.inMicroseconds.int)